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1 | \input texinfo |
2 | @setfilename ld.info | |
3 | @syncodeindex ky cp | |
4 | @include configdoc.texi | |
5 | @c (configdoc.texi is generated by the Makefile) | |
6 | @include ldver.texi | |
7 | ||
8 | @c @smallbook | |
9 | ||
10 | @ifinfo | |
11 | @format | |
12 | START-INFO-DIR-ENTRY | |
13 | * Ld: (ld). The GNU linker. | |
14 | END-INFO-DIR-ENTRY | |
15 | @end format | |
16 | @end ifinfo | |
17 | ||
18 | @ifinfo | |
19 | This file documents the @sc{gnu} linker LD version @value{VERSION}. | |
20 | ||
18625d54 | 21 | Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000 Free Software Foundation, Inc. |
252b5132 RH |
22 | |
23 | Permission is granted to make and distribute verbatim copies of | |
24 | this manual provided the copyright notice and this permission notice | |
25 | are preserved on all copies. | |
26 | ||
27 | Permission is granted to copy and distribute modified versions of this | |
28 | manual under the conditions for verbatim copying, provided also that | |
29 | the entire resulting derived work is distributed under the terms of a | |
30 | permission notice identical to this one. | |
31 | ||
32 | Permission is granted to copy and distribute translations of this manual | |
33 | into another language, under the above conditions for modified versions. | |
34 | ||
35 | @ignore | |
36 | Permission is granted to process this file through Tex and print the | |
37 | results, provided the printed document carries copying permission | |
38 | notice identical to this one except for the removal of this paragraph | |
39 | (this paragraph not being relevant to the printed manual). | |
40 | ||
41 | @end ignore | |
42 | @end ifinfo | |
43 | @iftex | |
44 | @finalout | |
45 | @setchapternewpage odd | |
46 | @settitle Using LD, the GNU linker | |
47 | @titlepage | |
48 | @title Using ld | |
49 | @subtitle The GNU linker | |
50 | @sp 1 | |
51 | @subtitle @code{ld} version 2 | |
52 | @subtitle Version @value{VERSION} | |
53 | @author Steve Chamberlain | |
54 | @author Ian Lance Taylor | |
55 | @author Cygnus Solutions | |
56 | @page | |
57 | ||
58 | @tex | |
59 | {\parskip=0pt | |
60 | \hfill Cygnus Solutions\par | |
61 | \hfill ian\@cygnus.com, doc\@cygnus.com\par | |
62 | \hfill {\it Using LD, the GNU linker}\par | |
63 | \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par | |
64 | } | |
65 | \global\parindent=0pt % Steve likes it this way. | |
66 | @end tex | |
67 | ||
68 | @vskip 0pt plus 1filll | |
69 | Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 98, 1999 Free Software Foundation, Inc. | |
70 | ||
71 | Permission is granted to make and distribute verbatim copies of | |
72 | this manual provided the copyright notice and this permission notice | |
73 | are preserved on all copies. | |
74 | ||
75 | Permission is granted to copy and distribute modified versions of this | |
76 | manual under the conditions for verbatim copying, provided also that | |
77 | the entire resulting derived work is distributed under the terms of a | |
78 | permission notice identical to this one. | |
79 | ||
80 | Permission is granted to copy and distribute translations of this manual | |
81 | into another language, under the above conditions for modified versions. | |
82 | @end titlepage | |
83 | @end iftex | |
84 | @c FIXME: Talk about importance of *order* of args, cmds to linker! | |
85 | ||
86 | @ifinfo | |
87 | @node Top | |
88 | @top Using ld | |
89 | This file documents the @sc{gnu} linker ld version @value{VERSION}. | |
90 | ||
91 | @menu | |
92 | * Overview:: Overview | |
93 | * Invocation:: Invocation | |
94 | * Scripts:: Linker Scripts | |
95 | @ifset GENERIC | |
96 | * Machine Dependent:: Machine Dependent Features | |
97 | @end ifset | |
98 | @ifclear GENERIC | |
99 | @ifset H8300 | |
100 | * H8/300:: ld and the H8/300 | |
101 | @end ifset | |
102 | @ifset Hitachi | |
103 | * Hitachi:: ld and other Hitachi micros | |
104 | @end ifset | |
105 | @ifset I960 | |
106 | * i960:: ld and the Intel 960 family | |
107 | @end ifset | |
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108 | @ifset TICOFF |
109 | * TI COFF:: ld and the TI COFF | |
110 | @end ifset | |
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111 | @end ifclear |
112 | @ifclear SingleFormat | |
113 | * BFD:: BFD | |
114 | @end ifclear | |
115 | @c Following blank line required for remaining bug in makeinfo conds/menus | |
116 | ||
117 | * Reporting Bugs:: Reporting Bugs | |
118 | * MRI:: MRI Compatible Script Files | |
119 | * Index:: Index | |
120 | @end menu | |
121 | @end ifinfo | |
122 | ||
123 | @node Overview | |
124 | @chapter Overview | |
125 | ||
126 | @cindex @sc{gnu} linker | |
127 | @cindex what is this? | |
128 | @code{ld} combines a number of object and archive files, relocates | |
129 | their data and ties up symbol references. Usually the last step in | |
130 | compiling a program is to run @code{ld}. | |
131 | ||
132 | @code{ld} accepts Linker Command Language files written in | |
133 | a superset of AT&T's Link Editor Command Language syntax, | |
134 | to provide explicit and total control over the linking process. | |
135 | ||
136 | @ifclear SingleFormat | |
137 | This version of @code{ld} uses the general purpose BFD libraries | |
138 | to operate on object files. This allows @code{ld} to read, combine, and | |
139 | write object files in many different formats---for example, COFF or | |
140 | @code{a.out}. Different formats may be linked together to produce any | |
141 | available kind of object file. @xref{BFD}, for more information. | |
142 | @end ifclear | |
143 | ||
144 | Aside from its flexibility, the @sc{gnu} linker is more helpful than other | |
145 | linkers in providing diagnostic information. Many linkers abandon | |
146 | execution immediately upon encountering an error; whenever possible, | |
147 | @code{ld} continues executing, allowing you to identify other errors | |
148 | (or, in some cases, to get an output file in spite of the error). | |
149 | ||
150 | @node Invocation | |
151 | @chapter Invocation | |
152 | ||
153 | The @sc{gnu} linker @code{ld} is meant to cover a broad range of situations, | |
154 | and to be as compatible as possible with other linkers. As a result, | |
155 | you have many choices to control its behavior. | |
156 | ||
157 | @ifset UsesEnvVars | |
158 | @menu | |
159 | * Options:: Command Line Options | |
160 | * Environment:: Environment Variables | |
161 | @end menu | |
162 | ||
163 | @node Options | |
164 | @section Command Line Options | |
165 | @end ifset | |
166 | ||
167 | @cindex command line | |
168 | @cindex options | |
169 | The linker supports a plethora of command-line options, but in actual | |
170 | practice few of them are used in any particular context. | |
171 | @cindex standard Unix system | |
172 | For instance, a frequent use of @code{ld} is to link standard Unix | |
173 | object files on a standard, supported Unix system. On such a system, to | |
174 | link a file @code{hello.o}: | |
175 | ||
176 | @smallexample | |
177 | ld -o @var{output} /lib/crt0.o hello.o -lc | |
178 | @end smallexample | |
179 | ||
180 | This tells @code{ld} to produce a file called @var{output} as the | |
181 | result of linking the file @code{/lib/crt0.o} with @code{hello.o} and | |
182 | the library @code{libc.a}, which will come from the standard search | |
183 | directories. (See the discussion of the @samp{-l} option below.) | |
184 | ||
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185 | Some of the command-line options to @code{ld} may be specified at any |
186 | point in the command line. However, options which refer to files, such | |
187 | as @samp{-l} or @samp{-T}, cause the file to be read at the point at | |
188 | which the option appears in the command line, relative to the object | |
189 | files and other file options. Repeating non-file options with a | |
190 | different argument will either have no further effect, or override prior | |
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191 | occurrences (those further to the left on the command line) of that |
192 | option. Options which may be meaningfully specified more than once are | |
193 | noted in the descriptions below. | |
194 | ||
195 | @cindex object files | |
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196 | Non-option arguments are object files or archives which are to be linked |
197 | together. They may follow, precede, or be mixed in with command-line | |
198 | options, except that an object file argument may not be placed between | |
199 | an option and its argument. | |
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200 | |
201 | Usually the linker is invoked with at least one object file, but you can | |
202 | specify other forms of binary input files using @samp{-l}, @samp{-R}, | |
203 | and the script command language. If @emph{no} binary input files at all | |
204 | are specified, the linker does not produce any output, and issues the | |
205 | message @samp{No input files}. | |
206 | ||
207 | If the linker can not recognize the format of an object file, it will | |
208 | assume that it is a linker script. A script specified in this way | |
209 | augments the main linker script used for the link (either the default | |
210 | linker script or the one specified by using @samp{-T}). This feature | |
211 | permits the linker to link against a file which appears to be an object | |
212 | or an archive, but actually merely defines some symbol values, or uses | |
213 | @code{INPUT} or @code{GROUP} to load other objects. Note that | |
214 | specifying a script in this way should only be used to augment the main | |
215 | linker script; if you want to use some command that logically can only | |
216 | appear once, such as the @code{SECTIONS} or @code{MEMORY} command, you | |
217 | must replace the default linker script using the @samp{-T} option. | |
218 | @xref{Scripts}. | |
219 | ||
220 | For options whose names are a single letter, | |
221 | option arguments must either follow the option letter without intervening | |
222 | whitespace, or be given as separate arguments immediately following the | |
223 | option that requires them. | |
224 | ||
225 | For options whose names are multiple letters, either one dash or two can | |
226 | precede the option name; for example, @samp{--oformat} and | |
227 | @samp{--oformat} are equivalent. Arguments to multiple-letter options | |
228 | must either be separated from the option name by an equals sign, or be | |
229 | given as separate arguments immediately following the option that | |
230 | requires them. For example, @samp{--oformat srec} and | |
231 | @samp{--oformat=srec} are equivalent. Unique abbreviations of the names | |
232 | of multiple-letter options are accepted. | |
233 | ||
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234 | Note - if the linker is being invoked indirectly, via a compiler driver |
235 | (eg @samp{gcc}) then all the linker command line options should be | |
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236 | prefixed by @samp{-Wl,} (or whatever is appropriate for the particular |
237 | compiler driver) like this: | |
4e53152f NC |
238 | |
239 | @smallexample | |
240 | gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup | |
241 | @end smallexample | |
242 | ||
243 | This is important, because otherwise the compiler driver program may | |
244 | silently drop the linker options, resulting in a bad link. | |
245 | ||
246 | Here is a table of the generic command line switches accepted by the GNU | |
247 | linker: | |
248 | ||
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249 | @table @code |
250 | @kindex -a@var{keyword} | |
251 | @item -a@var{keyword} | |
252 | This option is supported for HP/UX compatibility. The @var{keyword} | |
253 | argument must be one of the strings @samp{archive}, @samp{shared}, or | |
254 | @samp{default}. @samp{-aarchive} is functionally equivalent to | |
255 | @samp{-Bstatic}, and the other two keywords are functionally equivalent | |
256 | to @samp{-Bdynamic}. This option may be used any number of times. | |
257 | ||
258 | @ifset I960 | |
259 | @cindex architectures | |
260 | @kindex -A@var{arch} | |
261 | @item -A@var{architecture} | |
262 | @kindex --architecture=@var{arch} | |
263 | @itemx --architecture=@var{architecture} | |
264 | In the current release of @code{ld}, this option is useful only for the | |
265 | Intel 960 family of architectures. In that @code{ld} configuration, the | |
266 | @var{architecture} argument identifies the particular architecture in | |
267 | the 960 family, enabling some safeguards and modifying the | |
268 | archive-library search path. @xref{i960,,@code{ld} and the Intel 960 | |
269 | family}, for details. | |
270 | ||
271 | Future releases of @code{ld} may support similar functionality for | |
272 | other architecture families. | |
273 | @end ifset | |
274 | ||
275 | @ifclear SingleFormat | |
276 | @cindex binary input format | |
277 | @kindex -b @var{format} | |
278 | @kindex --format=@var{format} | |
279 | @cindex input format | |
280 | @cindex input format | |
281 | @item -b @var{input-format} | |
282 | @itemx --format=@var{input-format} | |
283 | @code{ld} may be configured to support more than one kind of object | |
284 | file. If your @code{ld} is configured this way, you can use the | |
285 | @samp{-b} option to specify the binary format for input object files | |
286 | that follow this option on the command line. Even when @code{ld} is | |
287 | configured to support alternative object formats, you don't usually need | |
288 | to specify this, as @code{ld} should be configured to expect as a | |
289 | default input format the most usual format on each machine. | |
290 | @var{input-format} is a text string, the name of a particular format | |
291 | supported by the BFD libraries. (You can list the available binary | |
292 | formats with @samp{objdump -i}.) | |
293 | @xref{BFD}. | |
294 | ||
295 | You may want to use this option if you are linking files with an unusual | |
296 | binary format. You can also use @samp{-b} to switch formats explicitly (when | |
297 | linking object files of different formats), by including | |
298 | @samp{-b @var{input-format}} before each group of object files in a | |
299 | particular format. | |
300 | ||
301 | The default format is taken from the environment variable | |
302 | @code{GNUTARGET}. | |
303 | @ifset UsesEnvVars | |
304 | @xref{Environment}. | |
305 | @end ifset | |
306 | You can also define the input format from a script, using the command | |
307 | @code{TARGET}; see @ref{Format Commands}. | |
308 | @end ifclear | |
309 | ||
310 | @kindex -c @var{MRI-cmdfile} | |
311 | @kindex --mri-script=@var{MRI-cmdfile} | |
312 | @cindex compatibility, MRI | |
313 | @item -c @var{MRI-commandfile} | |
314 | @itemx --mri-script=@var{MRI-commandfile} | |
315 | For compatibility with linkers produced by MRI, @code{ld} accepts script | |
316 | files written in an alternate, restricted command language, described in | |
317 | @ref{MRI,,MRI Compatible Script Files}. Introduce MRI script files with | |
318 | the option @samp{-c}; use the @samp{-T} option to run linker | |
319 | scripts written in the general-purpose @code{ld} scripting language. | |
320 | If @var{MRI-cmdfile} does not exist, @code{ld} looks for it in the directories | |
321 | specified by any @samp{-L} options. | |
322 | ||
323 | @cindex common allocation | |
324 | @kindex -d | |
325 | @kindex -dc | |
326 | @kindex -dp | |
327 | @item -d | |
328 | @itemx -dc | |
329 | @itemx -dp | |
330 | These three options are equivalent; multiple forms are supported for | |
331 | compatibility with other linkers. They assign space to common symbols | |
332 | even if a relocatable output file is specified (with @samp{-r}). The | |
333 | script command @code{FORCE_COMMON_ALLOCATION} has the same effect. | |
334 | @xref{Miscellaneous Commands}. | |
335 | ||
336 | @cindex entry point, from command line | |
337 | @kindex -e @var{entry} | |
338 | @kindex --entry=@var{entry} | |
339 | @item -e @var{entry} | |
340 | @itemx --entry=@var{entry} | |
341 | Use @var{entry} as the explicit symbol for beginning execution of your | |
342 | program, rather than the default entry point. If there is no symbol | |
343 | named @var{entry}, the linker will try to parse @var{entry} as a number, | |
344 | and use that as the entry address (the number will be interpreted in | |
345 | base 10; you may use a leading @samp{0x} for base 16, or a leading | |
346 | @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults | |
347 | and other ways of specifying the entry point. | |
348 | ||
349 | @cindex dynamic symbol table | |
350 | @kindex -E | |
351 | @kindex --export-dynamic | |
352 | @item -E | |
353 | @itemx --export-dynamic | |
354 | When creating a dynamically linked executable, add all symbols to the | |
355 | dynamic symbol table. The dynamic symbol table is the set of symbols | |
356 | which are visible from dynamic objects at run time. | |
357 | ||
358 | If you do not use this option, the dynamic symbol table will normally | |
359 | contain only those symbols which are referenced by some dynamic object | |
360 | mentioned in the link. | |
361 | ||
362 | If you use @code{dlopen} to load a dynamic object which needs to refer | |
363 | back to the symbols defined by the program, rather than some other | |
364 | dynamic object, then you will probably need to use this option when | |
365 | linking the program itself. | |
366 | ||
367 | @cindex big-endian objects | |
368 | @cindex endianness | |
369 | @kindex -EB | |
370 | @item -EB | |
371 | Link big-endian objects. This affects the default output format. | |
372 | ||
373 | @cindex little-endian objects | |
374 | @kindex -EL | |
375 | @item -EL | |
376 | Link little-endian objects. This affects the default output format. | |
377 | ||
378 | @kindex -f | |
379 | @kindex --auxiliary | |
380 | @item -f | |
381 | @itemx --auxiliary @var{name} | |
382 | When creating an ELF shared object, set the internal DT_AUXILIARY field | |
383 | to the specified name. This tells the dynamic linker that the symbol | |
384 | table of the shared object should be used as an auxiliary filter on the | |
385 | symbol table of the shared object @var{name}. | |
386 | ||
387 | If you later link a program against this filter object, then, when you | |
388 | run the program, the dynamic linker will see the DT_AUXILIARY field. If | |
389 | the dynamic linker resolves any symbols from the filter object, it will | |
390 | first check whether there is a definition in the shared object | |
391 | @var{name}. If there is one, it will be used instead of the definition | |
392 | in the filter object. The shared object @var{name} need not exist. | |
393 | Thus the shared object @var{name} may be used to provide an alternative | |
394 | implementation of certain functions, perhaps for debugging or for | |
395 | machine specific performance. | |
396 | ||
397 | This option may be specified more than once. The DT_AUXILIARY entries | |
398 | will be created in the order in which they appear on the command line. | |
399 | ||
400 | @kindex -F | |
401 | @kindex --filter | |
402 | @item -F @var{name} | |
403 | @itemx --filter @var{name} | |
404 | When creating an ELF shared object, set the internal DT_FILTER field to | |
405 | the specified name. This tells the dynamic linker that the symbol table | |
406 | of the shared object which is being created should be used as a filter | |
407 | on the symbol table of the shared object @var{name}. | |
408 | ||
409 | If you later link a program against this filter object, then, when you | |
410 | run the program, the dynamic linker will see the DT_FILTER field. The | |
411 | dynamic linker will resolve symbols according to the symbol table of the | |
412 | filter object as usual, but it will actually link to the definitions | |
413 | found in the shared object @var{name}. Thus the filter object can be | |
414 | used to select a subset of the symbols provided by the object | |
415 | @var{name}. | |
416 | ||
417 | Some older linkers used the @code{-F} option throughout a compilation | |
418 | toolchain for specifying object-file format for both input and output | |
419 | object files. The @sc{gnu} linker uses other mechanisms for this | |
420 | purpose: the @code{-b}, @code{--format}, @code{--oformat} options, the | |
421 | @code{TARGET} command in linker scripts, and the @code{GNUTARGET} | |
422 | environment variable. The @sc{gnu} linker will ignore the @code{-F} | |
423 | option when not creating an ELF shared object. | |
424 | ||
3dbf70a2 MM |
425 | @cindex finalization function |
426 | @kindex -fini | |
427 | @item -fini @var{name} | |
428 | When creating an ELF executable or shared object, call NAME when the | |
429 | executable or shared object is unloaded, by setting DT_FINI to the | |
430 | address of the function. By default, the linker uses @code{_fini} as | |
431 | the function to call. | |
432 | ||
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433 | @kindex -g |
434 | @item -g | |
435 | Ignored. Provided for compatibility with other tools. | |
436 | ||
437 | @kindex -G | |
438 | @kindex --gpsize | |
439 | @cindex object size | |
440 | @item -G@var{value} | |
441 | @itemx --gpsize=@var{value} | |
442 | Set the maximum size of objects to be optimized using the GP register to | |
443 | @var{size}. This is only meaningful for object file formats such as | |
444 | MIPS ECOFF which supports putting large and small objects into different | |
445 | sections. This is ignored for other object file formats. | |
446 | ||
447 | @cindex runtime library name | |
448 | @kindex -h@var{name} | |
449 | @kindex -soname=@var{name} | |
450 | @item -h@var{name} | |
451 | @itemx -soname=@var{name} | |
452 | When creating an ELF shared object, set the internal DT_SONAME field to | |
453 | the specified name. When an executable is linked with a shared object | |
454 | which has a DT_SONAME field, then when the executable is run the dynamic | |
455 | linker will attempt to load the shared object specified by the DT_SONAME | |
456 | field rather than the using the file name given to the linker. | |
457 | ||
458 | @kindex -i | |
459 | @cindex incremental link | |
460 | @item -i | |
461 | Perform an incremental link (same as option @samp{-r}). | |
462 | ||
3dbf70a2 MM |
463 | @cindex initialization function |
464 | @kindex -init | |
465 | @item -init @var{name} | |
466 | When creating an ELF executable or shared object, call NAME when the | |
467 | executable or shared object is loaded, by setting DT_INIT to the address | |
468 | of the function. By default, the linker uses @code{_init} as the | |
469 | function to call. | |
470 | ||
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471 | @cindex archive files, from cmd line |
472 | @kindex -l@var{archive} | |
473 | @kindex --library=@var{archive} | |
474 | @item -l@var{archive} | |
475 | @itemx --library=@var{archive} | |
476 | Add archive file @var{archive} to the list of files to link. This | |
477 | option may be used any number of times. @code{ld} will search its | |
478 | path-list for occurrences of @code{lib@var{archive}.a} for every | |
479 | @var{archive} specified. | |
480 | ||
481 | On systems which support shared libraries, @code{ld} may also search for | |
482 | libraries with extensions other than @code{.a}. Specifically, on ELF | |
483 | and SunOS systems, @code{ld} will search a directory for a library with | |
484 | an extension of @code{.so} before searching for one with an extension of | |
485 | @code{.a}. By convention, a @code{.so} extension indicates a shared | |
486 | library. | |
487 | ||
488 | The linker will search an archive only once, at the location where it is | |
489 | specified on the command line. If the archive defines a symbol which | |
490 | was undefined in some object which appeared before the archive on the | |
491 | command line, the linker will include the appropriate file(s) from the | |
492 | archive. However, an undefined symbol in an object appearing later on | |
493 | the command line will not cause the linker to search the archive again. | |
494 | ||
495 | See the @code{-(} option for a way to force the linker to search | |
496 | archives multiple times. | |
497 | ||
498 | You may list the same archive multiple times on the command line. | |
499 | ||
500 | @ifset GENERIC | |
501 | This type of archive searching is standard for Unix linkers. However, | |
502 | if you are using @code{ld} on AIX, note that it is different from the | |
503 | behaviour of the AIX linker. | |
504 | @end ifset | |
505 | ||
506 | @cindex search directory, from cmd line | |
507 | @kindex -L@var{dir} | |
508 | @kindex --library-path=@var{dir} | |
509 | @item -L@var{searchdir} | |
510 | @itemx --library-path=@var{searchdir} | |
511 | Add path @var{searchdir} to the list of paths that @code{ld} will search | |
512 | for archive libraries and @code{ld} control scripts. You may use this | |
513 | option any number of times. The directories are searched in the order | |
514 | in which they are specified on the command line. Directories specified | |
515 | on the command line are searched before the default directories. All | |
516 | @code{-L} options apply to all @code{-l} options, regardless of the | |
517 | order in which the options appear. | |
518 | ||
519 | @ifset UsesEnvVars | |
520 | The default set of paths searched (without being specified with | |
521 | @samp{-L}) depends on which emulation mode @code{ld} is using, and in | |
522 | some cases also on how it was configured. @xref{Environment}. | |
523 | @end ifset | |
524 | ||
525 | The paths can also be specified in a link script with the | |
526 | @code{SEARCH_DIR} command. Directories specified this way are searched | |
527 | at the point in which the linker script appears in the command line. | |
528 | ||
529 | @cindex emulation | |
530 | @kindex -m @var{emulation} | |
531 | @item -m@var{emulation} | |
532 | Emulate the @var{emulation} linker. You can list the available | |
533 | emulations with the @samp{--verbose} or @samp{-V} options. | |
534 | ||
535 | If the @samp{-m} option is not used, the emulation is taken from the | |
536 | @code{LDEMULATION} environment variable, if that is defined. | |
537 | ||
538 | Otherwise, the default emulation depends upon how the linker was | |
539 | configured. | |
540 | ||
541 | @cindex link map | |
542 | @kindex -M | |
543 | @kindex --print-map | |
544 | @item -M | |
545 | @itemx --print-map | |
546 | Print a link map to the standard output. A link map provides | |
547 | information about the link, including the following: | |
548 | ||
549 | @itemize @bullet | |
550 | @item | |
551 | Where object files and symbols are mapped into memory. | |
552 | @item | |
553 | How common symbols are allocated. | |
554 | @item | |
555 | All archive members included in the link, with a mention of the symbol | |
556 | which caused the archive member to be brought in. | |
557 | @end itemize | |
558 | ||
559 | @kindex -n | |
560 | @cindex read-only text | |
561 | @cindex NMAGIC | |
562 | @kindex --nmagic | |
563 | @item -n | |
564 | @itemx --nmagic | |
fa19fce0 NC |
565 | Turn off page alignment of sections, and mark the output as |
566 | @code{NMAGIC} if possible. | |
252b5132 RH |
567 | |
568 | @kindex -N | |
569 | @kindex --omagic | |
570 | @cindex read/write from cmd line | |
571 | @cindex OMAGIC | |
572 | @item -N | |
573 | @itemx --omagic | |
574 | Set the text and data sections to be readable and writable. Also, do | |
575 | not page-align the data segment. If the output format supports Unix | |
576 | style magic numbers, mark the output as @code{OMAGIC}. | |
577 | ||
578 | @kindex -o @var{output} | |
579 | @kindex --output=@var{output} | |
580 | @cindex naming the output file | |
581 | @item -o @var{output} | |
582 | @itemx --output=@var{output} | |
583 | Use @var{output} as the name for the program produced by @code{ld}; if this | |
584 | option is not specified, the name @file{a.out} is used by default. The | |
585 | script command @code{OUTPUT} can also specify the output file name. | |
586 | ||
587 | @kindex -O @var{level} | |
588 | @cindex generating optimized output | |
589 | @item -O @var{level} | |
590 | If @var{level} is a numeric values greater than zero @code{ld} optimizes | |
591 | the output. This might take significantly longer and therefore probably | |
592 | should only be enabled for the final binary. | |
593 | ||
a712da20 NC |
594 | @kindex -q |
595 | @kindex --emit-relocs | |
596 | @cindex retain relocations in final executable | |
597 | @item -q | |
598 | @itemx --emit-relocs | |
599 | Leave relocation sections and contents in fully linked exececutables. | |
600 | Post link analysis and optimization tools may need this information in | |
601 | order to perform correct modifications of executables. This results | |
602 | in larger executables. | |
603 | ||
252b5132 RH |
604 | @cindex partial link |
605 | @cindex relocatable output | |
606 | @kindex -r | |
607 | @kindex --relocateable | |
608 | @item -r | |
609 | @itemx --relocateable | |
610 | Generate relocatable output---i.e., generate an output file that can in | |
611 | turn serve as input to @code{ld}. This is often called @dfn{partial | |
612 | linking}. As a side effect, in environments that support standard Unix | |
613 | magic numbers, this option also sets the output file's magic number to | |
614 | @code{OMAGIC}. | |
615 | @c ; see @code{-N}. | |
616 | If this option is not specified, an absolute file is produced. When | |
617 | linking C++ programs, this option @emph{will not} resolve references to | |
618 | constructors; to do that, use @samp{-Ur}. | |
619 | ||
620 | This option does the same thing as @samp{-i}. | |
621 | ||
622 | @kindex -R @var{file} | |
623 | @kindex --just-symbols=@var{file} | |
624 | @cindex symbol-only input | |
625 | @item -R @var{filename} | |
626 | @itemx --just-symbols=@var{filename} | |
627 | Read symbol names and their addresses from @var{filename}, but do not | |
628 | relocate it or include it in the output. This allows your output file | |
629 | to refer symbolically to absolute locations of memory defined in other | |
630 | programs. You may use this option more than once. | |
631 | ||
632 | For compatibility with other ELF linkers, if the @code{-R} option is | |
633 | followed by a directory name, rather than a file name, it is treated as | |
634 | the @code{-rpath} option. | |
635 | ||
636 | @kindex -s | |
637 | @kindex --strip-all | |
638 | @cindex strip all symbols | |
639 | @item -s | |
640 | @itemx --strip-all | |
641 | Omit all symbol information from the output file. | |
642 | ||
643 | @kindex -S | |
644 | @kindex --strip-debug | |
645 | @cindex strip debugger symbols | |
646 | @item -S | |
647 | @itemx --strip-debug | |
648 | Omit debugger symbol information (but not all symbols) from the output file. | |
649 | ||
650 | @kindex -t | |
651 | @kindex --trace | |
652 | @cindex input files, displaying | |
653 | @item -t | |
654 | @itemx --trace | |
655 | Print the names of the input files as @code{ld} processes them. | |
656 | ||
657 | @kindex -T @var{script} | |
658 | @kindex --script=@var{script} | |
659 | @cindex script files | |
660 | @item -T @var{scriptfile} | |
661 | @itemx --script=@var{scriptfile} | |
662 | Use @var{scriptfile} as the linker script. This script replaces | |
663 | @code{ld}'s default linker script (rather than adding to it), so | |
664 | @var{commandfile} must specify everything necessary to describe the | |
665 | output file. You must use this option if you want to use a command | |
666 | which can only appear once in a linker script, such as the | |
667 | @code{SECTIONS} or @code{MEMORY} command. @xref{Scripts}. If | |
668 | @var{scriptfile} does not exist in the current directory, @code{ld} | |
669 | looks for it in the directories specified by any preceding @samp{-L} | |
670 | options. Multiple @samp{-T} options accumulate. | |
671 | ||
672 | @kindex -u @var{symbol} | |
673 | @kindex --undefined=@var{symbol} | |
674 | @cindex undefined symbol | |
675 | @item -u @var{symbol} | |
676 | @itemx --undefined=@var{symbol} | |
677 | Force @var{symbol} to be entered in the output file as an undefined | |
678 | symbol. Doing this may, for example, trigger linking of additional | |
679 | modules from standard libraries. @samp{-u} may be repeated with | |
680 | different option arguments to enter additional undefined symbols. This | |
681 | option is equivalent to the @code{EXTERN} linker script command. | |
682 | ||
683 | @kindex -Ur | |
684 | @cindex constructors | |
685 | @item -Ur | |
686 | For anything other than C++ programs, this option is equivalent to | |
687 | @samp{-r}: it generates relocatable output---i.e., an output file that can in | |
688 | turn serve as input to @code{ld}. When linking C++ programs, @samp{-Ur} | |
689 | @emph{does} resolve references to constructors, unlike @samp{-r}. | |
690 | It does not work to use @samp{-Ur} on files that were themselves linked | |
691 | with @samp{-Ur}; once the constructor table has been built, it cannot | |
692 | be added to. Use @samp{-Ur} only for the last partial link, and | |
693 | @samp{-r} for the others. | |
694 | ||
695 | @kindex -v | |
696 | @kindex -V | |
697 | @kindex --version | |
698 | @cindex version | |
699 | @item -v | |
700 | @itemx --version | |
701 | @itemx -V | |
702 | Display the version number for @code{ld}. The @code{-V} option also | |
703 | lists the supported emulations. | |
704 | ||
705 | @kindex -x | |
706 | @kindex --discard-all | |
707 | @cindex deleting local symbols | |
708 | @item -x | |
709 | @itemx --discard-all | |
710 | Delete all local symbols. | |
711 | ||
712 | @kindex -X | |
713 | @kindex --discard-locals | |
714 | @cindex local symbols, deleting | |
715 | @cindex L, deleting symbols beginning | |
716 | @item -X | |
717 | @itemx --discard-locals | |
718 | Delete all temporary local symbols. For most targets, this is all local | |
719 | symbols whose names begin with @samp{L}. | |
720 | ||
721 | @kindex -y @var{symbol} | |
722 | @kindex --trace-symbol=@var{symbol} | |
723 | @cindex symbol tracing | |
724 | @item -y @var{symbol} | |
725 | @itemx --trace-symbol=@var{symbol} | |
726 | Print the name of each linked file in which @var{symbol} appears. This | |
727 | option may be given any number of times. On many systems it is necessary | |
728 | to prepend an underscore. | |
729 | ||
730 | This option is useful when you have an undefined symbol in your link but | |
731 | don't know where the reference is coming from. | |
732 | ||
733 | @kindex -Y @var{path} | |
734 | @item -Y @var{path} | |
735 | Add @var{path} to the default library search path. This option exists | |
736 | for Solaris compatibility. | |
737 | ||
738 | @kindex -z @var{keyword} | |
739 | @item -z @var{keyword} | |
740 | This option is ignored for Solaris compatibility. | |
741 | ||
742 | @kindex -( | |
743 | @cindex groups of archives | |
744 | @item -( @var{archives} -) | |
745 | @itemx --start-group @var{archives} --end-group | |
746 | The @var{archives} should be a list of archive files. They may be | |
747 | either explicit file names, or @samp{-l} options. | |
748 | ||
749 | The specified archives are searched repeatedly until no new undefined | |
750 | references are created. Normally, an archive is searched only once in | |
751 | the order that it is specified on the command line. If a symbol in that | |
752 | archive is needed to resolve an undefined symbol referred to by an | |
753 | object in an archive that appears later on the command line, the linker | |
754 | would not be able to resolve that reference. By grouping the archives, | |
755 | they all be searched repeatedly until all possible references are | |
756 | resolved. | |
757 | ||
758 | Using this option has a significant performance cost. It is best to use | |
759 | it only when there are unavoidable circular references between two or | |
760 | more archives. | |
761 | ||
762 | @kindex -assert @var{keyword} | |
763 | @item -assert @var{keyword} | |
764 | This option is ignored for SunOS compatibility. | |
765 | ||
766 | @kindex -Bdynamic | |
767 | @kindex -dy | |
768 | @kindex -call_shared | |
769 | @item -Bdynamic | |
770 | @itemx -dy | |
771 | @itemx -call_shared | |
772 | Link against dynamic libraries. This is only meaningful on platforms | |
773 | for which shared libraries are supported. This option is normally the | |
774 | default on such platforms. The different variants of this option are | |
775 | for compatibility with various systems. You may use this option | |
776 | multiple times on the command line: it affects library searching for | |
777 | @code{-l} options which follow it. | |
778 | ||
779 | @kindex -Bstatic | |
780 | @kindex -dn | |
781 | @kindex -non_shared | |
782 | @kindex -static | |
783 | @item -Bstatic | |
784 | @itemx -dn | |
785 | @itemx -non_shared | |
786 | @itemx -static | |
787 | Do not link against shared libraries. This is only meaningful on | |
788 | platforms for which shared libraries are supported. The different | |
789 | variants of this option are for compatibility with various systems. You | |
790 | may use this option multiple times on the command line: it affects | |
791 | library searching for @code{-l} options which follow it. | |
792 | ||
793 | @kindex -Bsymbolic | |
794 | @item -Bsymbolic | |
795 | When creating a shared library, bind references to global symbols to the | |
796 | definition within the shared library, if any. Normally, it is possible | |
797 | for a program linked against a shared library to override the definition | |
798 | within the shared library. This option is only meaningful on ELF | |
799 | platforms which support shared libraries. | |
800 | ||
801 | @kindex --check-sections | |
802 | @kindex --no-check-sections | |
803 | @item --check-sections | |
308b1ffd | 804 | @itemx --no-check-sections |
252b5132 RH |
805 | Asks the linker @emph{not} to check section addresses after they have |
806 | been assigned to see if there any overlaps. Normally the linker will | |
807 | perform this check, and if it finds any overlaps it will produce | |
808 | suitable error messages. The linker does know about, and does make | |
809 | allowances for sections in overlays. The default behaviour can be | |
810 | restored by using the command line switch @samp{--check-sections}. | |
811 | ||
812 | @cindex cross reference table | |
813 | @kindex --cref | |
814 | @item --cref | |
815 | Output a cross reference table. If a linker map file is being | |
816 | generated, the cross reference table is printed to the map file. | |
817 | Otherwise, it is printed on the standard output. | |
818 | ||
819 | The format of the table is intentionally simple, so that it may be | |
820 | easily processed by a script if necessary. The symbols are printed out, | |
821 | sorted by name. For each symbol, a list of file names is given. If the | |
822 | symbol is defined, the first file listed is the location of the | |
823 | definition. The remaining files contain references to the symbol. | |
824 | ||
825 | @cindex symbols, from command line | |
826 | @kindex --defsym @var{symbol}=@var{exp} | |
827 | @item --defsym @var{symbol}=@var{expression} | |
828 | Create a global symbol in the output file, containing the absolute | |
829 | address given by @var{expression}. You may use this option as many | |
830 | times as necessary to define multiple symbols in the command line. A | |
831 | limited form of arithmetic is supported for the @var{expression} in this | |
832 | context: you may give a hexadecimal constant or the name of an existing | |
833 | symbol, or use @code{+} and @code{-} to add or subtract hexadecimal | |
834 | constants or symbols. If you need more elaborate expressions, consider | |
835 | using the linker command language from a script (@pxref{Assignments,, | |
836 | Assignment: Symbol Definitions}). @emph{Note:} there should be no white | |
837 | space between @var{symbol}, the equals sign (``@key{=}''), and | |
838 | @var{expression}. | |
839 | ||
840 | @cindex demangling, from command line | |
841 | @kindex --demangle | |
842 | @kindex --no-demangle | |
843 | @item --demangle | |
844 | @itemx --no-demangle | |
845 | These options control whether to demangle symbol names in error messages | |
846 | and other output. When the linker is told to demangle, it tries to | |
847 | present symbol names in a readable fashion: it strips leading | |
848 | underscores if they are used by the object file format, and converts C++ | |
849 | mangled symbol names into user readable names. The linker will demangle | |
850 | by default unless the environment variable @samp{COLLECT_NO_DEMANGLE} is | |
851 | set. These options may be used to override the default. | |
852 | ||
853 | @cindex dynamic linker, from command line | |
854 | @kindex --dynamic-linker @var{file} | |
855 | @item --dynamic-linker @var{file} | |
856 | Set the name of the dynamic linker. This is only meaningful when | |
857 | generating dynamically linked ELF executables. The default dynamic | |
858 | linker is normally correct; don't use this unless you know what you are | |
859 | doing. | |
860 | ||
861 | @cindex MIPS embedded PIC code | |
862 | @kindex --embedded-relocs | |
863 | @item --embedded-relocs | |
864 | This option is only meaningful when linking MIPS embedded PIC code, | |
865 | generated by the -membedded-pic option to the @sc{gnu} compiler and | |
866 | assembler. It causes the linker to create a table which may be used at | |
867 | runtime to relocate any data which was statically initialized to pointer | |
868 | values. See the code in testsuite/ld-empic for details. | |
869 | ||
870 | @kindex --force-exe-suffix | |
871 | @item --force-exe-suffix | |
872 | Make sure that an output file has a .exe suffix. | |
873 | ||
874 | If a successfully built fully linked output file does not have a | |
875 | @code{.exe} or @code{.dll} suffix, this option forces the linker to copy | |
876 | the output file to one of the same name with a @code{.exe} suffix. This | |
877 | option is useful when using unmodified Unix makefiles on a Microsoft | |
878 | Windows host, since some versions of Windows won't run an image unless | |
879 | it ends in a @code{.exe} suffix. | |
880 | ||
881 | @kindex --gc-sections | |
882 | @kindex --no-gc-sections | |
883 | @cindex garbage collection | |
884 | @item --no-gc-sections | |
885 | @itemx --gc-sections | |
886 | Enable garbage collection of unused input sections. It is ignored on | |
887 | targets that do not support this option. This option is not compatible | |
888 | with @samp{-r}, nor should it be used with dynamic linking. The default | |
889 | behaviour (of not performing this garbage collection) can be restored by | |
890 | specifying @samp{--no-gc-sections} on the command line. | |
891 | ||
892 | @cindex help | |
893 | @cindex usage | |
894 | @kindex --help | |
895 | @item --help | |
896 | Print a summary of the command-line options on the standard output and exit. | |
897 | ||
898 | @kindex -Map | |
899 | @item -Map @var{mapfile} | |
900 | Print a link map to the file @var{mapfile}. See the description of the | |
901 | @samp{-M} option, above. | |
902 | ||
903 | @cindex memory usage | |
904 | @kindex --no-keep-memory | |
905 | @item --no-keep-memory | |
906 | @code{ld} normally optimizes for speed over memory usage by caching the | |
907 | symbol tables of input files in memory. This option tells @code{ld} to | |
908 | instead optimize for memory usage, by rereading the symbol tables as | |
909 | necessary. This may be required if @code{ld} runs out of memory space | |
910 | while linking a large executable. | |
911 | ||
912 | @kindex --no-undefined | |
913 | @item --no-undefined | |
914 | Normally when creating a non-symbolic shared library, undefined symbols | |
915 | are allowed and left to be resolved by the runtime loader. This option | |
916 | disallows such undefined symbols. | |
917 | ||
918 | @kindex --no-warn-mismatch | |
919 | @item --no-warn-mismatch | |
920 | Normally @code{ld} will give an error if you try to link together input | |
921 | files that are mismatched for some reason, perhaps because they have | |
922 | been compiled for different processors or for different endiannesses. | |
923 | This option tells @code{ld} that it should silently permit such possible | |
924 | errors. This option should only be used with care, in cases when you | |
925 | have taken some special action that ensures that the linker errors are | |
926 | inappropriate. | |
927 | ||
928 | @kindex --no-whole-archive | |
929 | @item --no-whole-archive | |
930 | Turn off the effect of the @code{--whole-archive} option for subsequent | |
931 | archive files. | |
932 | ||
933 | @cindex output file after errors | |
934 | @kindex --noinhibit-exec | |
935 | @item --noinhibit-exec | |
936 | Retain the executable output file whenever it is still usable. | |
937 | Normally, the linker will not produce an output file if it encounters | |
938 | errors during the link process; it exits without writing an output file | |
939 | when it issues any error whatsoever. | |
940 | ||
941 | @ifclear SingleFormat | |
942 | @kindex --oformat | |
943 | @item --oformat @var{output-format} | |
944 | @code{ld} may be configured to support more than one kind of object | |
945 | file. If your @code{ld} is configured this way, you can use the | |
946 | @samp{--oformat} option to specify the binary format for the output | |
947 | object file. Even when @code{ld} is configured to support alternative | |
948 | object formats, you don't usually need to specify this, as @code{ld} | |
949 | should be configured to produce as a default output format the most | |
950 | usual format on each machine. @var{output-format} is a text string, the | |
951 | name of a particular format supported by the BFD libraries. (You can | |
952 | list the available binary formats with @samp{objdump -i}.) The script | |
953 | command @code{OUTPUT_FORMAT} can also specify the output format, but | |
954 | this option overrides it. @xref{BFD}. | |
955 | @end ifclear | |
956 | ||
957 | @kindex -qmagic | |
958 | @item -qmagic | |
959 | This option is ignored for Linux compatibility. | |
960 | ||
961 | @kindex -Qy | |
962 | @item -Qy | |
963 | This option is ignored for SVR4 compatibility. | |
964 | ||
965 | @kindex --relax | |
966 | @cindex synthesizing linker | |
967 | @cindex relaxing addressing modes | |
968 | @item --relax | |
969 | An option with machine dependent effects. | |
970 | @ifset GENERIC | |
971 | This option is only supported on a few targets. | |
972 | @end ifset | |
973 | @ifset H8300 | |
974 | @xref{H8/300,,@code{ld} and the H8/300}. | |
975 | @end ifset | |
976 | @ifset I960 | |
977 | @xref{i960,, @code{ld} and the Intel 960 family}. | |
978 | @end ifset | |
979 | ||
980 | ||
981 | On some platforms, the @samp{--relax} option performs global | |
982 | optimizations that become possible when the linker resolves addressing | |
983 | in the program, such as relaxing address modes and synthesizing new | |
984 | instructions in the output object file. | |
985 | ||
986 | On some platforms these link time global optimizations may make symbolic | |
987 | debugging of the resulting executable impossible. | |
988 | @ifset GENERIC | |
989 | This is known to be | |
990 | the case for the Matsushita MN10200 and MN10300 family of processors. | |
991 | @end ifset | |
992 | ||
993 | @ifset GENERIC | |
994 | On platforms where this is not supported, @samp{--relax} is accepted, | |
995 | but ignored. | |
996 | @end ifset | |
997 | ||
998 | @cindex retaining specified symbols | |
999 | @cindex stripping all but some symbols | |
1000 | @cindex symbols, retaining selectively | |
1001 | @item --retain-symbols-file @var{filename} | |
1002 | Retain @emph{only} the symbols listed in the file @var{filename}, | |
1003 | discarding all others. @var{filename} is simply a flat file, with one | |
1004 | symbol name per line. This option is especially useful in environments | |
1005 | @ifset GENERIC | |
1006 | (such as VxWorks) | |
1007 | @end ifset | |
1008 | where a large global symbol table is accumulated gradually, to conserve | |
1009 | run-time memory. | |
1010 | ||
1011 | @samp{--retain-symbols-file} does @emph{not} discard undefined symbols, | |
1012 | or symbols needed for relocations. | |
1013 | ||
1014 | You may only specify @samp{--retain-symbols-file} once in the command | |
1015 | line. It overrides @samp{-s} and @samp{-S}. | |
1016 | ||
1017 | @ifset GENERIC | |
1018 | @item -rpath @var{dir} | |
1019 | @cindex runtime library search path | |
1020 | @kindex -rpath | |
1021 | Add a directory to the runtime library search path. This is used when | |
1022 | linking an ELF executable with shared objects. All @code{-rpath} | |
1023 | arguments are concatenated and passed to the runtime linker, which uses | |
1024 | them to locate shared objects at runtime. The @code{-rpath} option is | |
1025 | also used when locating shared objects which are needed by shared | |
1026 | objects explicitly included in the link; see the description of the | |
1027 | @code{-rpath-link} option. If @code{-rpath} is not used when linking an | |
1028 | ELF executable, the contents of the environment variable | |
1029 | @code{LD_RUN_PATH} will be used if it is defined. | |
1030 | ||
1031 | The @code{-rpath} option may also be used on SunOS. By default, on | |
1032 | SunOS, the linker will form a runtime search patch out of all the | |
1033 | @code{-L} options it is given. If a @code{-rpath} option is used, the | |
1034 | runtime search path will be formed exclusively using the @code{-rpath} | |
1035 | options, ignoring the @code{-L} options. This can be useful when using | |
1036 | gcc, which adds many @code{-L} options which may be on NFS mounted | |
1037 | filesystems. | |
1038 | ||
1039 | For compatibility with other ELF linkers, if the @code{-R} option is | |
1040 | followed by a directory name, rather than a file name, it is treated as | |
1041 | the @code{-rpath} option. | |
1042 | @end ifset | |
1043 | ||
1044 | @ifset GENERIC | |
1045 | @cindex link-time runtime library search path | |
1046 | @kindex -rpath-link | |
1047 | @item -rpath-link @var{DIR} | |
1048 | When using ELF or SunOS, one shared library may require another. This | |
1049 | happens when an @code{ld -shared} link includes a shared library as one | |
1050 | of the input files. | |
1051 | ||
1052 | When the linker encounters such a dependency when doing a non-shared, | |
1053 | non-relocatable link, it will automatically try to locate the required | |
1054 | shared library and include it in the link, if it is not included | |
1055 | explicitly. In such a case, the @code{-rpath-link} option | |
1056 | specifies the first set of directories to search. The | |
1057 | @code{-rpath-link} option may specify a sequence of directory names | |
1058 | either by specifying a list of names separated by colons, or by | |
1059 | appearing multiple times. | |
1060 | ||
1061 | The linker uses the following search paths to locate required shared | |
1062 | libraries. | |
1063 | @enumerate | |
1064 | @item | |
1065 | Any directories specified by @code{-rpath-link} options. | |
1066 | @item | |
1067 | Any directories specified by @code{-rpath} options. The difference | |
1068 | between @code{-rpath} and @code{-rpath-link} is that directories | |
1069 | specified by @code{-rpath} options are included in the executable and | |
1070 | used at runtime, whereas the @code{-rpath-link} option is only effective | |
1071 | at link time. | |
1072 | @item | |
1073 | On an ELF system, if the @code{-rpath} and @code{rpath-link} options | |
1074 | were not used, search the contents of the environment variable | |
1075 | @code{LD_RUN_PATH}. | |
1076 | @item | |
1077 | On SunOS, if the @code{-rpath} option was not used, search any | |
1078 | directories specified using @code{-L} options. | |
1079 | @item | |
1080 | For a native linker, the contents of the environment variable | |
1081 | @code{LD_LIBRARY_PATH}. | |
1082 | @item | |
1083 | The default directories, normally @file{/lib} and @file{/usr/lib}. | |
1084 | @item | |
1085 | For a native linker on an ELF system, if the file @file{/etc/ld.so.conf} | |
1086 | exists, the list of directories found in that file. | |
1087 | @end enumerate | |
1088 | ||
1089 | If the required shared library is not found, the linker will issue a | |
1090 | warning and continue with the link. | |
1091 | @end ifset | |
1092 | ||
1093 | @kindex -shared | |
1094 | @kindex -Bshareable | |
1095 | @item -shared | |
1096 | @itemx -Bshareable | |
1097 | @cindex shared libraries | |
1098 | Create a shared library. This is currently only supported on ELF, XCOFF | |
1099 | and SunOS platforms. On SunOS, the linker will automatically create a | |
1100 | shared library if the @code{-e} option is not used and there are | |
1101 | undefined symbols in the link. | |
1102 | ||
1103 | @item --sort-common | |
1104 | @kindex --sort-common | |
1105 | This option tells @code{ld} to sort the common symbols by size when it | |
1106 | places them in the appropriate output sections. First come all the one | |
1107 | byte symbols, then all the two bytes, then all the four bytes, and then | |
1108 | everything else. This is to prevent gaps between symbols due to | |
1109 | alignment constraints. | |
1110 | ||
1111 | @kindex --split-by-file | |
1112 | @item --split-by-file | |
1113 | Similar to @code{--split-by-reloc} but creates a new output section for | |
1114 | each input file. | |
1115 | ||
1116 | @kindex --split-by-reloc | |
1117 | @item --split-by-reloc @var{count} | |
1118 | Trys to creates extra sections in the output file so that no single | |
1119 | output section in the file contains more than @var{count} relocations. | |
1120 | This is useful when generating huge relocatable for downloading into | |
1121 | certain real time kernels with the COFF object file format; since COFF | |
1122 | cannot represent more than 65535 relocations in a single section. Note | |
1123 | that this will fail to work with object file formats which do not | |
1124 | support arbitrary sections. The linker will not split up individual | |
1125 | input sections for redistribution, so if a single input section contains | |
1126 | more than @var{count} relocations one output section will contain that | |
1127 | many relocations. | |
1128 | ||
1129 | @kindex --stats | |
1130 | @item --stats | |
1131 | Compute and display statistics about the operation of the linker, such | |
1132 | as execution time and memory usage. | |
1133 | ||
1134 | @kindex --traditional-format | |
1135 | @cindex traditional format | |
1136 | @item --traditional-format | |
1137 | For some targets, the output of @code{ld} is different in some ways from | |
1138 | the output of some existing linker. This switch requests @code{ld} to | |
1139 | use the traditional format instead. | |
1140 | ||
1141 | @cindex dbx | |
1142 | For example, on SunOS, @code{ld} combines duplicate entries in the | |
1143 | symbol string table. This can reduce the size of an output file with | |
1144 | full debugging information by over 30 percent. Unfortunately, the SunOS | |
1145 | @code{dbx} program can not read the resulting program (@code{gdb} has no | |
1146 | trouble). The @samp{--traditional-format} switch tells @code{ld} to not | |
1147 | combine duplicate entries. | |
1148 | ||
176355da NC |
1149 | @kindex --section-start @var{sectionname}=@var{org} |
1150 | @item --section-start @var{sectionname}=@var{org} | |
1151 | Locate a section in the output file at the absolute | |
1152 | address given by @var{org}. You may use this option as many | |
1153 | times as necessary to locate multiple sections in the command | |
1154 | line. | |
1155 | @var{org} must be a single hexadecimal integer; | |
1156 | for compatibility with other linkers, you may omit the leading | |
1157 | @samp{0x} usually associated with hexadecimal values. @emph{Note:} there | |
1158 | should be no white space between @var{sectionname}, the equals | |
1159 | sign (``@key{=}''), and @var{org}. | |
1160 | ||
252b5132 RH |
1161 | @kindex -Tbss @var{org} |
1162 | @kindex -Tdata @var{org} | |
1163 | @kindex -Ttext @var{org} | |
1164 | @cindex segment origins, cmd line | |
1165 | @item -Tbss @var{org} | |
1166 | @itemx -Tdata @var{org} | |
1167 | @itemx -Ttext @var{org} | |
1168 | Use @var{org} as the starting address for---respectively---the | |
1169 | @code{bss}, @code{data}, or the @code{text} segment of the output file. | |
1170 | @var{org} must be a single hexadecimal integer; | |
1171 | for compatibility with other linkers, you may omit the leading | |
1172 | @samp{0x} usually associated with hexadecimal values. | |
1173 | ||
1174 | @kindex --verbose | |
1175 | @cindex verbose | |
1176 | @item --dll-verbose | |
308b1ffd | 1177 | @itemx --verbose |
252b5132 RH |
1178 | Display the version number for @code{ld} and list the linker emulations |
1179 | supported. Display which input files can and cannot be opened. Display | |
1180 | the linker script if using a default builtin script. | |
1181 | ||
1182 | @kindex --version-script=@var{version-scriptfile} | |
1183 | @cindex version script, symbol versions | |
1184 | @itemx --version-script=@var{version-scriptfile} | |
1185 | Specify the name of a version script to the linker. This is typically | |
1186 | used when creating shared libraries to specify additional information | |
1187 | about the version heirarchy for the library being created. This option | |
1188 | is only meaningful on ELF platforms which support shared libraries. | |
1189 | @xref{VERSION}. | |
1190 | ||
1191 | @kindex --warn-comon | |
1192 | @cindex warnings, on combining symbols | |
1193 | @cindex combining symbols, warnings on | |
1194 | @item --warn-common | |
1195 | Warn when a common symbol is combined with another common symbol or with | |
1196 | a symbol definition. Unix linkers allow this somewhat sloppy practice, | |
1197 | but linkers on some other operating systems do not. This option allows | |
1198 | you to find potential problems from combining global symbols. | |
1199 | Unfortunately, some C libraries use this practice, so you may get some | |
1200 | warnings about symbols in the libraries as well as in your programs. | |
1201 | ||
1202 | There are three kinds of global symbols, illustrated here by C examples: | |
1203 | ||
1204 | @table @samp | |
1205 | @item int i = 1; | |
1206 | A definition, which goes in the initialized data section of the output | |
1207 | file. | |
1208 | ||
1209 | @item extern int i; | |
1210 | An undefined reference, which does not allocate space. | |
1211 | There must be either a definition or a common symbol for the | |
1212 | variable somewhere. | |
1213 | ||
1214 | @item int i; | |
1215 | A common symbol. If there are only (one or more) common symbols for a | |
1216 | variable, it goes in the uninitialized data area of the output file. | |
1217 | The linker merges multiple common symbols for the same variable into a | |
1218 | single symbol. If they are of different sizes, it picks the largest | |
1219 | size. The linker turns a common symbol into a declaration, if there is | |
1220 | a definition of the same variable. | |
1221 | @end table | |
1222 | ||
1223 | The @samp{--warn-common} option can produce five kinds of warnings. | |
1224 | Each warning consists of a pair of lines: the first describes the symbol | |
1225 | just encountered, and the second describes the previous symbol | |
1226 | encountered with the same name. One or both of the two symbols will be | |
1227 | a common symbol. | |
1228 | ||
1229 | @enumerate | |
1230 | @item | |
1231 | Turning a common symbol into a reference, because there is already a | |
1232 | definition for the symbol. | |
1233 | @smallexample | |
1234 | @var{file}(@var{section}): warning: common of `@var{symbol}' | |
1235 | overridden by definition | |
1236 | @var{file}(@var{section}): warning: defined here | |
1237 | @end smallexample | |
1238 | ||
1239 | @item | |
1240 | Turning a common symbol into a reference, because a later definition for | |
1241 | the symbol is encountered. This is the same as the previous case, | |
1242 | except that the symbols are encountered in a different order. | |
1243 | @smallexample | |
1244 | @var{file}(@var{section}): warning: definition of `@var{symbol}' | |
1245 | overriding common | |
1246 | @var{file}(@var{section}): warning: common is here | |
1247 | @end smallexample | |
1248 | ||
1249 | @item | |
1250 | Merging a common symbol with a previous same-sized common symbol. | |
1251 | @smallexample | |
1252 | @var{file}(@var{section}): warning: multiple common | |
1253 | of `@var{symbol}' | |
1254 | @var{file}(@var{section}): warning: previous common is here | |
1255 | @end smallexample | |
1256 | ||
1257 | @item | |
1258 | Merging a common symbol with a previous larger common symbol. | |
1259 | @smallexample | |
1260 | @var{file}(@var{section}): warning: common of `@var{symbol}' | |
1261 | overridden by larger common | |
1262 | @var{file}(@var{section}): warning: larger common is here | |
1263 | @end smallexample | |
1264 | ||
1265 | @item | |
1266 | Merging a common symbol with a previous smaller common symbol. This is | |
1267 | the same as the previous case, except that the symbols are | |
1268 | encountered in a different order. | |
1269 | @smallexample | |
1270 | @var{file}(@var{section}): warning: common of `@var{symbol}' | |
1271 | overriding smaller common | |
1272 | @var{file}(@var{section}): warning: smaller common is here | |
1273 | @end smallexample | |
1274 | @end enumerate | |
1275 | ||
1276 | @kindex --warn-constructors | |
1277 | @item --warn-constructors | |
1278 | Warn if any global constructors are used. This is only useful for a few | |
1279 | object file formats. For formats like COFF or ELF, the linker can not | |
1280 | detect the use of global constructors. | |
1281 | ||
1282 | @kindex --warn-multiple-gp | |
1283 | @item --warn-multiple-gp | |
1284 | Warn if multiple global pointer values are required in the output file. | |
1285 | This is only meaningful for certain processors, such as the Alpha. | |
1286 | Specifically, some processors put large-valued constants in a special | |
1287 | section. A special register (the global pointer) points into the middle | |
1288 | of this section, so that constants can be loaded efficiently via a | |
1289 | base-register relative addressing mode. Since the offset in | |
1290 | base-register relative mode is fixed and relatively small (e.g., 16 | |
1291 | bits), this limits the maximum size of the constant pool. Thus, in | |
1292 | large programs, it is often necessary to use multiple global pointer | |
1293 | values in order to be able to address all possible constants. This | |
1294 | option causes a warning to be issued whenever this case occurs. | |
1295 | ||
1296 | @kindex --warn-once | |
1297 | @cindex warnings, on undefined symbols | |
1298 | @cindex undefined symbols, warnings on | |
1299 | @item --warn-once | |
1300 | Only warn once for each undefined symbol, rather than once per module | |
1301 | which refers to it. | |
1302 | ||
1303 | @kindex --warn-section-align | |
1304 | @cindex warnings, on section alignment | |
1305 | @cindex section alignment, warnings on | |
1306 | @item --warn-section-align | |
1307 | Warn if the address of an output section is changed because of | |
1308 | alignment. Typically, the alignment will be set by an input section. | |
1309 | The address will only be changed if it not explicitly specified; that | |
1310 | is, if the @code{SECTIONS} command does not specify a start address for | |
1311 | the section (@pxref{SECTIONS}). | |
1312 | ||
1313 | @kindex --whole-archive | |
1314 | @cindex including an entire archive | |
1315 | @item --whole-archive | |
1316 | For each archive mentioned on the command line after the | |
1317 | @code{--whole-archive} option, include every object file in the archive | |
1318 | in the link, rather than searching the archive for the required object | |
1319 | files. This is normally used to turn an archive file into a shared | |
1320 | library, forcing every object to be included in the resulting shared | |
1321 | library. This option may be used more than once. | |
1322 | ||
1323 | @kindex --wrap | |
1324 | @item --wrap @var{symbol} | |
1325 | Use a wrapper function for @var{symbol}. Any undefined reference to | |
1326 | @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any | |
1327 | undefined reference to @code{__real_@var{symbol}} will be resolved to | |
1328 | @var{symbol}. | |
1329 | ||
1330 | This can be used to provide a wrapper for a system function. The | |
1331 | wrapper function should be called @code{__wrap_@var{symbol}}. If it | |
1332 | wishes to call the system function, it should call | |
1333 | @code{__real_@var{symbol}}. | |
1334 | ||
1335 | Here is a trivial example: | |
1336 | ||
1337 | @smallexample | |
1338 | void * | |
1339 | __wrap_malloc (int c) | |
1340 | @{ | |
1341 | printf ("malloc called with %ld\n", c); | |
1342 | return __real_malloc (c); | |
1343 | @} | |
1344 | @end smallexample | |
1345 | ||
1346 | If you link other code with this file using @code{--wrap malloc}, then | |
1347 | all calls to @code{malloc} will call the function @code{__wrap_malloc} | |
1348 | instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will | |
1349 | call the real @code{malloc} function. | |
1350 | ||
1351 | You may wish to provide a @code{__real_malloc} function as well, so that | |
1352 | links without the @code{--wrap} option will succeed. If you do this, | |
1353 | you should not put the definition of @code{__real_malloc} in the same | |
1354 | file as @code{__wrap_malloc}; if you do, the assembler may resolve the | |
1355 | call before the linker has a chance to wrap it to @code{malloc}. | |
1356 | ||
1357 | @end table | |
1358 | ||
1359 | @subsection Options specific to i386 PE targets | |
1360 | ||
1361 | The i386 PE linker supports the @code{-shared} option, which causes | |
1362 | the output to be a dynamically linked library (DLL) instead of a | |
1363 | normal executable. You should name the output @code{*.dll} when you | |
1364 | use this option. In addition, the linker fully supports the standard | |
1365 | @code{*.def} files, which may be specified on the linker command line | |
1366 | like an object file (in fact, it should precede archives it exports | |
1367 | symbols from, to ensure that they get linked in, just like a normal | |
1368 | object file). | |
1369 | ||
1370 | In addition to the options common to all targets, the i386 PE linker | |
1371 | support additional command line options that are specific to the i386 | |
1372 | PE target. Options that take values may be separated from their | |
1373 | values by either a space or an equals sign. | |
1374 | ||
1375 | @table @code | |
1376 | ||
1377 | @kindex --add-stdcall-alias | |
1378 | @item --add-stdcall-alias | |
1379 | If given, symbols with a stdcall suffix (@@@var{nn}) will be exported | |
1380 | as-is and also with the suffix stripped. | |
1381 | ||
1382 | @kindex --base-file | |
1383 | @item --base-file @var{file} | |
1384 | Use @var{file} as the name of a file in which to save the base | |
1385 | addresses of all the relocations needed for generating DLLs with | |
1386 | @file{dlltool}. | |
1387 | ||
1388 | @kindex --dll | |
1389 | @item --dll | |
1390 | Create a DLL instead of a regular executable. You may also use | |
1391 | @code{-shared} or specify a @code{LIBRARY} in a given @code{.def} | |
1392 | file. | |
1393 | ||
1394 | @kindex --enable-stdcall-fixup | |
1395 | @kindex --disable-stdcall-fixup | |
1396 | @item --enable-stdcall-fixup | |
1397 | @itemx --disable-stdcall-fixup | |
1398 | If the link finds a symbol that it cannot resolve, it will attempt to | |
1399 | do "fuzzy linking" by looking for another defined symbol that differs | |
1400 | only in the format of the symbol name (cdecl vs stdcall) and will | |
1401 | resolve that symbol by linking to the match. For example, the | |
1402 | undefined symbol @code{_foo} might be linked to the function | |
1403 | @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked | |
1404 | to the function @code{_bar}. When the linker does this, it prints a | |
1405 | warning, since it normally should have failed to link, but sometimes | |
1406 | import libraries generated from third-party dlls may need this feature | |
1407 | to be usable. If you specify @code{--enable-stdcall-fixup}, this | |
1408 | feature is fully enabled and warnings are not printed. If you specify | |
1409 | @code{--disable-stdcall-fixup}, this feature is disabled and such | |
1410 | mismatches are considered to be errors. | |
1411 | ||
1412 | @cindex DLLs, creating | |
1413 | @kindex --export-all-symbols | |
1414 | @item --export-all-symbols | |
1415 | If given, all global symbols in the objects used to build a DLL will | |
1416 | be exported by the DLL. Note that this is the default if there | |
1417 | otherwise wouldn't be any exported symbols. When symbols are | |
1418 | explicitly exported via DEF files or implicitly exported via function | |
1419 | attributes, the default is to not export anything else unless this | |
1420 | option is given. Note that the symbols @code{DllMain@@12}, | |
1421 | @code{DllEntryPoint@@0}, and @code{impure_ptr} will not be automatically | |
1422 | exported. | |
1423 | ||
1424 | @kindex --exclude-symbols | |
1d0a3c9c | 1425 | @item --exclude-symbols @var{symbol},@var{symbol},... |
252b5132 RH |
1426 | Specifies a list of symbols which should not be automatically |
1427 | exported. The symbol names may be delimited by commas or colons. | |
1428 | ||
1429 | @kindex --file-alignment | |
1430 | @item --file-alignment | |
1431 | Specify the file alignment. Sections in the file will always begin at | |
1432 | file offsets which are multiples of this number. This defaults to | |
1433 | 512. | |
1434 | ||
1435 | @cindex heap size | |
1436 | @kindex --heap | |
1437 | @item --heap @var{reserve} | |
1438 | @itemx --heap @var{reserve},@var{commit} | |
1439 | Specify the amount of memory to reserve (and optionally commit) to be | |
1440 | used as heap for this program. The default is 1Mb reserved, 4K | |
1441 | committed. | |
1442 | ||
1443 | @cindex image base | |
1444 | @kindex --image-base | |
1445 | @item --image-base @var{value} | |
1446 | Use @var{value} as the base address of your program or dll. This is | |
1447 | the lowest memory location that will be used when your program or dll | |
1448 | is loaded. To reduce the need to relocate and improve performance of | |
1449 | your dlls, each should have a unique base address and not overlap any | |
1450 | other dlls. The default is 0x400000 for executables, and 0x10000000 | |
1451 | for dlls. | |
1452 | ||
1453 | @kindex --kill-at | |
1454 | @item --kill-at | |
1455 | If given, the stdcall suffixes (@@@var{nn}) will be stripped from | |
1456 | symbols before they are exported. | |
1457 | ||
1458 | @kindex --major-image-version | |
1459 | @item --major-image-version @var{value} | |
1460 | Sets the major number of the "image version". Defaults to 1. | |
1461 | ||
1462 | @kindex --major-os-version | |
1463 | @item --major-os-version @var{value} | |
1464 | Sets the major number of the "os version". Defaults to 4. | |
1465 | ||
1466 | @kindex --major-subsystem-version | |
1467 | @item --major-subsystem-version @var{value} | |
1468 | Sets the major number of the "subsystem version". Defaults to 4. | |
1469 | ||
1470 | @kindex --minor-image-version | |
1471 | @item --minor-image-version @var{value} | |
1472 | Sets the minor number of the "image version". Defaults to 0. | |
1473 | ||
1474 | @kindex --minor-os-version | |
1475 | @item --minor-os-version @var{value} | |
1476 | Sets the minor number of the "os version". Defaults to 0. | |
1477 | ||
1478 | @kindex --minor-subsystem-version | |
1479 | @item --minor-subsystem-version @var{value} | |
1480 | Sets the minor number of the "subsystem version". Defaults to 0. | |
1481 | ||
1482 | @cindex DEF files, creating | |
1483 | @cindex DLLs, creating | |
1484 | @kindex --output-def | |
1485 | @item --output-def @var{file} | |
1486 | The linker will create the file @var{file} which will contain a DEF | |
1487 | file corresponding to the DLL the linker is generating. This DEF file | |
1488 | (which should be called @code{*.def}) may be used to create an import | |
1489 | library with @code{dlltool} or may be used as a reference to | |
1490 | automatically or implicitly exported symbols. | |
1491 | ||
1492 | @kindex --section-alignment | |
1493 | @item --section-alignment | |
1494 | Sets the section alignment. Sections in memory will always begin at | |
1495 | addresses which are a multiple of this number. Defaults to 0x1000. | |
1496 | ||
1497 | @cindex stack size | |
1498 | @kindex --stack | |
1499 | @item --stack @var{reserve} | |
1500 | @itemx --stack @var{reserve},@var{commit} | |
1501 | Specify the amount of memory to reserve (and optionally commit) to be | |
1502 | used as stack for this program. The default is 32Mb reserved, 4K | |
1503 | committed. | |
1504 | ||
1505 | @kindex --subsystem | |
1506 | @item --subsystem @var{which} | |
1507 | @itemx --subsystem @var{which}:@var{major} | |
1508 | @itemx --subsystem @var{which}:@var{major}.@var{minor} | |
1509 | Specifies the subsystem under which your program will execute. The | |
1510 | legal values for @var{which} are @code{native}, @code{windows}, | |
1511 | @code{console}, and @code{posix}. You may optionally set the | |
1512 | subsystem version also. | |
1513 | ||
1514 | @end table | |
1515 | ||
1516 | @ifset UsesEnvVars | |
1517 | @node Environment | |
1518 | @section Environment Variables | |
1519 | ||
1520 | You can change the behavior of @code{ld} with the environment variables | |
1521 | @code{GNUTARGET}, @code{LDEMULATION}, and @code{COLLECT_NO_DEMANGLE}. | |
1522 | ||
1523 | @kindex GNUTARGET | |
1524 | @cindex default input format | |
1525 | @code{GNUTARGET} determines the input-file object format if you don't | |
1526 | use @samp{-b} (or its synonym @samp{--format}). Its value should be one | |
1527 | of the BFD names for an input format (@pxref{BFD}). If there is no | |
1528 | @code{GNUTARGET} in the environment, @code{ld} uses the natural format | |
1529 | of the target. If @code{GNUTARGET} is set to @code{default} then BFD | |
1530 | attempts to discover the input format by examining binary input files; | |
1531 | this method often succeeds, but there are potential ambiguities, since | |
1532 | there is no method of ensuring that the magic number used to specify | |
1533 | object-file formats is unique. However, the configuration procedure for | |
1534 | BFD on each system places the conventional format for that system first | |
1535 | in the search-list, so ambiguities are resolved in favor of convention. | |
1536 | ||
1537 | @kindex LDEMULATION | |
1538 | @cindex default emulation | |
1539 | @cindex emulation, default | |
1540 | @code{LDEMULATION} determines the default emulation if you don't use the | |
1541 | @samp{-m} option. The emulation can affect various aspects of linker | |
1542 | behaviour, particularly the default linker script. You can list the | |
1543 | available emulations with the @samp{--verbose} or @samp{-V} options. If | |
1544 | the @samp{-m} option is not used, and the @code{LDEMULATION} environment | |
1545 | variable is not defined, the default emulation depends upon how the | |
1546 | linker was configured. | |
1547 | @end ifset | |
1548 | ||
1549 | @kindex COLLECT_NO_DEMANGLE | |
1550 | @cindex demangling, default | |
1551 | Normally, the linker will default to demangling symbols. However, if | |
1552 | @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will | |
1553 | default to not demangling symbols. This environment variable is used in | |
1554 | a similar fashion by the @code{gcc} linker wrapper program. The default | |
1555 | may be overridden by the @samp{--demangle} and @samp{--no-demangle} | |
1556 | options. | |
1557 | ||
1558 | @node Scripts | |
1559 | @chapter Linker Scripts | |
1560 | ||
1561 | @cindex scripts | |
1562 | @cindex linker scripts | |
1563 | @cindex command files | |
1564 | Every link is controlled by a @dfn{linker script}. This script is | |
1565 | written in the linker command language. | |
1566 | ||
1567 | The main purpose of the linker script is to describe how the sections in | |
1568 | the input files should be mapped into the output file, and to control | |
1569 | the memory layout of the output file. Most linker scripts do nothing | |
1570 | more than this. However, when necessary, the linker script can also | |
1571 | direct the linker to perform many other operations, using the commands | |
1572 | described below. | |
1573 | ||
1574 | The linker always uses a linker script. If you do not supply one | |
1575 | yourself, the linker will use a default script that is compiled into the | |
1576 | linker executable. You can use the @samp{--verbose} command line option | |
1577 | to display the default linker script. Certain command line options, | |
1578 | such as @samp{-r} or @samp{-N}, will affect the default linker script. | |
1579 | ||
1580 | You may supply your own linker script by using the @samp{-T} command | |
1581 | line option. When you do this, your linker script will replace the | |
1582 | default linker script. | |
1583 | ||
1584 | You may also use linker scripts implicitly by naming them as input files | |
1585 | to the linker, as though they were files to be linked. @xref{Implicit | |
1586 | Linker Scripts}. | |
1587 | ||
1588 | @menu | |
1589 | * Basic Script Concepts:: Basic Linker Script Concepts | |
1590 | * Script Format:: Linker Script Format | |
1591 | * Simple Example:: Simple Linker Script Example | |
1592 | * Simple Commands:: Simple Linker Script Commands | |
1593 | * Assignments:: Assigning Values to Symbols | |
1594 | * SECTIONS:: SECTIONS Command | |
1595 | * MEMORY:: MEMORY Command | |
1596 | * PHDRS:: PHDRS Command | |
1597 | * VERSION:: VERSION Command | |
1598 | * Expressions:: Expressions in Linker Scripts | |
1599 | * Implicit Linker Scripts:: Implicit Linker Scripts | |
1600 | @end menu | |
1601 | ||
1602 | @node Basic Script Concepts | |
1603 | @section Basic Linker Script Concepts | |
1604 | @cindex linker script concepts | |
1605 | We need to define some basic concepts and vocabulary in order to | |
1606 | describe the linker script language. | |
1607 | ||
1608 | The linker combines input files into a single output file. The output | |
1609 | file and each input file are in a special data format known as an | |
1610 | @dfn{object file format}. Each file is called an @dfn{object file}. | |
1611 | The output file is often called an @dfn{executable}, but for our | |
1612 | purposes we will also call it an object file. Each object file has, | |
1613 | among other things, a list of @dfn{sections}. We sometimes refer to a | |
1614 | section in an input file as an @dfn{input section}; similarly, a section | |
1615 | in the output file is an @dfn{output section}. | |
1616 | ||
1617 | Each section in an object file has a name and a size. Most sections | |
1618 | also have an associated block of data, known as the @dfn{section | |
1619 | contents}. A section may be marked as @dfn{loadable}, which mean that | |
1620 | the contents should be loaded into memory when the output file is run. | |
1621 | A section with no contents may be @dfn{allocatable}, which means that an | |
1622 | area in memory should be set aside, but nothing in particular should be | |
1623 | loaded there (in some cases this memory must be zeroed out). A section | |
1624 | which is neither loadable nor allocatable typically contains some sort | |
1625 | of debugging information. | |
1626 | ||
1627 | Every loadable or allocatable output section has two addresses. The | |
1628 | first is the @dfn{VMA}, or virtual memory address. This is the address | |
1629 | the section will have when the output file is run. The second is the | |
1630 | @dfn{LMA}, or load memory address. This is the address at which the | |
1631 | section will be loaded. In most cases the two addresses will be the | |
1632 | same. An example of when they might be different is when a data section | |
1633 | is loaded into ROM, and then copied into RAM when the program starts up | |
1634 | (this technique is often used to initialize global variables in a ROM | |
1635 | based system). In this case the ROM address would be the LMA, and the | |
1636 | RAM address would be the VMA. | |
1637 | ||
1638 | You can see the sections in an object file by using the @code{objdump} | |
1639 | program with the @samp{-h} option. | |
1640 | ||
1641 | Every object file also has a list of @dfn{symbols}, known as the | |
1642 | @dfn{symbol table}. A symbol may be defined or undefined. Each symbol | |
1643 | has a name, and each defined symbol has an address, among other | |
1644 | information. If you compile a C or C++ program into an object file, you | |
1645 | will get a defined symbol for every defined function and global or | |
1646 | static variable. Every undefined function or global variable which is | |
1647 | referenced in the input file will become an undefined symbol. | |
1648 | ||
1649 | You can see the symbols in an object file by using the @code{nm} | |
1650 | program, or by using the @code{objdump} program with the @samp{-t} | |
1651 | option. | |
1652 | ||
1653 | @node Script Format | |
1654 | @section Linker Script Format | |
1655 | @cindex linker script format | |
1656 | Linker scripts are text files. | |
1657 | ||
1658 | You write a linker script as a series of commands. Each command is | |
1659 | either a keyword, possibly followed by arguments, or an assignment to a | |
1660 | symbol. You may separate commands using semicolons. Whitespace is | |
1661 | generally ignored. | |
1662 | ||
1663 | Strings such as file or format names can normally be entered directly. | |
1664 | If the file name contains a character such as a comma which would | |
1665 | otherwise serve to separate file names, you may put the file name in | |
1666 | double quotes. There is no way to use a double quote character in a | |
1667 | file name. | |
1668 | ||
1669 | You may include comments in linker scripts just as in C, delimited by | |
1670 | @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent | |
1671 | to whitespace. | |
1672 | ||
1673 | @node Simple Example | |
1674 | @section Simple Linker Script Example | |
1675 | @cindex linker script example | |
1676 | @cindex example of linker script | |
1677 | Many linker scripts are fairly simple. | |
1678 | ||
1679 | The simplest possible linker script has just one command: | |
1680 | @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the | |
1681 | memory layout of the output file. | |
1682 | ||
1683 | The @samp{SECTIONS} command is a powerful command. Here we will | |
1684 | describe a simple use of it. Let's assume your program consists only of | |
1685 | code, initialized data, and uninitialized data. These will be in the | |
1686 | @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively. | |
1687 | Let's assume further that these are the only sections which appear in | |
1688 | your input files. | |
1689 | ||
1690 | For this example, let's say that the code should be loaded at address | |
1691 | 0x10000, and that the data should start at address 0x8000000. Here is a | |
1692 | linker script which will do that: | |
1693 | @smallexample | |
1694 | SECTIONS | |
1695 | @{ | |
1696 | . = 0x10000; | |
1697 | .text : @{ *(.text) @} | |
1698 | . = 0x8000000; | |
1699 | .data : @{ *(.data) @} | |
1700 | .bss : @{ *(.bss) @} | |
1701 | @} | |
1702 | @end smallexample | |
1703 | ||
1704 | You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS}, | |
1705 | followed by a series of symbol assignments and output section | |
1706 | descriptions enclosed in curly braces. | |
1707 | ||
252b5132 RH |
1708 | The first line inside the @samp{SECTIONS} command of the above example |
1709 | sets the value of the special symbol @samp{.}, which is the location | |
1710 | counter. If you do not specify the address of an output section in some | |
1711 | other way (other ways are described later), the address is set from the | |
1712 | current value of the location counter. The location counter is then | |
1713 | incremented by the size of the output section. At the start of the | |
1714 | @samp{SECTIONS} command, the location counter has the value @samp{0}. | |
1715 | ||
1716 | The second line defines an output section, @samp{.text}. The colon is | |
1717 | required syntax which may be ignored for now. Within the curly braces | |
1718 | after the output section name, you list the names of the input sections | |
1719 | which should be placed into this output section. The @samp{*} is a | |
1720 | wildcard which matches any file name. The expression @samp{*(.text)} | |
1721 | means all @samp{.text} input sections in all input files. | |
1722 | ||
1723 | Since the location counter is @samp{0x10000} when the output section | |
1724 | @samp{.text} is defined, the linker will set the address of the | |
1725 | @samp{.text} section in the output file to be @samp{0x10000}. | |
1726 | ||
1727 | The remaining lines define the @samp{.data} and @samp{.bss} sections in | |
1728 | the output file. The linker will place the @samp{.data} output section | |
1729 | at address @samp{0x8000000}. After the linker places the @samp{.data} | |
1730 | output section, the value of the location counter will be | |
1731 | @samp{0x8000000} plus the size of the @samp{.data} output section. The | |
1732 | effect is that the linker will place the @samp{.bss} output section | |
1733 | immediately after the @samp{.data} output section in memory | |
1734 | ||
1735 | The linker will ensure that each output section has the required | |
1736 | alignment, by increasing the location counter if necessary. In this | |
1737 | example, the specified addresses for the @samp{.text} and @samp{.data} | |
1738 | sections will probably satisfy any alignment constraints, but the linker | |
1739 | may have to create a small gap between the @samp{.data} and @samp{.bss} | |
1740 | sections. | |
1741 | ||
1742 | That's it! That's a simple and complete linker script. | |
1743 | ||
1744 | @node Simple Commands | |
1745 | @section Simple Linker Script Commands | |
1746 | @cindex linker script simple commands | |
1747 | In this section we describe the simple linker script commands. | |
1748 | ||
1749 | @menu | |
1750 | * Entry Point:: Setting the entry point | |
1751 | * File Commands:: Commands dealing with files | |
1752 | @ifclear SingleFormat | |
1753 | * Format Commands:: Commands dealing with object file formats | |
1754 | @end ifclear | |
1755 | ||
1756 | * Miscellaneous Commands:: Other linker script commands | |
1757 | @end menu | |
1758 | ||
1759 | @node Entry Point | |
1760 | @subsection Setting the entry point | |
1761 | @kindex ENTRY(@var{symbol}) | |
1762 | @cindex start of execution | |
1763 | @cindex first instruction | |
1764 | @cindex entry point | |
1765 | The first instruction to execute in a program is called the @dfn{entry | |
1766 | point}. You can use the @code{ENTRY} linker script command to set the | |
1767 | entry point. The argument is a symbol name: | |
1768 | @smallexample | |
1769 | ENTRY(@var{symbol}) | |
1770 | @end smallexample | |
1771 | ||
1772 | There are several ways to set the entry point. The linker will set the | |
1773 | entry point by trying each of the following methods in order, and | |
1774 | stopping when one of them succeeds: | |
1775 | @itemize @bullet | |
1776 | @item | |
1777 | the @samp{-e} @var{entry} command-line option; | |
1778 | @item | |
1779 | the @code{ENTRY(@var{symbol})} command in a linker script; | |
1780 | @item | |
1781 | the value of the symbol @code{start}, if defined; | |
1782 | @item | |
1783 | the address of the first byte of the @samp{.text} section, if present; | |
1784 | @item | |
1785 | The address @code{0}. | |
1786 | @end itemize | |
1787 | ||
1788 | @node File Commands | |
1789 | @subsection Commands dealing with files | |
1790 | @cindex linker script file commands | |
1791 | Several linker script commands deal with files. | |
1792 | ||
1793 | @table @code | |
1794 | @item INCLUDE @var{filename} | |
1795 | @kindex INCLUDE @var{filename} | |
1796 | @cindex including a linker script | |
1797 | Include the linker script @var{filename} at this point. The file will | |
1798 | be searched for in the current directory, and in any directory specified | |
1799 | with the @code{-L} option. You can nest calls to @code{INCLUDE} up to | |
1800 | 10 levels deep. | |
1801 | ||
1802 | @item INPUT(@var{file}, @var{file}, @dots{}) | |
1803 | @itemx INPUT(@var{file} @var{file} @dots{}) | |
1804 | @kindex INPUT(@var{files}) | |
1805 | @cindex input files in linker scripts | |
1806 | @cindex input object files in linker scripts | |
1807 | @cindex linker script input object files | |
1808 | The @code{INPUT} command directs the linker to include the named files | |
1809 | in the link, as though they were named on the command line. | |
1810 | ||
1811 | For example, if you always want to include @file{subr.o} any time you do | |
1812 | a link, but you can't be bothered to put it on every link command line, | |
1813 | then you can put @samp{INPUT (subr.o)} in your linker script. | |
1814 | ||
1815 | In fact, if you like, you can list all of your input files in the linker | |
1816 | script, and then invoke the linker with nothing but a @samp{-T} option. | |
1817 | ||
1818 | The linker will first try to open the file in the current directory. If | |
1819 | it is not found, the linker will search through the archive library | |
1820 | search path. See the description of @samp{-L} in @ref{Options,,Command | |
1821 | Line Options}. | |
1822 | ||
1823 | If you use @samp{INPUT (-l@var{file})}, @code{ld} will transform the | |
1824 | name to @code{lib@var{file}.a}, as with the command line argument | |
1825 | @samp{-l}. | |
1826 | ||
1827 | When you use the @code{INPUT} command in an implicit linker script, the | |
1828 | files will be included in the link at the point at which the linker | |
1829 | script file is included. This can affect archive searching. | |
1830 | ||
1831 | @item GROUP(@var{file}, @var{file}, @dots{}) | |
1832 | @itemx GROUP(@var{file} @var{file} @dots{}) | |
1833 | @kindex GROUP(@var{files}) | |
1834 | @cindex grouping input files | |
1835 | The @code{GROUP} command is like @code{INPUT}, except that the named | |
1836 | files should all be archives, and they are searched repeatedly until no | |
1837 | new undefined references are created. See the description of @samp{-(} | |
1838 | in @ref{Options,,Command Line Options}. | |
1839 | ||
1840 | @item OUTPUT(@var{filename}) | |
1841 | @kindex OUTPUT(@var{filename}) | |
1842 | @cindex output file name in linker scripot | |
1843 | The @code{OUTPUT} command names the output file. Using | |
1844 | @code{OUTPUT(@var{filename})} in the linker script is exactly like using | |
1845 | @samp{-o @var{filename}} on the command line (@pxref{Options,,Command | |
1846 | Line Options}). If both are used, the command line option takes | |
1847 | precedence. | |
1848 | ||
1849 | You can use the @code{OUTPUT} command to define a default name for the | |
1850 | output file other than the usual default of @file{a.out}. | |
1851 | ||
1852 | @item SEARCH_DIR(@var{path}) | |
1853 | @kindex SEARCH_DIR(@var{path}) | |
1854 | @cindex library search path in linker script | |
1855 | @cindex archive search path in linker script | |
1856 | @cindex search path in linker script | |
1857 | The @code{SEARCH_DIR} command adds @var{path} to the list of paths where | |
1858 | @code{ld} looks for archive libraries. Using | |
1859 | @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}} | |
1860 | on the command line (@pxref{Options,,Command Line Options}). If both | |
1861 | are used, then the linker will search both paths. Paths specified using | |
1862 | the command line option are searched first. | |
1863 | ||
1864 | @item STARTUP(@var{filename}) | |
1865 | @kindex STARTUP(@var{filename}) | |
1866 | @cindex first input file | |
1867 | The @code{STARTUP} command is just like the @code{INPUT} command, except | |
1868 | that @var{filename} will become the first input file to be linked, as | |
1869 | though it were specified first on the command line. This may be useful | |
1870 | when using a system in which the entry point is always the start of the | |
1871 | first file. | |
1872 | @end table | |
1873 | ||
1874 | @ifclear SingleFormat | |
1875 | @node Format Commands | |
1876 | @subsection Commands dealing with object file formats | |
1877 | A couple of linker script commands deal with object file formats. | |
1878 | ||
1879 | @table @code | |
1880 | @item OUTPUT_FORMAT(@var{bfdname}) | |
1881 | @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little}) | |
1882 | @kindex OUTPUT_FORMAT(@var{bfdname}) | |
1883 | @cindex output file format in linker script | |
1884 | The @code{OUTPUT_FORMAT} command names the BFD format to use for the | |
1885 | output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is | |
1886 | exactly like using @samp{-oformat @var{bfdname}} on the command line | |
1887 | (@pxref{Options,,Command Line Options}). If both are used, the command | |
1888 | line option takes precedence. | |
1889 | ||
1890 | You can use @code{OUTPUT_FORMAT} with three arguments to use different | |
1891 | formats based on the @samp{-EB} and @samp{-EL} command line options. | |
1892 | This permits the linker script to set the output format based on the | |
1893 | desired endianness. | |
1894 | ||
1895 | If neither @samp{-EB} nor @samp{-EL} are used, then the output format | |
1896 | will be the first argument, @var{default}. If @samp{-EB} is used, the | |
1897 | output format will be the second argument, @var{big}. If @samp{-EL} is | |
1898 | used, the output format will be the third argument, @var{little}. | |
1899 | ||
1900 | For example, the default linker script for the MIPS ELF target uses this | |
1901 | command: | |
1902 | @smallexample | |
1903 | OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) | |
1904 | @end smallexample | |
1905 | This says that the default format for the output file is | |
1906 | @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line | |
1907 | option, the output file will be created in the @samp{elf32-littlemips} | |
1908 | format. | |
1909 | ||
1910 | @item TARGET(@var{bfdname}) | |
1911 | @kindex TARGET(@var{bfdname}) | |
1912 | @cindex input file format in linker script | |
1913 | The @code{TARGET} command names the BFD format to use when reading input | |
1914 | files. It affects subsequent @code{INPUT} and @code{GROUP} commands. | |
1915 | This command is like using @samp{-b @var{bfdname}} on the command line | |
1916 | (@pxref{Options,,Command Line Options}). If the @code{TARGET} command | |
1917 | is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET} | |
1918 | command is also used to set the format for the output file. @xref{BFD}. | |
1919 | @end table | |
1920 | @end ifclear | |
1921 | ||
1922 | @node Miscellaneous Commands | |
1923 | @subsection Other linker script commands | |
1924 | There are a few other linker scripts commands. | |
1925 | ||
1926 | @table @code | |
1927 | @item ASSERT(@var{exp}, @var{message}) | |
1928 | @kindex ASSERT | |
1929 | @cindex assertion in linker script | |
1930 | Ensure that @var{exp} is non-zero. If it is zero, then exit the linker | |
1931 | with an error code, and print @var{message}. | |
1932 | ||
1933 | @item EXTERN(@var{symbol} @var{symbol} @dots{}) | |
1934 | @kindex EXTERN | |
1935 | @cindex undefined symbol in linker script | |
1936 | Force @var{symbol} to be entered in the output file as an undefined | |
1937 | symbol. Doing this may, for example, trigger linking of additional | |
1938 | modules from standard libraries. You may list several @var{symbol}s for | |
1939 | each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This | |
1940 | command has the same effect as the @samp{-u} command-line option. | |
1941 | ||
1942 | @item FORCE_COMMON_ALLOCATION | |
1943 | @kindex FORCE_COMMON_ALLOCATION | |
1944 | @cindex common allocation in linker script | |
1945 | This command has the same effect as the @samp{-d} command-line option: | |
1946 | to make @code{ld} assign space to common symbols even if a relocatable | |
1947 | output file is specified (@samp{-r}). | |
1948 | ||
1949 | @item NOCROSSREFS(@var{section} @var{section} @dots{}) | |
1950 | @kindex NOCROSSREFS(@var{sections}) | |
1951 | @cindex cross references | |
1952 | This command may be used to tell @code{ld} to issue an error about any | |
1953 | references among certain output sections. | |
1954 | ||
1955 | In certain types of programs, particularly on embedded systems when | |
1956 | using overlays, when one section is loaded into memory, another section | |
1957 | will not be. Any direct references between the two sections would be | |
1958 | errors. For example, it would be an error if code in one section called | |
1959 | a function defined in the other section. | |
1960 | ||
1961 | The @code{NOCROSSREFS} command takes a list of output section names. If | |
1962 | @code{ld} detects any cross references between the sections, it reports | |
1963 | an error and returns a non-zero exit status. Note that the | |
1964 | @code{NOCROSSREFS} command uses output section names, not input section | |
1965 | names. | |
1966 | ||
1967 | @ifclear SingleFormat | |
1968 | @item OUTPUT_ARCH(@var{bfdarch}) | |
1969 | @kindex OUTPUT_ARCH(@var{bfdarch}) | |
1970 | @cindex machine architecture | |
1971 | @cindex architecture | |
1972 | Specify a particular output machine architecture. The argument is one | |
1973 | of the names used by the BFD library (@pxref{BFD}). You can see the | |
1974 | architecture of an object file by using the @code{objdump} program with | |
1975 | the @samp{-f} option. | |
1976 | @end ifclear | |
1977 | @end table | |
1978 | ||
1979 | @node Assignments | |
1980 | @section Assigning Values to Symbols | |
1981 | @cindex assignment in scripts | |
1982 | @cindex symbol definition, scripts | |
1983 | @cindex variables, defining | |
1984 | You may assign a value to a symbol in a linker script. This will define | |
1985 | the symbol as a global symbol. | |
1986 | ||
1987 | @menu | |
1988 | * Simple Assignments:: Simple Assignments | |
1989 | * PROVIDE:: PROVIDE | |
1990 | @end menu | |
1991 | ||
1992 | @node Simple Assignments | |
1993 | @subsection Simple Assignments | |
1994 | ||
1995 | You may assign to a symbol using any of the C assignment operators: | |
1996 | ||
1997 | @table @code | |
1998 | @item @var{symbol} = @var{expression} ; | |
1999 | @itemx @var{symbol} += @var{expression} ; | |
2000 | @itemx @var{symbol} -= @var{expression} ; | |
2001 | @itemx @var{symbol} *= @var{expression} ; | |
2002 | @itemx @var{symbol} /= @var{expression} ; | |
2003 | @itemx @var{symbol} <<= @var{expression} ; | |
2004 | @itemx @var{symbol} >>= @var{expression} ; | |
2005 | @itemx @var{symbol} &= @var{expression} ; | |
2006 | @itemx @var{symbol} |= @var{expression} ; | |
2007 | @end table | |
2008 | ||
2009 | The first case will define @var{symbol} to the value of | |
2010 | @var{expression}. In the other cases, @var{symbol} must already be | |
2011 | defined, and the value will be adjusted accordingly. | |
2012 | ||
2013 | The special symbol name @samp{.} indicates the location counter. You | |
2014 | may only use this within a @code{SECTIONS} command. | |
2015 | ||
2016 | The semicolon after @var{expression} is required. | |
2017 | ||
2018 | Expressions are defined below; see @ref{Expressions}. | |
2019 | ||
2020 | You may write symbol assignments as commands in their own right, or as | |
2021 | statements within a @code{SECTIONS} command, or as part of an output | |
2022 | section description in a @code{SECTIONS} command. | |
2023 | ||
2024 | The section of the symbol will be set from the section of the | |
2025 | expression; for more information, see @ref{Expression Section}. | |
2026 | ||
2027 | Here is an example showing the three different places that symbol | |
2028 | assignments may be used: | |
2029 | ||
2030 | @smallexample | |
2031 | floating_point = 0; | |
2032 | SECTIONS | |
2033 | @{ | |
2034 | .text : | |
2035 | @{ | |
2036 | *(.text) | |
2037 | _etext = .; | |
2038 | @} | |
2039 | _bdata = (. + 3) & ~ 4; | |
2040 | .data : @{ *(.data) @} | |
2041 | @} | |
2042 | @end smallexample | |
2043 | @noindent | |
2044 | In this example, the symbol @samp{floating_point} will be defined as | |
2045 | zero. The symbol @samp{_etext} will be defined as the address following | |
2046 | the last @samp{.text} input section. The symbol @samp{_bdata} will be | |
2047 | defined as the address following the @samp{.text} output section aligned | |
2048 | upward to a 4 byte boundary. | |
2049 | ||
2050 | @node PROVIDE | |
2051 | @subsection PROVIDE | |
2052 | @cindex PROVIDE | |
2053 | In some cases, it is desirable for a linker script to define a symbol | |
2054 | only if it is referenced and is not defined by any object included in | |
2055 | the link. For example, traditional linkers defined the symbol | |
2056 | @samp{etext}. However, ANSI C requires that the user be able to use | |
2057 | @samp{etext} as a function name without encountering an error. The | |
2058 | @code{PROVIDE} keyword may be used to define a symbol, such as | |
2059 | @samp{etext}, only if it is referenced but not defined. The syntax is | |
2060 | @code{PROVIDE(@var{symbol} = @var{expression})}. | |
2061 | ||
2062 | Here is an example of using @code{PROVIDE} to define @samp{etext}: | |
2063 | @smallexample | |
2064 | SECTIONS | |
2065 | @{ | |
2066 | .text : | |
2067 | @{ | |
2068 | *(.text) | |
2069 | _etext = .; | |
2070 | PROVIDE(etext = .); | |
2071 | @} | |
2072 | @} | |
2073 | @end smallexample | |
2074 | ||
2075 | In this example, if the program defines @samp{_etext} (with a leading | |
2076 | underscore), the linker will give a multiple definition error. If, on | |
2077 | the other hand, the program defines @samp{etext} (with no leading | |
2078 | underscore), the linker will silently use the definition in the program. | |
2079 | If the program references @samp{etext} but does not define it, the | |
2080 | linker will use the definition in the linker script. | |
2081 | ||
2082 | @node SECTIONS | |
2083 | @section SECTIONS command | |
2084 | @kindex SECTIONS | |
2085 | The @code{SECTIONS} command tells the linker how to map input sections | |
2086 | into output sections, and how to place the output sections in memory. | |
2087 | ||
2088 | The format of the @code{SECTIONS} command is: | |
2089 | @smallexample | |
2090 | SECTIONS | |
2091 | @{ | |
2092 | @var{sections-command} | |
2093 | @var{sections-command} | |
2094 | @dots{} | |
2095 | @} | |
2096 | @end smallexample | |
2097 | ||
2098 | Each @var{sections-command} may of be one of the following: | |
2099 | ||
2100 | @itemize @bullet | |
2101 | @item | |
2102 | an @code{ENTRY} command (@pxref{Entry Point,,Entry command}) | |
2103 | @item | |
2104 | a symbol assignment (@pxref{Assignments}) | |
2105 | @item | |
2106 | an output section description | |
2107 | @item | |
2108 | an overlay description | |
2109 | @end itemize | |
2110 | ||
2111 | The @code{ENTRY} command and symbol assignments are permitted inside the | |
2112 | @code{SECTIONS} command for convenience in using the location counter in | |
2113 | those commands. This can also make the linker script easier to | |
2114 | understand because you can use those commands at meaningful points in | |
2115 | the layout of the output file. | |
2116 | ||
2117 | Output section descriptions and overlay descriptions are described | |
2118 | below. | |
2119 | ||
2120 | If you do not use a @code{SECTIONS} command in your linker script, the | |
2121 | linker will place each input section into an identically named output | |
2122 | section in the order that the sections are first encountered in the | |
2123 | input files. If all input sections are present in the first file, for | |
2124 | example, the order of sections in the output file will match the order | |
2125 | in the first input file. The first section will be at address zero. | |
2126 | ||
2127 | @menu | |
2128 | * Output Section Description:: Output section description | |
2129 | * Output Section Name:: Output section name | |
2130 | * Output Section Address:: Output section address | |
2131 | * Input Section:: Input section description | |
2132 | * Output Section Data:: Output section data | |
2133 | * Output Section Keywords:: Output section keywords | |
2134 | * Output Section Discarding:: Output section discarding | |
2135 | * Output Section Attributes:: Output section attributes | |
2136 | * Overlay Description:: Overlay description | |
2137 | @end menu | |
2138 | ||
2139 | @node Output Section Description | |
2140 | @subsection Output section description | |
2141 | The full description of an output section looks like this: | |
2142 | @smallexample | |
2143 | @group | |
2144 | @var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] | |
2145 | @{ | |
2146 | @var{output-section-command} | |
2147 | @var{output-section-command} | |
2148 | @dots{} | |
562d3460 | 2149 | @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
252b5132 RH |
2150 | @end group |
2151 | @end smallexample | |
2152 | ||
2153 | Most output sections do not use most of the optional section attributes. | |
2154 | ||
2155 | The whitespace around @var{section} is required, so that the section | |
2156 | name is unambiguous. The colon and the curly braces are also required. | |
2157 | The line breaks and other white space are optional. | |
2158 | ||
2159 | Each @var{output-section-command} may be one of the following: | |
2160 | ||
2161 | @itemize @bullet | |
2162 | @item | |
2163 | a symbol assignment (@pxref{Assignments}) | |
2164 | @item | |
2165 | an input section description (@pxref{Input Section}) | |
2166 | @item | |
2167 | data values to include directly (@pxref{Output Section Data}) | |
2168 | @item | |
2169 | a special output section keyword (@pxref{Output Section Keywords}) | |
2170 | @end itemize | |
2171 | ||
2172 | @node Output Section Name | |
2173 | @subsection Output section name | |
2174 | @cindex name, section | |
2175 | @cindex section name | |
2176 | The name of the output section is @var{section}. @var{section} must | |
2177 | meet the constraints of your output format. In formats which only | |
2178 | support a limited number of sections, such as @code{a.out}, the name | |
2179 | must be one of the names supported by the format (@code{a.out}, for | |
2180 | example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the | |
2181 | output format supports any number of sections, but with numbers and not | |
2182 | names (as is the case for Oasys), the name should be supplied as a | |
2183 | quoted numeric string. A section name may consist of any sequence of | |
2184 | characters, but a name which contains any unusual characters such as | |
2185 | commas must be quoted. | |
2186 | ||
2187 | The output section name @samp{/DISCARD/} is special; @ref{Output Section | |
2188 | Discarding}. | |
2189 | ||
2190 | @node Output Section Address | |
2191 | @subsection Output section address | |
2192 | @cindex address, section | |
2193 | @cindex section address | |
2194 | The @var{address} is an expression for the VMA (the virtual memory | |
2195 | address) of the output section. If you do not provide @var{address}, | |
2196 | the linker will set it based on @var{region} if present, or otherwise | |
2197 | based on the current value of the location counter. | |
2198 | ||
2199 | If you provide @var{address}, the address of the output section will be | |
2200 | set to precisely that. If you provide neither @var{address} nor | |
2201 | @var{region}, then the address of the output section will be set to the | |
2202 | current value of the location counter aligned to the alignment | |
2203 | requirements of the output section. The alignment requirement of the | |
2204 | output section is the strictest alignment of any input section contained | |
2205 | within the output section. | |
2206 | ||
2207 | For example, | |
2208 | @smallexample | |
2209 | .text . : @{ *(.text) @} | |
2210 | @end smallexample | |
2211 | @noindent | |
2212 | and | |
2213 | @smallexample | |
2214 | .text : @{ *(.text) @} | |
2215 | @end smallexample | |
2216 | @noindent | |
2217 | are subtly different. The first will set the address of the | |
2218 | @samp{.text} output section to the current value of the location | |
2219 | counter. The second will set it to the current value of the location | |
2220 | counter aligned to the strictest alignment of a @samp{.text} input | |
2221 | section. | |
2222 | ||
2223 | The @var{address} may be an arbitrary expression; @ref{Expressions}. | |
2224 | For example, if you want to align the section on a 0x10 byte boundary, | |
2225 | so that the lowest four bits of the section address are zero, you could | |
2226 | do something like this: | |
2227 | @smallexample | |
2228 | .text ALIGN(0x10) : @{ *(.text) @} | |
2229 | @end smallexample | |
2230 | @noindent | |
2231 | This works because @code{ALIGN} returns the current location counter | |
2232 | aligned upward to the specified value. | |
2233 | ||
2234 | Specifying @var{address} for a section will change the value of the | |
2235 | location counter. | |
2236 | ||
2237 | @node Input Section | |
2238 | @subsection Input section description | |
2239 | @cindex input sections | |
2240 | @cindex mapping input sections to output sections | |
2241 | The most common output section command is an input section description. | |
2242 | ||
2243 | The input section description is the most basic linker script operation. | |
2244 | You use output sections to tell the linker how to lay out your program | |
2245 | in memory. You use input section descriptions to tell the linker how to | |
2246 | map the input files into your memory layout. | |
2247 | ||
2248 | @menu | |
2249 | * Input Section Basics:: Input section basics | |
2250 | * Input Section Wildcards:: Input section wildcard patterns | |
2251 | * Input Section Common:: Input section for common symbols | |
2252 | * Input Section Keep:: Input section and garbage collection | |
2253 | * Input Section Example:: Input section example | |
2254 | @end menu | |
2255 | ||
2256 | @node Input Section Basics | |
2257 | @subsubsection Input section basics | |
2258 | @cindex input section basics | |
2259 | An input section description consists of a file name optionally followed | |
2260 | by a list of section names in parentheses. | |
2261 | ||
2262 | The file name and the section name may be wildcard patterns, which we | |
2263 | describe further below (@pxref{Input Section Wildcards}). | |
2264 | ||
2265 | The most common input section description is to include all input | |
2266 | sections with a particular name in the output section. For example, to | |
2267 | include all input @samp{.text} sections, you would write: | |
2268 | @smallexample | |
2269 | *(.text) | |
2270 | @end smallexample | |
2271 | @noindent | |
18625d54 CM |
2272 | Here the @samp{*} is a wildcard which matches any file name. To exclude a list |
2273 | of files from matching the file name wildcard, EXCLUDE_FILE may be used to | |
2274 | match all files except the ones specified in the EXCLUDE_FILE list. For | |
2275 | example: | |
252b5132 | 2276 | @smallexample |
765b7cbe | 2277 | (*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)) |
252b5132 | 2278 | @end smallexample |
765b7cbe JB |
2279 | will cause all .ctors sections from all files except @file{crtend.o} and |
2280 | @file{otherfile.o} to be included. | |
252b5132 RH |
2281 | |
2282 | There are two ways to include more than one section: | |
2283 | @smallexample | |
2284 | *(.text .rdata) | |
2285 | *(.text) *(.rdata) | |
2286 | @end smallexample | |
2287 | @noindent | |
2288 | The difference between these is the order in which the @samp{.text} and | |
2289 | @samp{.rdata} input sections will appear in the output section. In the | |
2290 | first example, they will be intermingled. In the second example, all | |
2291 | @samp{.text} input sections will appear first, followed by all | |
2292 | @samp{.rdata} input sections. | |
2293 | ||
2294 | You can specify a file name to include sections from a particular file. | |
2295 | You would do this if one or more of your files contain special data that | |
2296 | needs to be at a particular location in memory. For example: | |
2297 | @smallexample | |
2298 | data.o(.data) | |
2299 | @end smallexample | |
2300 | ||
2301 | If you use a file name without a list of sections, then all sections in | |
2302 | the input file will be included in the output section. This is not | |
2303 | commonly done, but it may by useful on occasion. For example: | |
2304 | @smallexample | |
2305 | data.o | |
2306 | @end smallexample | |
2307 | ||
2308 | When you use a file name which does not contain any wild card | |
2309 | characters, the linker will first see if you also specified the file | |
2310 | name on the linker command line or in an @code{INPUT} command. If you | |
2311 | did not, the linker will attempt to open the file as an input file, as | |
2312 | though it appeared on the command line. Note that this differs from an | |
2313 | @code{INPUT} command, because the linker will not search for the file in | |
2314 | the archive search path. | |
2315 | ||
2316 | @node Input Section Wildcards | |
2317 | @subsubsection Input section wildcard patterns | |
2318 | @cindex input section wildcards | |
2319 | @cindex wildcard file name patterns | |
2320 | @cindex file name wildcard patterns | |
2321 | @cindex section name wildcard patterns | |
2322 | In an input section description, either the file name or the section | |
2323 | name or both may be wildcard patterns. | |
2324 | ||
2325 | The file name of @samp{*} seen in many examples is a simple wildcard | |
2326 | pattern for the file name. | |
2327 | ||
2328 | The wildcard patterns are like those used by the Unix shell. | |
2329 | ||
2330 | @table @samp | |
2331 | @item * | |
2332 | matches any number of characters | |
2333 | @item ? | |
2334 | matches any single character | |
2335 | @item [@var{chars}] | |
2336 | matches a single instance of any of the @var{chars}; the @samp{-} | |
2337 | character may be used to specify a range of characters, as in | |
2338 | @samp{[a-z]} to match any lower case letter | |
2339 | @item \ | |
2340 | quotes the following character | |
2341 | @end table | |
2342 | ||
2343 | When a file name is matched with a wildcard, the wildcard characters | |
2344 | will not match a @samp{/} character (used to separate directory names on | |
2345 | Unix). A pattern consisting of a single @samp{*} character is an | |
2346 | exception; it will always match any file name, whether it contains a | |
2347 | @samp{/} or not. In a section name, the wildcard characters will match | |
2348 | a @samp{/} character. | |
2349 | ||
2350 | File name wildcard patterns only match files which are explicitly | |
2351 | specified on the command line or in an @code{INPUT} command. The linker | |
2352 | does not search directories to expand wildcards. | |
2353 | ||
2354 | If a file name matches more than one wildcard pattern, or if a file name | |
2355 | appears explicitly and is also matched by a wildcard pattern, the linker | |
2356 | will use the first match in the linker script. For example, this | |
2357 | sequence of input section descriptions is probably in error, because the | |
2358 | @file{data.o} rule will not be used: | |
2359 | @smallexample | |
2360 | .data : @{ *(.data) @} | |
2361 | .data1 : @{ data.o(.data) @} | |
2362 | @end smallexample | |
2363 | ||
2364 | @cindex SORT | |
2365 | Normally, the linker will place files and sections matched by wildcards | |
2366 | in the order in which they are seen during the link. You can change | |
2367 | this by using the @code{SORT} keyword, which appears before a wildcard | |
2368 | pattern in parentheses (e.g., @code{SORT(.text*)}). When the | |
2369 | @code{SORT} keyword is used, the linker will sort the files or sections | |
2370 | into ascending order by name before placing them in the output file. | |
2371 | ||
2372 | If you ever get confused about where input sections are going, use the | |
2373 | @samp{-M} linker option to generate a map file. The map file shows | |
2374 | precisely how input sections are mapped to output sections. | |
2375 | ||
2376 | This example shows how wildcard patterns might be used to partition | |
2377 | files. This linker script directs the linker to place all @samp{.text} | |
2378 | sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}. | |
2379 | The linker will place the @samp{.data} section from all files beginning | |
2380 | with an upper case character in @samp{.DATA}; for all other files, the | |
2381 | linker will place the @samp{.data} section in @samp{.data}. | |
2382 | @smallexample | |
2383 | @group | |
2384 | SECTIONS @{ | |
2385 | .text : @{ *(.text) @} | |
2386 | .DATA : @{ [A-Z]*(.data) @} | |
2387 | .data : @{ *(.data) @} | |
2388 | .bss : @{ *(.bss) @} | |
2389 | @} | |
2390 | @end group | |
2391 | @end smallexample | |
2392 | ||
2393 | @node Input Section Common | |
2394 | @subsubsection Input section for common symbols | |
2395 | @cindex common symbol placement | |
2396 | @cindex uninitialized data placement | |
2397 | A special notation is needed for common symbols, because in many object | |
2398 | file formats common symbols do not have a particular input section. The | |
2399 | linker treats common symbols as though they are in an input section | |
2400 | named @samp{COMMON}. | |
2401 | ||
2402 | You may use file names with the @samp{COMMON} section just as with any | |
2403 | other input sections. You can use this to place common symbols from a | |
2404 | particular input file in one section while common symbols from other | |
2405 | input files are placed in another section. | |
2406 | ||
2407 | In most cases, common symbols in input files will be placed in the | |
2408 | @samp{.bss} section in the output file. For example: | |
2409 | @smallexample | |
2410 | .bss @{ *(.bss) *(COMMON) @} | |
2411 | @end smallexample | |
2412 | ||
2413 | @cindex scommon section | |
2414 | @cindex small common symbols | |
2415 | Some object file formats have more than one type of common symbol. For | |
2416 | example, the MIPS ELF object file format distinguishes standard common | |
2417 | symbols and small common symbols. In this case, the linker will use a | |
2418 | different special section name for other types of common symbols. In | |
2419 | the case of MIPS ELF, the linker uses @samp{COMMON} for standard common | |
2420 | symbols and @samp{.scommon} for small common symbols. This permits you | |
2421 | to map the different types of common symbols into memory at different | |
2422 | locations. | |
2423 | ||
2424 | @cindex [COMMON] | |
2425 | You will sometimes see @samp{[COMMON]} in old linker scripts. This | |
2426 | notation is now considered obsolete. It is equivalent to | |
2427 | @samp{*(COMMON)}. | |
2428 | ||
2429 | @node Input Section Keep | |
2430 | @subsubsection Input section and garbage collection | |
2431 | @cindex KEEP | |
2432 | @cindex garbage collection | |
2433 | When link-time garbage collection is in use (@samp{--gc-sections}), | |
2434 | it is often useful to mark sections that should not be eliminated. | |
2435 | This is accomplished by surrounding an input section's wildcard entry | |
2436 | with @code{KEEP()}, as in @code{KEEP(*(.init))} or | |
2437 | @code{KEEP(SORT(*)(.ctors))}. | |
2438 | ||
2439 | @node Input Section Example | |
2440 | @subsubsection Input section example | |
2441 | The following example is a complete linker script. It tells the linker | |
2442 | to read all of the sections from file @file{all.o} and place them at the | |
2443 | start of output section @samp{outputa} which starts at location | |
2444 | @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o} | |
2445 | follows immediately, in the same output section. All of section | |
2446 | @samp{.input2} from @file{foo.o} goes into output section | |
2447 | @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}. | |
2448 | All of the remaining @samp{.input1} and @samp{.input2} sections from any | |
2449 | files are written to output section @samp{outputc}. | |
2450 | ||
2451 | @smallexample | |
2452 | @group | |
2453 | SECTIONS @{ | |
2454 | outputa 0x10000 : | |
2455 | @{ | |
2456 | all.o | |
2457 | foo.o (.input1) | |
2458 | @} | |
2459 | outputb : | |
2460 | @{ | |
2461 | foo.o (.input2) | |
2462 | foo1.o (.input1) | |
2463 | @} | |
2464 | outputc : | |
2465 | @{ | |
2466 | *(.input1) | |
2467 | *(.input2) | |
2468 | @} | |
2469 | @} | |
2470 | @end group | |
2471 | @end smallexample | |
2472 | ||
2473 | @node Output Section Data | |
2474 | @subsection Output section data | |
2475 | @cindex data | |
2476 | @cindex section data | |
2477 | @cindex output section data | |
2478 | @kindex BYTE(@var{expression}) | |
2479 | @kindex SHORT(@var{expression}) | |
2480 | @kindex LONG(@var{expression}) | |
2481 | @kindex QUAD(@var{expression}) | |
2482 | @kindex SQUAD(@var{expression}) | |
2483 | You can include explicit bytes of data in an output section by using | |
2484 | @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as | |
2485 | an output section command. Each keyword is followed by an expression in | |
2486 | parentheses providing the value to store (@pxref{Expressions}). The | |
2487 | value of the expression is stored at the current value of the location | |
2488 | counter. | |
2489 | ||
2490 | The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands | |
2491 | store one, two, four, and eight bytes (respectively). After storing the | |
2492 | bytes, the location counter is incremented by the number of bytes | |
2493 | stored. | |
2494 | ||
2495 | For example, this will store the byte 1 followed by the four byte value | |
2496 | of the symbol @samp{addr}: | |
2497 | @smallexample | |
2498 | BYTE(1) | |
2499 | LONG(addr) | |
2500 | @end smallexample | |
2501 | ||
2502 | When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the | |
2503 | same; they both store an 8 byte, or 64 bit, value. When both host and | |
2504 | target are 32 bits, an expression is computed as 32 bits. In this case | |
2505 | @code{QUAD} stores a 32 bit value zero extended to 64 bits, and | |
2506 | @code{SQUAD} stores a 32 bit value sign extended to 64 bits. | |
2507 | ||
2508 | If the object file format of the output file has an explicit endianness, | |
2509 | which is the normal case, the value will be stored in that endianness. | |
2510 | When the object file format does not have an explicit endianness, as is | |
2511 | true of, for example, S-records, the value will be stored in the | |
2512 | endianness of the first input object file. | |
2513 | ||
2b5fc1f5 NC |
2514 | Note - these commands only work inside a section description and not |
2515 | between them, so the following will produce an error from the linker: | |
2516 | @smallexample | |
2517 | SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@ | |
2518 | @end smallexample | |
2519 | whereas this will work: | |
2520 | @smallexample | |
2521 | SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@ | |
2522 | @end smallexample | |
2523 | ||
252b5132 RH |
2524 | @kindex FILL(@var{expression}) |
2525 | @cindex holes, filling | |
2526 | @cindex unspecified memory | |
2527 | You may use the @code{FILL} command to set the fill pattern for the | |
2528 | current section. It is followed by an expression in parentheses. Any | |
2529 | otherwise unspecified regions of memory within the section (for example, | |
2530 | gaps left due to the required alignment of input sections) are filled | |
2531 | with the two least significant bytes of the expression, repeated as | |
2532 | necessary. A @code{FILL} statement covers memory locations after the | |
2533 | point at which it occurs in the section definition; by including more | |
2534 | than one @code{FILL} statement, you can have different fill patterns in | |
2535 | different parts of an output section. | |
2536 | ||
2537 | This example shows how to fill unspecified regions of memory with the | |
2538 | value @samp{0x9090}: | |
2539 | @smallexample | |
2540 | FILL(0x9090) | |
2541 | @end smallexample | |
2542 | ||
2543 | The @code{FILL} command is similar to the @samp{=@var{fillexp}} output | |
2544 | section attribute (@pxref{Output Section Fill}), but it only affects the | |
2545 | part of the section following the @code{FILL} command, rather than the | |
2546 | entire section. If both are used, the @code{FILL} command takes | |
2547 | precedence. | |
2548 | ||
2549 | @node Output Section Keywords | |
2550 | @subsection Output section keywords | |
2551 | There are a couple of keywords which can appear as output section | |
2552 | commands. | |
2553 | ||
2554 | @table @code | |
2555 | @kindex CREATE_OBJECT_SYMBOLS | |
2556 | @cindex input filename symbols | |
2557 | @cindex filename symbols | |
2558 | @item CREATE_OBJECT_SYMBOLS | |
2559 | The command tells the linker to create a symbol for each input file. | |
2560 | The name of each symbol will be the name of the corresponding input | |
2561 | file. The section of each symbol will be the output section in which | |
2562 | the @code{CREATE_OBJECT_SYMBOLS} command appears. | |
2563 | ||
2564 | This is conventional for the a.out object file format. It is not | |
2565 | normally used for any other object file format. | |
2566 | ||
2567 | @kindex CONSTRUCTORS | |
2568 | @cindex C++ constructors, arranging in link | |
2569 | @cindex constructors, arranging in link | |
2570 | @item CONSTRUCTORS | |
2571 | When linking using the a.out object file format, the linker uses an | |
2572 | unusual set construct to support C++ global constructors and | |
2573 | destructors. When linking object file formats which do not support | |
2574 | arbitrary sections, such as ECOFF and XCOFF, the linker will | |
2575 | automatically recognize C++ global constructors and destructors by name. | |
2576 | For these object file formats, the @code{CONSTRUCTORS} command tells the | |
2577 | linker to place constructor information in the output section where the | |
2578 | @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is | |
2579 | ignored for other object file formats. | |
2580 | ||
2581 | The symbol @w{@code{__CTOR_LIST__}} marks the start of the global | |
2582 | constructors, and the symbol @w{@code{__DTOR_LIST}} marks the end. The | |
2583 | first word in the list is the number of entries, followed by the address | |
2584 | of each constructor or destructor, followed by a zero word. The | |
2585 | compiler must arrange to actually run the code. For these object file | |
2586 | formats @sc{gnu} C++ normally calls constructors from a subroutine | |
2587 | @code{__main}; a call to @code{__main} is automatically inserted into | |
2588 | the startup code for @code{main}. @sc{gnu} C++ normally runs | |
2589 | destructors either by using @code{atexit}, or directly from the function | |
2590 | @code{exit}. | |
2591 | ||
2592 | For object file formats such as @code{COFF} or @code{ELF} which support | |
2593 | arbitrary section names, @sc{gnu} C++ will normally arrange to put the | |
2594 | addresses of global constructors and destructors into the @code{.ctors} | |
2595 | and @code{.dtors} sections. Placing the following sequence into your | |
2596 | linker script will build the sort of table which the @sc{gnu} C++ | |
2597 | runtime code expects to see. | |
2598 | ||
2599 | @smallexample | |
2600 | __CTOR_LIST__ = .; | |
2601 | LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2) | |
2602 | *(.ctors) | |
2603 | LONG(0) | |
2604 | __CTOR_END__ = .; | |
2605 | __DTOR_LIST__ = .; | |
2606 | LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2) | |
2607 | *(.dtors) | |
2608 | LONG(0) | |
2609 | __DTOR_END__ = .; | |
2610 | @end smallexample | |
2611 | ||
2612 | If you are using the @sc{gnu} C++ support for initialization priority, | |
2613 | which provides some control over the order in which global constructors | |
2614 | are run, you must sort the constructors at link time to ensure that they | |
2615 | are executed in the correct order. When using the @code{CONSTRUCTORS} | |
2616 | command, use @samp{SORT(CONSTRUCTORS)} instead. When using the | |
2617 | @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT(.ctors))} and | |
2618 | @samp{*(SORT(.dtors))} instead of just @samp{*(.ctors)} and | |
2619 | @samp{*(.dtors)}. | |
2620 | ||
2621 | Normally the compiler and linker will handle these issues automatically, | |
2622 | and you will not need to concern yourself with them. However, you may | |
2623 | need to consider this if you are using C++ and writing your own linker | |
2624 | scripts. | |
2625 | ||
2626 | @end table | |
2627 | ||
2628 | @node Output Section Discarding | |
2629 | @subsection Output section discarding | |
2630 | @cindex discarding sections | |
2631 | @cindex sections, discarding | |
2632 | @cindex removing sections | |
2633 | The linker will not create output section which do not have any | |
2634 | contents. This is for convenience when referring to input sections that | |
2635 | may or may not be present in any of the input files. For example: | |
2636 | @smallexample | |
2637 | .foo @{ *(.foo) @} | |
2638 | @end smallexample | |
2639 | @noindent | |
2640 | will only create a @samp{.foo} section in the output file if there is a | |
2641 | @samp{.foo} section in at least one input file. | |
2642 | ||
2643 | If you use anything other than an input section description as an output | |
2644 | section command, such as a symbol assignment, then the output section | |
2645 | will always be created, even if there are no matching input sections. | |
2646 | ||
2647 | @cindex /DISCARD/ | |
2648 | The special output section name @samp{/DISCARD/} may be used to discard | |
2649 | input sections. Any input sections which are assigned to an output | |
2650 | section named @samp{/DISCARD/} are not included in the output file. | |
2651 | ||
2652 | @node Output Section Attributes | |
2653 | @subsection Output section attributes | |
2654 | @cindex output section attributes | |
2655 | We showed above that the full description of an output section looked | |
2656 | like this: | |
2657 | @smallexample | |
2658 | @group | |
2659 | @var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] | |
2660 | @{ | |
2661 | @var{output-section-command} | |
2662 | @var{output-section-command} | |
2663 | @dots{} | |
562d3460 | 2664 | @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
252b5132 RH |
2665 | @end group |
2666 | @end smallexample | |
2667 | We've already described @var{section}, @var{address}, and | |
2668 | @var{output-section-command}. In this section we will describe the | |
2669 | remaining section attributes. | |
2670 | ||
2671 | @menu | |
2672 | * Output Section Type:: Output section type | |
2673 | * Output Section LMA:: Output section LMA | |
2674 | * Output Section Region:: Output section region | |
2675 | * Output Section Phdr:: Output section phdr | |
2676 | * Output Section Fill:: Output section fill | |
2677 | @end menu | |
2678 | ||
2679 | @node Output Section Type | |
2680 | @subsubsection Output section type | |
2681 | Each output section may have a type. The type is a keyword in | |
2682 | parentheses. The following types are defined: | |
2683 | ||
2684 | @table @code | |
2685 | @item NOLOAD | |
2686 | The section should be marked as not loadable, so that it will not be | |
2687 | loaded into memory when the program is run. | |
2688 | @item DSECT | |
2689 | @itemx COPY | |
2690 | @itemx INFO | |
2691 | @itemx OVERLAY | |
2692 | These type names are supported for backward compatibility, and are | |
2693 | rarely used. They all have the same effect: the section should be | |
2694 | marked as not allocatable, so that no memory is allocated for the | |
2695 | section when the program is run. | |
2696 | @end table | |
2697 | ||
2698 | @kindex NOLOAD | |
2699 | @cindex prevent unnecessary loading | |
2700 | @cindex loading, preventing | |
2701 | The linker normally sets the attributes of an output section based on | |
2702 | the input sections which map into it. You can override this by using | |
2703 | the section type. For example, in the script sample below, the | |
2704 | @samp{ROM} section is addressed at memory location @samp{0} and does not | |
2705 | need to be loaded when the program is run. The contents of the | |
2706 | @samp{ROM} section will appear in the linker output file as usual. | |
2707 | @smallexample | |
2708 | @group | |
2709 | SECTIONS @{ | |
2710 | ROM 0 (NOLOAD) : @{ @dots{} @} | |
2711 | @dots{} | |
2712 | @} | |
2713 | @end group | |
2714 | @end smallexample | |
2715 | ||
2716 | @node Output Section LMA | |
2717 | @subsubsection Output section LMA | |
562d3460 | 2718 | @kindex AT>@var{lma_region} |
252b5132 RH |
2719 | @kindex AT(@var{lma}) |
2720 | @cindex load address | |
2721 | @cindex section load address | |
2722 | Every section has a virtual address (VMA) and a load address (LMA); see | |
2723 | @ref{Basic Script Concepts}. The address expression which may appear in | |
2724 | an output section description sets the VMA (@pxref{Output Section | |
2725 | Address}). | |
2726 | ||
2727 | The linker will normally set the LMA equal to the VMA. You can change | |
2728 | that by using the @code{AT} keyword. The expression @var{lma} that | |
562d3460 TW |
2729 | follows the @code{AT} keyword specifies the load address of the |
2730 | section. Alternatively, with @samp{AT>@var{lma_region}} expression, | |
2731 | you may specify a memory region for the section's load address. @xref{MEMORY}. | |
252b5132 RH |
2732 | |
2733 | @cindex ROM initialized data | |
2734 | @cindex initialized data in ROM | |
2735 | This feature is designed to make it easy to build a ROM image. For | |
2736 | example, the following linker script creates three output sections: one | |
2737 | called @samp{.text}, which starts at @code{0x1000}, one called | |
2738 | @samp{.mdata}, which is loaded at the end of the @samp{.text} section | |
2739 | even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold | |
2740 | uninitialized data at address @code{0x3000}. The symbol @code{_data} is | |
2741 | defined with the value @code{0x2000}, which shows that the location | |
2742 | counter holds the VMA value, not the LMA value. | |
2743 | ||
2744 | @smallexample | |
2745 | @group | |
2746 | SECTIONS | |
2747 | @{ | |
2748 | .text 0x1000 : @{ *(.text) _etext = . ; @} | |
2749 | .mdata 0x2000 : | |
2750 | AT ( ADDR (.text) + SIZEOF (.text) ) | |
2751 | @{ _data = . ; *(.data); _edata = . ; @} | |
2752 | .bss 0x3000 : | |
2753 | @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} | |
2754 | @} | |
2755 | @end group | |
2756 | @end smallexample | |
2757 | ||
2758 | The run-time initialization code for use with a program generated with | |
2759 | this linker script would include something like the following, to copy | |
2760 | the initialized data from the ROM image to its runtime address. Notice | |
2761 | how this code takes advantage of the symbols defined by the linker | |
2762 | script. | |
2763 | ||
2764 | @smallexample | |
2765 | @group | |
2766 | extern char _etext, _data, _edata, _bstart, _bend; | |
2767 | char *src = &_etext; | |
2768 | char *dst = &_data; | |
2769 | ||
2770 | /* ROM has data at end of text; copy it. */ | |
2771 | while (dst < &_edata) @{ | |
2772 | *dst++ = *src++; | |
2773 | @} | |
2774 | ||
2775 | /* Zero bss */ | |
2776 | for (dst = &_bstart; dst< &_bend; dst++) | |
2777 | *dst = 0; | |
2778 | @end group | |
2779 | @end smallexample | |
2780 | ||
2781 | @node Output Section Region | |
2782 | @subsubsection Output section region | |
2783 | @kindex >@var{region} | |
2784 | @cindex section, assigning to memory region | |
2785 | @cindex memory regions and sections | |
2786 | You can assign a section to a previously defined region of memory by | |
2787 | using @samp{>@var{region}}. @xref{MEMORY}. | |
2788 | ||
2789 | Here is a simple example: | |
2790 | @smallexample | |
2791 | @group | |
2792 | MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @} | |
2793 | SECTIONS @{ ROM : @{ *(.text) @} >rom @} | |
2794 | @end group | |
2795 | @end smallexample | |
2796 | ||
2797 | @node Output Section Phdr | |
2798 | @subsubsection Output section phdr | |
2799 | @kindex :@var{phdr} | |
2800 | @cindex section, assigning to program header | |
2801 | @cindex program headers and sections | |
2802 | You can assign a section to a previously defined program segment by | |
2803 | using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to | |
2804 | one or more segments, then all subsequent allocated sections will be | |
2805 | assigned to those segments as well, unless they use an explicitly | |
2806 | @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the | |
2807 | linker to not put the section in any segment at all. | |
2808 | ||
2809 | Here is a simple example: | |
2810 | @smallexample | |
2811 | @group | |
2812 | PHDRS @{ text PT_LOAD ; @} | |
2813 | SECTIONS @{ .text : @{ *(.text) @} :text @} | |
2814 | @end group | |
2815 | @end smallexample | |
2816 | ||
2817 | @node Output Section Fill | |
2818 | @subsubsection Output section fill | |
2819 | @kindex =@var{fillexp} | |
2820 | @cindex section fill pattern | |
2821 | @cindex fill pattern, entire section | |
2822 | You can set the fill pattern for an entire section by using | |
2823 | @samp{=@var{fillexp}}. @var{fillexp} is an expression | |
2824 | (@pxref{Expressions}). Any otherwise unspecified regions of memory | |
2825 | within the output section (for example, gaps left due to the required | |
2826 | alignment of input sections) will be filled with the two least | |
2827 | significant bytes of the value, repeated as necessary. | |
2828 | ||
2829 | You can also change the fill value with a @code{FILL} command in the | |
2830 | output section commands; see @ref{Output Section Data}. | |
2831 | ||
2832 | Here is a simple example: | |
2833 | @smallexample | |
2834 | @group | |
2835 | SECTIONS @{ .text : @{ *(.text) @} =0x9090 @} | |
2836 | @end group | |
2837 | @end smallexample | |
2838 | ||
2839 | @node Overlay Description | |
2840 | @subsection Overlay description | |
2841 | @kindex OVERLAY | |
2842 | @cindex overlays | |
2843 | An overlay description provides an easy way to describe sections which | |
2844 | are to be loaded as part of a single memory image but are to be run at | |
2845 | the same memory address. At run time, some sort of overlay manager will | |
2846 | copy the overlaid sections in and out of the runtime memory address as | |
2847 | required, perhaps by simply manipulating addressing bits. This approach | |
2848 | can be useful, for example, when a certain region of memory is faster | |
2849 | than another. | |
2850 | ||
2851 | Overlays are described using the @code{OVERLAY} command. The | |
2852 | @code{OVERLAY} command is used within a @code{SECTIONS} command, like an | |
2853 | output section description. The full syntax of the @code{OVERLAY} | |
2854 | command is as follows: | |
2855 | @smallexample | |
2856 | @group | |
2857 | OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )] | |
2858 | @{ | |
2859 | @var{secname1} | |
2860 | @{ | |
2861 | @var{output-section-command} | |
2862 | @var{output-section-command} | |
2863 | @dots{} | |
2864 | @} [:@var{phdr}@dots{}] [=@var{fill}] | |
2865 | @var{secname2} | |
2866 | @{ | |
2867 | @var{output-section-command} | |
2868 | @var{output-section-command} | |
2869 | @dots{} | |
2870 | @} [:@var{phdr}@dots{}] [=@var{fill}] | |
2871 | @dots{} | |
2872 | @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] | |
2873 | @end group | |
2874 | @end smallexample | |
2875 | ||
2876 | Everything is optional except @code{OVERLAY} (a keyword), and each | |
2877 | section must have a name (@var{secname1} and @var{secname2} above). The | |
2878 | section definitions within the @code{OVERLAY} construct are identical to | |
2879 | those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}), | |
2880 | except that no addresses and no memory regions may be defined for | |
2881 | sections within an @code{OVERLAY}. | |
2882 | ||
2883 | The sections are all defined with the same starting address. The load | |
2884 | addresses of the sections are arranged such that they are consecutive in | |
2885 | memory starting at the load address used for the @code{OVERLAY} as a | |
2886 | whole (as with normal section definitions, the load address is optional, | |
2887 | and defaults to the start address; the start address is also optional, | |
2888 | and defaults to the current value of the location counter). | |
2889 | ||
2890 | If the @code{NOCROSSREFS} keyword is used, and there any references | |
2891 | among the sections, the linker will report an error. Since the sections | |
2892 | all run at the same address, it normally does not make sense for one | |
2893 | section to refer directly to another. @xref{Miscellaneous Commands, | |
2894 | NOCROSSREFS}. | |
2895 | ||
2896 | For each section within the @code{OVERLAY}, the linker automatically | |
2897 | defines two symbols. The symbol @code{__load_start_@var{secname}} is | |
2898 | defined as the starting load address of the section. The symbol | |
2899 | @code{__load_stop_@var{secname}} is defined as the final load address of | |
2900 | the section. Any characters within @var{secname} which are not legal | |
2901 | within C identifiers are removed. C (or assembler) code may use these | |
2902 | symbols to move the overlaid sections around as necessary. | |
2903 | ||
2904 | At the end of the overlay, the value of the location counter is set to | |
2905 | the start address of the overlay plus the size of the largest section. | |
2906 | ||
2907 | Here is an example. Remember that this would appear inside a | |
2908 | @code{SECTIONS} construct. | |
2909 | @smallexample | |
2910 | @group | |
2911 | OVERLAY 0x1000 : AT (0x4000) | |
2912 | @{ | |
2913 | .text0 @{ o1/*.o(.text) @} | |
2914 | .text1 @{ o2/*.o(.text) @} | |
2915 | @} | |
2916 | @end group | |
2917 | @end smallexample | |
2918 | @noindent | |
2919 | This will define both @samp{.text0} and @samp{.text1} to start at | |
2920 | address 0x1000. @samp{.text0} will be loaded at address 0x4000, and | |
2921 | @samp{.text1} will be loaded immediately after @samp{.text0}. The | |
2922 | following symbols will be defined: @code{__load_start_text0}, | |
2923 | @code{__load_stop_text0}, @code{__load_start_text1}, | |
2924 | @code{__load_stop_text1}. | |
2925 | ||
2926 | C code to copy overlay @code{.text1} into the overlay area might look | |
2927 | like the following. | |
2928 | ||
2929 | @smallexample | |
2930 | @group | |
2931 | extern char __load_start_text1, __load_stop_text1; | |
2932 | memcpy ((char *) 0x1000, &__load_start_text1, | |
2933 | &__load_stop_text1 - &__load_start_text1); | |
2934 | @end group | |
2935 | @end smallexample | |
2936 | ||
2937 | Note that the @code{OVERLAY} command is just syntactic sugar, since | |
2938 | everything it does can be done using the more basic commands. The above | |
2939 | example could have been written identically as follows. | |
2940 | ||
2941 | @smallexample | |
2942 | @group | |
2943 | .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @} | |
2944 | __load_start_text0 = LOADADDR (.text0); | |
2945 | __load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0); | |
2946 | .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @} | |
2947 | __load_start_text1 = LOADADDR (.text1); | |
2948 | __load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1); | |
2949 | . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); | |
2950 | @end group | |
2951 | @end smallexample | |
2952 | ||
2953 | @node MEMORY | |
2954 | @section MEMORY command | |
2955 | @kindex MEMORY | |
2956 | @cindex memory regions | |
2957 | @cindex regions of memory | |
2958 | @cindex allocating memory | |
2959 | @cindex discontinuous memory | |
2960 | The linker's default configuration permits allocation of all available | |
2961 | memory. You can override this by using the @code{MEMORY} command. | |
2962 | ||
2963 | The @code{MEMORY} command describes the location and size of blocks of | |
2964 | memory in the target. You can use it to describe which memory regions | |
2965 | may be used by the linker, and which memory regions it must avoid. You | |
2966 | can then assign sections to particular memory regions. The linker will | |
2967 | set section addresses based on the memory regions, and will warn about | |
2968 | regions that become too full. The linker will not shuffle sections | |
2969 | around to fit into the available regions. | |
2970 | ||
2971 | A linker script may contain at most one use of the @code{MEMORY} | |
2972 | command. However, you can define as many blocks of memory within it as | |
2973 | you wish. The syntax is: | |
2974 | @smallexample | |
2975 | @group | |
2976 | MEMORY | |
2977 | @{ | |
2978 | @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len} | |
2979 | @dots{} | |
2980 | @} | |
2981 | @end group | |
2982 | @end smallexample | |
2983 | ||
2984 | The @var{name} is a name used in the linker script to refer to the | |
2985 | region. The region name has no meaning outside of the linker script. | |
2986 | Region names are stored in a separate name space, and will not conflict | |
2987 | with symbol names, file names, or section names. Each memory region | |
2988 | must have a distinct name. | |
2989 | ||
2990 | @cindex memory region attributes | |
2991 | The @var{attr} string is an optional list of attributes that specify | |
2992 | whether to use a particular memory region for an input section which is | |
2993 | not explicitly mapped in the linker script. As described in | |
2994 | @ref{SECTIONS}, if you do not specify an output section for some input | |
2995 | section, the linker will create an output section with the same name as | |
2996 | the input section. If you define region attributes, the linker will use | |
2997 | them to select the memory region for the output section that it creates. | |
2998 | ||
2999 | The @var{attr} string must consist only of the following characters: | |
3000 | @table @samp | |
3001 | @item R | |
3002 | Read-only section | |
3003 | @item W | |
3004 | Read/write section | |
3005 | @item X | |
3006 | Executable section | |
3007 | @item A | |
3008 | Allocatable section | |
3009 | @item I | |
3010 | Initialized section | |
3011 | @item L | |
3012 | Same as @samp{I} | |
3013 | @item ! | |
3014 | Invert the sense of any of the preceding attributes | |
3015 | @end table | |
3016 | ||
3017 | If a unmapped section matches any of the listed attributes other than | |
3018 | @samp{!}, it will be placed in the memory region. The @samp{!} | |
3019 | attribute reverses this test, so that an unmapped section will be placed | |
3020 | in the memory region only if it does not match any of the listed | |
3021 | attributes. | |
3022 | ||
3023 | @kindex ORIGIN = | |
3024 | @kindex o = | |
3025 | @kindex org = | |
3026 | The @var{origin} is an expression for the start address of the memory | |
3027 | region. The expression must evaluate to a constant before memory | |
3028 | allocation is performed, which means that you may not use any section | |
3029 | relative symbols. The keyword @code{ORIGIN} may be abbreviated to | |
3030 | @code{org} or @code{o} (but not, for example, @code{ORG}). | |
3031 | ||
3032 | @kindex LENGTH = | |
3033 | @kindex len = | |
3034 | @kindex l = | |
3035 | The @var{len} is an expression for the size in bytes of the memory | |
3036 | region. As with the @var{origin} expression, the expression must | |
3037 | evaluate to a constant before memory allocation is performed. The | |
3038 | keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}. | |
3039 | ||
3040 | In the following example, we specify that there are two memory regions | |
3041 | available for allocation: one starting at @samp{0} for 256 kilobytes, | |
3042 | and the other starting at @samp{0x40000000} for four megabytes. The | |
3043 | linker will place into the @samp{rom} memory region every section which | |
3044 | is not explicitly mapped into a memory region, and is either read-only | |
3045 | or executable. The linker will place other sections which are not | |
3046 | explicitly mapped into a memory region into the @samp{ram} memory | |
3047 | region. | |
3048 | ||
3049 | @smallexample | |
3050 | @group | |
3051 | MEMORY | |
3052 | @{ | |
3053 | rom (rx) : ORIGIN = 0, LENGTH = 256K | |
3054 | ram (!rx) : org = 0x40000000, l = 4M | |
3055 | @} | |
3056 | @end group | |
3057 | @end smallexample | |
3058 | ||
3059 | Once you define a memory region, you can direct the linker to place | |
3060 | specific output sections into that memory region by using the | |
3061 | @samp{>@var{region}} output section attribute. For example, if you have | |
3062 | a memory region named @samp{mem}, you would use @samp{>mem} in the | |
3063 | output section definition. @xref{Output Section Region}. If no address | |
3064 | was specified for the output section, the linker will set the address to | |
3065 | the next available address within the memory region. If the combined | |
3066 | output sections directed to a memory region are too large for the | |
3067 | region, the linker will issue an error message. | |
3068 | ||
3069 | @node PHDRS | |
3070 | @section PHDRS Command | |
3071 | @kindex PHDRS | |
3072 | @cindex program headers | |
3073 | @cindex ELF program headers | |
3074 | @cindex program segments | |
3075 | @cindex segments, ELF | |
3076 | The ELF object file format uses @dfn{program headers}, also knows as | |
3077 | @dfn{segments}. The program headers describe how the program should be | |
3078 | loaded into memory. You can print them out by using the @code{objdump} | |
3079 | program with the @samp{-p} option. | |
3080 | ||
3081 | When you run an ELF program on a native ELF system, the system loader | |
3082 | reads the program headers in order to figure out how to load the | |
3083 | program. This will only work if the program headers are set correctly. | |
3084 | This manual does not describe the details of how the system loader | |
3085 | interprets program headers; for more information, see the ELF ABI. | |
3086 | ||
3087 | The linker will create reasonable program headers by default. However, | |
3088 | in some cases, you may need to specify the program headers more | |
3089 | precisely. You may use the @code{PHDRS} command for this purpose. When | |
3090 | the linker sees the @code{PHDRS} command in the linker script, it will | |
3091 | not create any program headers other than the ones specified. | |
3092 | ||
3093 | The linker only pays attention to the @code{PHDRS} command when | |
3094 | generating an ELF output file. In other cases, the linker will simply | |
3095 | ignore @code{PHDRS}. | |
3096 | ||
3097 | This is the syntax of the @code{PHDRS} command. The words @code{PHDRS}, | |
3098 | @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords. | |
3099 | ||
3100 | @smallexample | |
3101 | @group | |
3102 | PHDRS | |
3103 | @{ | |
3104 | @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ] | |
3105 | [ FLAGS ( @var{flags} ) ] ; | |
3106 | @} | |
3107 | @end group | |
3108 | @end smallexample | |
3109 | ||
3110 | The @var{name} is used only for reference in the @code{SECTIONS} command | |
3111 | of the linker script. It is not put into the output file. Program | |
3112 | header names are stored in a separate name space, and will not conflict | |
3113 | with symbol names, file names, or section names. Each program header | |
3114 | must have a distinct name. | |
3115 | ||
3116 | Certain program header types describe segments of memory which the | |
3117 | system loader will load from the file. In the linker script, you | |
3118 | specify the contents of these segments by placing allocatable output | |
3119 | sections in the segments. You use the @samp{:@var{phdr}} output section | |
3120 | attribute to place a section in a particular segment. @xref{Output | |
3121 | Section Phdr}. | |
3122 | ||
3123 | It is normal to put certain sections in more than one segment. This | |
3124 | merely implies that one segment of memory contains another. You may | |
3125 | repeat @samp{:@var{phdr}}, using it once for each segment which should | |
3126 | contain the section. | |
3127 | ||
3128 | If you place a section in one or more segments using @samp{:@var{phdr}}, | |
3129 | then the linker will place all subsequent allocatable sections which do | |
3130 | not specify @samp{:@var{phdr}} in the same segments. This is for | |
3131 | convenience, since generally a whole set of contiguous sections will be | |
3132 | placed in a single segment. You can use @code{:NONE} to override the | |
3133 | default segment and tell the linker to not put the section in any | |
3134 | segment at all. | |
3135 | ||
3136 | @kindex FILEHDR | |
3137 | @kindex PHDRS | |
3138 | You may use the @code{FILEHDR} and @code{PHDRS} keywords appear after | |
3139 | the program header type to further describe the contents of the segment. | |
3140 | The @code{FILEHDR} keyword means that the segment should include the ELF | |
3141 | file header. The @code{PHDRS} keyword means that the segment should | |
3142 | include the ELF program headers themselves. | |
3143 | ||
3144 | The @var{type} may be one of the following. The numbers indicate the | |
3145 | value of the keyword. | |
3146 | ||
3147 | @table @asis | |
3148 | @item @code{PT_NULL} (0) | |
3149 | Indicates an unused program header. | |
3150 | ||
3151 | @item @code{PT_LOAD} (1) | |
3152 | Indicates that this program header describes a segment to be loaded from | |
3153 | the file. | |
3154 | ||
3155 | @item @code{PT_DYNAMIC} (2) | |
3156 | Indicates a segment where dynamic linking information can be found. | |
3157 | ||
3158 | @item @code{PT_INTERP} (3) | |
3159 | Indicates a segment where the name of the program interpreter may be | |
3160 | found. | |
3161 | ||
3162 | @item @code{PT_NOTE} (4) | |
3163 | Indicates a segment holding note information. | |
3164 | ||
3165 | @item @code{PT_SHLIB} (5) | |
3166 | A reserved program header type, defined but not specified by the ELF | |
3167 | ABI. | |
3168 | ||
3169 | @item @code{PT_PHDR} (6) | |
3170 | Indicates a segment where the program headers may be found. | |
3171 | ||
3172 | @item @var{expression} | |
3173 | An expression giving the numeric type of the program header. This may | |
3174 | be used for types not defined above. | |
3175 | @end table | |
3176 | ||
3177 | You can specify that a segment should be loaded at a particular address | |
3178 | in memory by using an @code{AT} expression. This is identical to the | |
3179 | @code{AT} command used as an output section attribute (@pxref{Output | |
3180 | Section LMA}). The @code{AT} command for a program header overrides the | |
3181 | output section attribute. | |
3182 | ||
3183 | The linker will normally set the segment flags based on the sections | |
3184 | which comprise the segment. You may use the @code{FLAGS} keyword to | |
3185 | explicitly specify the segment flags. The value of @var{flags} must be | |
3186 | an integer. It is used to set the @code{p_flags} field of the program | |
3187 | header. | |
3188 | ||
3189 | Here is an example of @code{PHDRS}. This shows a typical set of program | |
3190 | headers used on a native ELF system. | |
3191 | ||
3192 | @example | |
3193 | @group | |
3194 | PHDRS | |
3195 | @{ | |
3196 | headers PT_PHDR PHDRS ; | |
3197 | interp PT_INTERP ; | |
3198 | text PT_LOAD FILEHDR PHDRS ; | |
3199 | data PT_LOAD ; | |
3200 | dynamic PT_DYNAMIC ; | |
3201 | @} | |
3202 | ||
3203 | SECTIONS | |
3204 | @{ | |
3205 | . = SIZEOF_HEADERS; | |
3206 | .interp : @{ *(.interp) @} :text :interp | |
3207 | .text : @{ *(.text) @} :text | |
3208 | .rodata : @{ *(.rodata) @} /* defaults to :text */ | |
3209 | @dots{} | |
3210 | . = . + 0x1000; /* move to a new page in memory */ | |
3211 | .data : @{ *(.data) @} :data | |
3212 | .dynamic : @{ *(.dynamic) @} :data :dynamic | |
3213 | @dots{} | |
3214 | @} | |
3215 | @end group | |
3216 | @end example | |
3217 | ||
3218 | @node VERSION | |
3219 | @section VERSION Command | |
3220 | @kindex VERSION @{script text@} | |
3221 | @cindex symbol versions | |
3222 | @cindex version script | |
3223 | @cindex versions of symbols | |
3224 | The linker supports symbol versions when using ELF. Symbol versions are | |
3225 | only useful when using shared libraries. The dynamic linker can use | |
3226 | symbol versions to select a specific version of a function when it runs | |
3227 | a program that may have been linked against an earlier version of the | |
3228 | shared library. | |
3229 | ||
3230 | You can include a version script directly in the main linker script, or | |
3231 | you can supply the version script as an implicit linker script. You can | |
3232 | also use the @samp{--version-script} linker option. | |
3233 | ||
3234 | The syntax of the @code{VERSION} command is simply | |
3235 | @smallexample | |
3236 | VERSION @{ version-script-commands @} | |
3237 | @end smallexample | |
3238 | ||
3239 | The format of the version script commands is identical to that used by | |
3240 | Sun's linker in Solaris 2.5. The version script defines a tree of | |
3241 | version nodes. You specify the node names and interdependencies in the | |
3242 | version script. You can specify which symbols are bound to which | |
3243 | version nodes, and you can reduce a specified set of symbols to local | |
3244 | scope so that they are not globally visible outside of the shared | |
3245 | library. | |
3246 | ||
3247 | The easiest way to demonstrate the version script language is with a few | |
3248 | examples. | |
3249 | ||
3250 | @smallexample | |
3251 | VERS_1.1 @{ | |
3252 | global: | |
3253 | foo1; | |
3254 | local: | |
3255 | old*; | |
3256 | original*; | |
3257 | new*; | |
3258 | @}; | |
3259 | ||
3260 | VERS_1.2 @{ | |
3261 | foo2; | |
3262 | @} VERS_1.1; | |
3263 | ||
3264 | VERS_2.0 @{ | |
3265 | bar1; bar2; | |
3266 | @} VERS_1.2; | |
3267 | @end smallexample | |
3268 | ||
3269 | This example version script defines three version nodes. The first | |
3270 | version node defined is @samp{VERS_1.1}; it has no other dependencies. | |
3271 | The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces | |
3272 | a number of symbols to local scope so that they are not visible outside | |
3273 | of the shared library. | |
3274 | ||
3275 | Next, the version script defines node @samp{VERS_1.2}. This node | |
3276 | depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2} | |
3277 | to the version node @samp{VERS_1.2}. | |
3278 | ||
3279 | Finally, the version script defines node @samp{VERS_2.0}. This node | |
3280 | depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1} | |
3281 | and @samp{bar2} are bound to the version node @samp{VERS_2.0}. | |
3282 | ||
3283 | When the linker finds a symbol defined in a library which is not | |
3284 | specifically bound to a version node, it will effectively bind it to an | |
3285 | unspecified base version of the library. You can bind all otherwise | |
3286 | unspecified symbols to a given version node by using @samp{global: *} | |
3287 | somewhere in the version script. | |
3288 | ||
3289 | The names of the version nodes have no specific meaning other than what | |
3290 | they might suggest to the person reading them. The @samp{2.0} version | |
3291 | could just as well have appeared in between @samp{1.1} and @samp{1.2}. | |
3292 | However, this would be a confusing way to write a version script. | |
3293 | ||
3294 | When you link an application against a shared library that has versioned | |
3295 | symbols, the application itself knows which version of each symbol it | |
3296 | requires, and it also knows which version nodes it needs from each | |
3297 | shared library it is linked against. Thus at runtime, the dynamic | |
3298 | loader can make a quick check to make sure that the libraries you have | |
3299 | linked against do in fact supply all of the version nodes that the | |
3300 | application will need to resolve all of the dynamic symbols. In this | |
3301 | way it is possible for the dynamic linker to know with certainty that | |
3302 | all external symbols that it needs will be resolvable without having to | |
3303 | search for each symbol reference. | |
3304 | ||
3305 | The symbol versioning is in effect a much more sophisticated way of | |
3306 | doing minor version checking that SunOS does. The fundamental problem | |
3307 | that is being addressed here is that typically references to external | |
3308 | functions are bound on an as-needed basis, and are not all bound when | |
3309 | the application starts up. If a shared library is out of date, a | |
3310 | required interface may be missing; when the application tries to use | |
3311 | that interface, it may suddenly and unexpectedly fail. With symbol | |
3312 | versioning, the user will get a warning when they start their program if | |
3313 | the libraries being used with the application are too old. | |
3314 | ||
3315 | There are several GNU extensions to Sun's versioning approach. The | |
3316 | first of these is the ability to bind a symbol to a version node in the | |
3317 | source file where the symbol is defined instead of in the versioning | |
3318 | script. This was done mainly to reduce the burden on the library | |
3319 | maintainer. You can do this by putting something like: | |
3320 | @smallexample | |
3321 | __asm__(".symver original_foo,foo@@VERS_1.1"); | |
3322 | @end smallexample | |
3323 | @noindent | |
3324 | in the C source file. This renames the function @samp{original_foo} to | |
3325 | be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}. | |
3326 | The @samp{local:} directive can be used to prevent the symbol | |
3327 | @samp{original_foo} from being exported. | |
3328 | ||
3329 | The second GNU extension is to allow multiple versions of the same | |
3330 | function to appear in a given shared library. In this way you can make | |
3331 | an incompatible change to an interface without increasing the major | |
3332 | version number of the shared library, while still allowing applications | |
3333 | linked against the old interface to continue to function. | |
3334 | ||
3335 | To do this, you must use multiple @samp{.symver} directives in the | |
3336 | source file. Here is an example: | |
3337 | ||
3338 | @smallexample | |
3339 | __asm__(".symver original_foo,foo@@"); | |
3340 | __asm__(".symver old_foo,foo@@VERS_1.1"); | |
3341 | __asm__(".symver old_foo1,foo@@VERS_1.2"); | |
3342 | __asm__(".symver new_foo,foo@@@@VERS_2.0"); | |
3343 | @end smallexample | |
3344 | ||
3345 | In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the | |
3346 | unspecified base version of the symbol. The source file that contains this | |
3347 | example would define 4 C functions: @samp{original_foo}, @samp{old_foo}, | |
3348 | @samp{old_foo1}, and @samp{new_foo}. | |
3349 | ||
3350 | When you have multiple definitions of a given symbol, there needs to be | |
3351 | some way to specify a default version to which external references to | |
3352 | this symbol will be bound. You can do this with the | |
3353 | @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only | |
3354 | declare one version of a symbol as the default in this manner; otherwise | |
3355 | you would effectively have multiple definitions of the same symbol. | |
3356 | ||
3357 | If you wish to bind a reference to a specific version of the symbol | |
3358 | within the shared library, you can use the aliases of convenience | |
3359 | (i.e. @samp{old_foo}), or you can use the @samp{.symver} directive to | |
3360 | specifically bind to an external version of the function in question. | |
3361 | ||
3362 | @node Expressions | |
3363 | @section Expressions in Linker Scripts | |
3364 | @cindex expressions | |
3365 | @cindex arithmetic | |
3366 | The syntax for expressions in the linker script language is identical to | |
3367 | that of C expressions. All expressions are evaluated as integers. All | |
3368 | expressions are evaluated in the same size, which is 32 bits if both the | |
3369 | host and target are 32 bits, and is otherwise 64 bits. | |
3370 | ||
3371 | You can use and set symbol values in expressions. | |
3372 | ||
3373 | The linker defines several special purpose builtin functions for use in | |
3374 | expressions. | |
3375 | ||
3376 | @menu | |
3377 | * Constants:: Constants | |
3378 | * Symbols:: Symbol Names | |
3379 | * Location Counter:: The Location Counter | |
3380 | * Operators:: Operators | |
3381 | * Evaluation:: Evaluation | |
3382 | * Expression Section:: The Section of an Expression | |
3383 | * Builtin Functions:: Builtin Functions | |
3384 | @end menu | |
3385 | ||
3386 | @node Constants | |
3387 | @subsection Constants | |
3388 | @cindex integer notation | |
3389 | @cindex constants in linker scripts | |
3390 | All constants are integers. | |
3391 | ||
3392 | As in C, the linker considers an integer beginning with @samp{0} to be | |
3393 | octal, and an integer beginning with @samp{0x} or @samp{0X} to be | |
3394 | hexadecimal. The linker considers other integers to be decimal. | |
3395 | ||
3396 | @cindex scaled integers | |
3397 | @cindex K and M integer suffixes | |
3398 | @cindex M and K integer suffixes | |
3399 | @cindex suffixes for integers | |
3400 | @cindex integer suffixes | |
3401 | In addition, you can use the suffixes @code{K} and @code{M} to scale a | |
3402 | constant by | |
3403 | @c TEXI2ROFF-KILL | |
3404 | @ifinfo | |
3405 | @c END TEXI2ROFF-KILL | |
3406 | @code{1024} or @code{1024*1024} | |
3407 | @c TEXI2ROFF-KILL | |
3408 | @end ifinfo | |
3409 | @tex | |
3410 | ${\rm 1024}$ or ${\rm 1024}^2$ | |
3411 | @end tex | |
3412 | @c END TEXI2ROFF-KILL | |
3413 | respectively. For example, the following all refer to the same quantity: | |
3414 | @smallexample | |
3415 | _fourk_1 = 4K; | |
3416 | _fourk_2 = 4096; | |
3417 | _fourk_3 = 0x1000; | |
3418 | @end smallexample | |
3419 | ||
3420 | @node Symbols | |
3421 | @subsection Symbol Names | |
3422 | @cindex symbol names | |
3423 | @cindex names | |
3424 | @cindex quoted symbol names | |
3425 | @kindex " | |
3426 | Unless quoted, symbol names start with a letter, underscore, or period | |
3427 | and may include letters, digits, underscores, periods, and hyphens. | |
3428 | Unquoted symbol names must not conflict with any keywords. You can | |
3429 | specify a symbol which contains odd characters or has the same name as a | |
3430 | keyword by surrounding the symbol name in double quotes: | |
3431 | @smallexample | |
3432 | "SECTION" = 9; | |
3433 | "with a space" = "also with a space" + 10; | |
3434 | @end smallexample | |
3435 | ||
3436 | Since symbols can contain many non-alphabetic characters, it is safest | |
3437 | to delimit symbols with spaces. For example, @samp{A-B} is one symbol, | |
3438 | whereas @samp{A - B} is an expression involving subtraction. | |
3439 | ||
3440 | @node Location Counter | |
3441 | @subsection The Location Counter | |
3442 | @kindex . | |
3443 | @cindex dot | |
3444 | @cindex location counter | |
3445 | @cindex current output location | |
3446 | The special linker variable @dfn{dot} @samp{.} always contains the | |
3447 | current output location counter. Since the @code{.} always refers to a | |
3448 | location in an output section, it may only appear in an expression | |
3449 | within a @code{SECTIONS} command. The @code{.} symbol may appear | |
3450 | anywhere that an ordinary symbol is allowed in an expression. | |
3451 | ||
3452 | @cindex holes | |
3453 | Assigning a value to @code{.} will cause the location counter to be | |
3454 | moved. This may be used to create holes in the output section. The | |
3455 | location counter may never be moved backwards. | |
3456 | ||
3457 | @smallexample | |
3458 | SECTIONS | |
3459 | @{ | |
3460 | output : | |
3461 | @{ | |
3462 | file1(.text) | |
3463 | . = . + 1000; | |
3464 | file2(.text) | |
3465 | . += 1000; | |
3466 | file3(.text) | |
3467 | @} = 0x1234; | |
3468 | @} | |
3469 | @end smallexample | |
3470 | @noindent | |
3471 | In the previous example, the @samp{.text} section from @file{file1} is | |
3472 | located at the beginning of the output section @samp{output}. It is | |
3473 | followed by a 1000 byte gap. Then the @samp{.text} section from | |
3474 | @file{file2} appears, also with a 1000 byte gap following before the | |
3475 | @samp{.text} section from @file{file3}. The notation @samp{= 0x1234} | |
3476 | specifies what data to write in the gaps (@pxref{Output Section Fill}). | |
3477 | ||
5c6bbab8 NC |
3478 | @cindex dot inside sections |
3479 | Note: @code{.} actually refers to the byte offset from the start of the | |
3480 | current containing object. Normally this is the @code{SECTIONS} | |
3481 | statement, whoes start address is 0, hence @code{.} can be used as an | |
3482 | absolute address. If @code{.} is used inside a section description | |
3483 | however, it refers to the byte offset from the start of that section, | |
3484 | not an absolute address. Thus in a script like this: | |
3485 | ||
3486 | @smallexample | |
3487 | SECTIONS | |
3488 | @{ | |
3489 | . = 0x100 | |
3490 | .text: @{ | |
3491 | *(.text) | |
3492 | . = 0x200 | |
3493 | @} | |
3494 | . = 0x500 | |
3495 | .data: @{ | |
3496 | *(.data) | |
3497 | . += 0x600 | |
3498 | @} | |
3499 | @} | |
3500 | @end smallexample | |
3501 | ||
3502 | The @samp{.text} section will be assigned a starting address of 0x100 | |
3503 | and a size of exactly 0x200 bytes, even if there is not enough data in | |
3504 | the @samp{.text} input sections to fill this area. (If there is too | |
3505 | much data, an error will be produced because this would be an attempt to | |
3506 | move @code{.} backwards). The @samp{.data} section will start at 0x500 | |
3507 | and it will have an extra 0x600 bytes worth of space after the end of | |
3508 | the values from the @samp{.data} input sections and before the end of | |
3509 | the @samp{.data} output section itself. | |
3510 | ||
252b5132 RH |
3511 | @need 2000 |
3512 | @node Operators | |
3513 | @subsection Operators | |
3514 | @cindex operators for arithmetic | |
3515 | @cindex arithmetic operators | |
3516 | @cindex precedence in expressions | |
3517 | The linker recognizes the standard C set of arithmetic operators, with | |
3518 | the standard bindings and precedence levels: | |
3519 | @c TEXI2ROFF-KILL | |
3520 | @ifinfo | |
3521 | @c END TEXI2ROFF-KILL | |
3522 | @smallexample | |
3523 | precedence associativity Operators Notes | |
3524 | (highest) | |
3525 | 1 left ! - ~ (1) | |
3526 | 2 left * / % | |
3527 | 3 left + - | |
3528 | 4 left >> << | |
3529 | 5 left == != > < <= >= | |
3530 | 6 left & | |
3531 | 7 left | | |
3532 | 8 left && | |
3533 | 9 left || | |
3534 | 10 right ? : | |
3535 | 11 right &= += -= *= /= (2) | |
3536 | (lowest) | |
3537 | @end smallexample | |
3538 | Notes: | |
3539 | (1) Prefix operators | |
3540 | (2) @xref{Assignments}. | |
3541 | @c TEXI2ROFF-KILL | |
3542 | @end ifinfo | |
3543 | @tex | |
3544 | \vskip \baselineskip | |
3545 | %"lispnarrowing" is the extra indent used generally for smallexample | |
3546 | \hskip\lispnarrowing\vbox{\offinterlineskip | |
3547 | \hrule | |
3548 | \halign | |
3549 | {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr | |
3550 | height2pt&\omit&&\omit&&\omit&\cr | |
3551 | &Precedence&& Associativity &&{\rm Operators}&\cr | |
3552 | height2pt&\omit&&\omit&&\omit&\cr | |
3553 | \noalign{\hrule} | |
3554 | height2pt&\omit&&\omit&&\omit&\cr | |
3555 | &highest&&&&&\cr | |
3556 | % '176 is tilde, '~' in tt font | |
3557 | &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr | |
3558 | &2&&left&&* / \%&\cr | |
3559 | &3&&left&&+ -&\cr | |
3560 | &4&&left&&>> <<&\cr | |
3561 | &5&&left&&== != > < <= >=&\cr | |
3562 | &6&&left&&\&&\cr | |
3563 | &7&&left&&|&\cr | |
3564 | &8&&left&&{\&\&}&\cr | |
3565 | &9&&left&&||&\cr | |
3566 | &10&&right&&? :&\cr | |
3567 | &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr | |
3568 | &lowest&&&&&\cr | |
3569 | height2pt&\omit&&\omit&&\omit&\cr} | |
3570 | \hrule} | |
3571 | @end tex | |
3572 | @iftex | |
3573 | { | |
3574 | @obeylines@parskip=0pt@parindent=0pt | |
3575 | @dag@quad Prefix operators. | |
3576 | @ddag@quad @xref{Assignments}. | |
3577 | } | |
3578 | @end iftex | |
3579 | @c END TEXI2ROFF-KILL | |
3580 | ||
3581 | @node Evaluation | |
3582 | @subsection Evaluation | |
3583 | @cindex lazy evaluation | |
3584 | @cindex expression evaluation order | |
3585 | The linker evaluates expressions lazily. It only computes the value of | |
3586 | an expression when absolutely necessary. | |
3587 | ||
3588 | The linker needs some information, such as the value of the start | |
3589 | address of the first section, and the origins and lengths of memory | |
3590 | regions, in order to do any linking at all. These values are computed | |
3591 | as soon as possible when the linker reads in the linker script. | |
3592 | ||
3593 | However, other values (such as symbol values) are not known or needed | |
3594 | until after storage allocation. Such values are evaluated later, when | |
3595 | other information (such as the sizes of output sections) is available | |
3596 | for use in the symbol assignment expression. | |
3597 | ||
3598 | The sizes of sections cannot be known until after allocation, so | |
3599 | assignments dependent upon these are not performed until after | |
3600 | allocation. | |
3601 | ||
3602 | Some expressions, such as those depending upon the location counter | |
3603 | @samp{.}, must be evaluated during section allocation. | |
3604 | ||
3605 | If the result of an expression is required, but the value is not | |
3606 | available, then an error results. For example, a script like the | |
3607 | following | |
3608 | @smallexample | |
3609 | @group | |
3610 | SECTIONS | |
3611 | @{ | |
3612 | .text 9+this_isnt_constant : | |
3613 | @{ *(.text) @} | |
3614 | @} | |
3615 | @end group | |
3616 | @end smallexample | |
3617 | @noindent | |
3618 | will cause the error message @samp{non constant expression for initial | |
3619 | address}. | |
3620 | ||
3621 | @node Expression Section | |
3622 | @subsection The Section of an Expression | |
3623 | @cindex expression sections | |
3624 | @cindex absolute expressions | |
3625 | @cindex relative expressions | |
3626 | @cindex absolute and relocatable symbols | |
3627 | @cindex relocatable and absolute symbols | |
3628 | @cindex symbols, relocatable and absolute | |
3629 | When the linker evaluates an expression, the result is either absolute | |
3630 | or relative to some section. A relative expression is expressed as a | |
3631 | fixed offset from the base of a section. | |
3632 | ||
3633 | The position of the expression within the linker script determines | |
3634 | whether it is absolute or relative. An expression which appears within | |
3635 | an output section definition is relative to the base of the output | |
3636 | section. An expression which appears elsewhere will be absolute. | |
3637 | ||
3638 | A symbol set to a relative expression will be relocatable if you request | |
3639 | relocatable output using the @samp{-r} option. That means that a | |
3640 | further link operation may change the value of the symbol. The symbol's | |
3641 | section will be the section of the relative expression. | |
3642 | ||
3643 | A symbol set to an absolute expression will retain the same value | |
3644 | through any further link operation. The symbol will be absolute, and | |
3645 | will not have any particular associated section. | |
3646 | ||
3647 | You can use the builtin function @code{ABSOLUTE} to force an expression | |
3648 | to be absolute when it would otherwise be relative. For example, to | |
3649 | create an absolute symbol set to the address of the end of the output | |
3650 | section @samp{.data}: | |
3651 | @smallexample | |
3652 | SECTIONS | |
3653 | @{ | |
3654 | .data : @{ *(.data) _edata = ABSOLUTE(.); @} | |
3655 | @} | |
3656 | @end smallexample | |
3657 | @noindent | |
3658 | If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the | |
3659 | @samp{.data} section. | |
3660 | ||
3661 | @node Builtin Functions | |
3662 | @subsection Builtin Functions | |
3663 | @cindex functions in expressions | |
3664 | The linker script language includes a number of builtin functions for | |
3665 | use in linker script expressions. | |
3666 | ||
3667 | @table @code | |
3668 | @item ABSOLUTE(@var{exp}) | |
3669 | @kindex ABSOLUTE(@var{exp}) | |
3670 | @cindex expression, absolute | |
3671 | Return the absolute (non-relocatable, as opposed to non-negative) value | |
3672 | of the expression @var{exp}. Primarily useful to assign an absolute | |
3673 | value to a symbol within a section definition, where symbol values are | |
3674 | normally section relative. @xref{Expression Section}. | |
3675 | ||
3676 | @item ADDR(@var{section}) | |
3677 | @kindex ADDR(@var{section}) | |
3678 | @cindex section address in expression | |
3679 | Return the absolute address (the VMA) of the named @var{section}. Your | |
3680 | script must previously have defined the location of that section. In | |
3681 | the following example, @code{symbol_1} and @code{symbol_2} are assigned | |
3682 | identical values: | |
3683 | @smallexample | |
3684 | @group | |
3685 | SECTIONS @{ @dots{} | |
3686 | .output1 : | |
3687 | @{ | |
3688 | start_of_output_1 = ABSOLUTE(.); | |
3689 | @dots{} | |
3690 | @} | |
3691 | .output : | |
3692 | @{ | |
3693 | symbol_1 = ADDR(.output1); | |
3694 | symbol_2 = start_of_output_1; | |
3695 | @} | |
3696 | @dots{} @} | |
3697 | @end group | |
3698 | @end smallexample | |
3699 | ||
3700 | @item ALIGN(@var{exp}) | |
3701 | @kindex ALIGN(@var{exp}) | |
3702 | @cindex round up location counter | |
3703 | @cindex align location counter | |
3704 | Return the location counter (@code{.}) aligned to the next @var{exp} | |
3705 | boundary. @var{exp} must be an expression whose value is a power of | |
3706 | two. This is equivalent to | |
3707 | @smallexample | |
3708 | (. + @var{exp} - 1) & ~(@var{exp} - 1) | |
3709 | @end smallexample | |
3710 | ||
3711 | @code{ALIGN} doesn't change the value of the location counter---it just | |
3712 | does arithmetic on it. Here is an example which aligns the output | |
3713 | @code{.data} section to the next @code{0x2000} byte boundary after the | |
3714 | preceding section and sets a variable within the section to the next | |
3715 | @code{0x8000} boundary after the input sections: | |
3716 | @smallexample | |
3717 | @group | |
3718 | SECTIONS @{ @dots{} | |
3719 | .data ALIGN(0x2000): @{ | |
3720 | *(.data) | |
3721 | variable = ALIGN(0x8000); | |
3722 | @} | |
3723 | @dots{} @} | |
3724 | @end group | |
3725 | @end smallexample | |
3726 | @noindent | |
3727 | The first use of @code{ALIGN} in this example specifies the location of | |
3728 | a section because it is used as the optional @var{address} attribute of | |
3729 | a section definition (@pxref{Output Section Address}). The second use | |
3730 | of @code{ALIGN} is used to defines the value of a symbol. | |
3731 | ||
3732 | The builtin function @code{NEXT} is closely related to @code{ALIGN}. | |
3733 | ||
3734 | @item BLOCK(@var{exp}) | |
3735 | @kindex BLOCK(@var{exp}) | |
3736 | This is a synonym for @code{ALIGN}, for compatibility with older linker | |
3737 | scripts. It is most often seen when setting the address of an output | |
3738 | section. | |
3739 | ||
3740 | @item DEFINED(@var{symbol}) | |
3741 | @kindex DEFINED(@var{symbol}) | |
3742 | @cindex symbol defaults | |
3743 | Return 1 if @var{symbol} is in the linker global symbol table and is | |
3744 | defined, otherwise return 0. You can use this function to provide | |
3745 | default values for symbols. For example, the following script fragment | |
3746 | shows how to set a global symbol @samp{begin} to the first location in | |
3747 | the @samp{.text} section---but if a symbol called @samp{begin} already | |
3748 | existed, its value is preserved: | |
3749 | ||
3750 | @smallexample | |
3751 | @group | |
3752 | SECTIONS @{ @dots{} | |
3753 | .text : @{ | |
3754 | begin = DEFINED(begin) ? begin : . ; | |
3755 | @dots{} | |
3756 | @} | |
3757 | @dots{} | |
3758 | @} | |
3759 | @end group | |
3760 | @end smallexample | |
3761 | ||
3762 | @item LOADADDR(@var{section}) | |
3763 | @kindex LOADADDR(@var{section}) | |
3764 | @cindex section load address in expression | |
3765 | Return the absolute LMA of the named @var{section}. This is normally | |
3766 | the same as @code{ADDR}, but it may be different if the @code{AT} | |
3767 | attribute is used in the output section definition (@pxref{Output | |
3768 | Section LMA}). | |
3769 | ||
3770 | @kindex MAX | |
3771 | @item MAX(@var{exp1}, @var{exp2}) | |
3772 | Returns the maximum of @var{exp1} and @var{exp2}. | |
3773 | ||
3774 | @kindex MIN | |
3775 | @item MIN(@var{exp1}, @var{exp2}) | |
3776 | Returns the minimum of @var{exp1} and @var{exp2}. | |
3777 | ||
3778 | @item NEXT(@var{exp}) | |
3779 | @kindex NEXT(@var{exp}) | |
3780 | @cindex unallocated address, next | |
3781 | Return the next unallocated address that is a multiple of @var{exp}. | |
3782 | This function is closely related to @code{ALIGN(@var{exp})}; unless you | |
3783 | use the @code{MEMORY} command to define discontinuous memory for the | |
3784 | output file, the two functions are equivalent. | |
3785 | ||
3786 | @item SIZEOF(@var{section}) | |
3787 | @kindex SIZEOF(@var{section}) | |
3788 | @cindex section size | |
3789 | Return the size in bytes of the named @var{section}, if that section has | |
3790 | been allocated. If the section has not been allocated when this is | |
3791 | evaluated, the linker will report an error. In the following example, | |
3792 | @code{symbol_1} and @code{symbol_2} are assigned identical values: | |
3793 | @smallexample | |
3794 | @group | |
3795 | SECTIONS@{ @dots{} | |
3796 | .output @{ | |
3797 | .start = . ; | |
3798 | @dots{} | |
3799 | .end = . ; | |
3800 | @} | |
3801 | symbol_1 = .end - .start ; | |
3802 | symbol_2 = SIZEOF(.output); | |
3803 | @dots{} @} | |
3804 | @end group | |
3805 | @end smallexample | |
3806 | ||
3807 | @item SIZEOF_HEADERS | |
3808 | @itemx sizeof_headers | |
3809 | @kindex SIZEOF_HEADERS | |
3810 | @cindex header size | |
3811 | Return the size in bytes of the output file's headers. This is | |
3812 | information which appears at the start of the output file. You can use | |
3813 | this number when setting the start address of the first section, if you | |
3814 | choose, to facilitate paging. | |
3815 | ||
3816 | @cindex not enough room for program headers | |
3817 | @cindex program headers, not enough room | |
3818 | When producing an ELF output file, if the linker script uses the | |
3819 | @code{SIZEOF_HEADERS} builtin function, the linker must compute the | |
3820 | number of program headers before it has determined all the section | |
3821 | addresses and sizes. If the linker later discovers that it needs | |
3822 | additional program headers, it will report an error @samp{not enough | |
3823 | room for program headers}. To avoid this error, you must avoid using | |
3824 | the @code{SIZEOF_HEADERS} function, or you must rework your linker | |
3825 | script to avoid forcing the linker to use additional program headers, or | |
3826 | you must define the program headers yourself using the @code{PHDRS} | |
3827 | command (@pxref{PHDRS}). | |
3828 | @end table | |
3829 | ||
3830 | @node Implicit Linker Scripts | |
3831 | @section Implicit Linker Scripts | |
3832 | @cindex implicit linker scripts | |
3833 | If you specify a linker input file which the linker can not recognize as | |
3834 | an object file or an archive file, it will try to read the file as a | |
3835 | linker script. If the file can not be parsed as a linker script, the | |
3836 | linker will report an error. | |
3837 | ||
3838 | An implicit linker script will not replace the default linker script. | |
3839 | ||
3840 | Typically an implicit linker script would contain only symbol | |
3841 | assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION} | |
3842 | commands. | |
3843 | ||
3844 | Any input files read because of an implicit linker script will be read | |
3845 | at the position in the command line where the implicit linker script was | |
3846 | read. This can affect archive searching. | |
3847 | ||
3848 | @ifset GENERIC | |
3849 | @node Machine Dependent | |
3850 | @chapter Machine Dependent Features | |
3851 | ||
3852 | @cindex machine dependencies | |
3853 | @code{ld} has additional features on some platforms; the following | |
3854 | sections describe them. Machines where @code{ld} has no additional | |
3855 | functionality are not listed. | |
3856 | ||
3857 | @menu | |
3858 | * H8/300:: @code{ld} and the H8/300 | |
3859 | * i960:: @code{ld} and the Intel 960 family | |
3860 | * ARM:: @code{ld} and the ARM family | |
74459f0e TW |
3861 | @ifset TICOFF |
3862 | * TI COFF:: @code{ld} and TI COFF | |
3863 | @end ifset | |
252b5132 RH |
3864 | @end menu |
3865 | @end ifset | |
3866 | ||
3867 | @c FIXME! This could use @raisesections/@lowersections, but there seems to be a conflict | |
3868 | @c between those and node-defaulting. | |
3869 | @ifset H8300 | |
3870 | @ifclear GENERIC | |
3871 | @raisesections | |
3872 | @end ifclear | |
3873 | ||
3874 | @node H8/300 | |
3875 | @section @code{ld} and the H8/300 | |
3876 | ||
3877 | @cindex H8/300 support | |
3878 | For the H8/300, @code{ld} can perform these global optimizations when | |
3879 | you specify the @samp{--relax} command-line option. | |
3880 | ||
3881 | @table @emph | |
3882 | @cindex relaxing on H8/300 | |
3883 | @item relaxing address modes | |
3884 | @code{ld} finds all @code{jsr} and @code{jmp} instructions whose | |
3885 | targets are within eight bits, and turns them into eight-bit | |
3886 | program-counter relative @code{bsr} and @code{bra} instructions, | |
3887 | respectively. | |
3888 | ||
3889 | @cindex synthesizing on H8/300 | |
3890 | @item synthesizing instructions | |
3891 | @c FIXME: specifically mov.b, or any mov instructions really? | |
3892 | @code{ld} finds all @code{mov.b} instructions which use the | |
3893 | sixteen-bit absolute address form, but refer to the top | |
3894 | page of memory, and changes them to use the eight-bit address form. | |
3895 | (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into | |
3896 | @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the | |
3897 | top page of memory). | |
3898 | @end table | |
3899 | ||
3900 | @ifclear GENERIC | |
3901 | @lowersections | |
3902 | @end ifclear | |
3903 | @end ifset | |
3904 | ||
3905 | @ifclear GENERIC | |
3906 | @ifset Hitachi | |
3907 | @c This stuff is pointless to say unless you're especially concerned | |
3908 | @c with Hitachi chips; don't enable it for generic case, please. | |
3909 | @node Hitachi | |
3910 | @chapter @code{ld} and other Hitachi chips | |
3911 | ||
3912 | @code{ld} also supports the H8/300H, the H8/500, and the Hitachi SH. No | |
3913 | special features, commands, or command-line options are required for | |
3914 | these chips. | |
3915 | @end ifset | |
3916 | @end ifclear | |
3917 | ||
3918 | @ifset I960 | |
3919 | @ifclear GENERIC | |
3920 | @raisesections | |
3921 | @end ifclear | |
3922 | ||
3923 | @node i960 | |
3924 | @section @code{ld} and the Intel 960 family | |
3925 | ||
3926 | @cindex i960 support | |
3927 | ||
3928 | You can use the @samp{-A@var{architecture}} command line option to | |
3929 | specify one of the two-letter names identifying members of the 960 | |
3930 | family; the option specifies the desired output target, and warns of any | |
3931 | incompatible instructions in the input files. It also modifies the | |
3932 | linker's search strategy for archive libraries, to support the use of | |
3933 | libraries specific to each particular architecture, by including in the | |
3934 | search loop names suffixed with the string identifying the architecture. | |
3935 | ||
3936 | For example, if your @code{ld} command line included @w{@samp{-ACA}} as | |
3937 | well as @w{@samp{-ltry}}, the linker would look (in its built-in search | |
3938 | paths, and in any paths you specify with @samp{-L}) for a library with | |
3939 | the names | |
3940 | ||
3941 | @smallexample | |
3942 | @group | |
3943 | try | |
3944 | libtry.a | |
3945 | tryca | |
3946 | libtryca.a | |
3947 | @end group | |
3948 | @end smallexample | |
3949 | ||
3950 | @noindent | |
3951 | The first two possibilities would be considered in any event; the last | |
3952 | two are due to the use of @w{@samp{-ACA}}. | |
3953 | ||
3954 | You can meaningfully use @samp{-A} more than once on a command line, since | |
3955 | the 960 architecture family allows combination of target architectures; each | |
3956 | use will add another pair of name variants to search for when @w{@samp{-l}} | |
3957 | specifies a library. | |
3958 | ||
3959 | @cindex @code{--relax} on i960 | |
3960 | @cindex relaxing on i960 | |
3961 | @code{ld} supports the @samp{--relax} option for the i960 family. If | |
3962 | you specify @samp{--relax}, @code{ld} finds all @code{balx} and | |
3963 | @code{calx} instructions whose targets are within 24 bits, and turns | |
3964 | them into 24-bit program-counter relative @code{bal} and @code{cal} | |
3965 | instructions, respectively. @code{ld} also turns @code{cal} | |
3966 | instructions into @code{bal} instructions when it determines that the | |
3967 | target subroutine is a leaf routine (that is, the target subroutine does | |
3968 | not itself call any subroutines). | |
3969 | ||
3970 | @ifclear GENERIC | |
3971 | @lowersections | |
3972 | @end ifclear | |
3973 | @end ifset | |
3974 | ||
3975 | @ifclear GENERIC | |
3976 | @raisesections | |
3977 | @end ifclear | |
3978 | ||
3979 | @node ARM | |
3980 | @section @code{ld}'s support for interworking between ARM and Thumb code | |
3981 | ||
3982 | @cindex ARM interworking support | |
6f798e5c | 3983 | @kindex --support-old-code |
252b5132 RH |
3984 | For the ARM, @code{ld} will generate code stubs to allow functions calls |
3985 | betweem ARM and Thumb code. These stubs only work with code that has | |
3986 | been compiled and assembled with the @samp{-mthumb-interwork} command | |
3987 | line option. If it is necessary to link with old ARM object files or | |
3988 | libraries, which have not been compiled with the -mthumb-interwork | |
3989 | option then the @samp{--support-old-code} command line switch should be | |
3990 | given to the linker. This will make it generate larger stub functions | |
3991 | which will work with non-interworking aware ARM code. Note, however, | |
3992 | the linker does not support generating stubs for function calls to | |
3993 | non-interworking aware Thumb code. | |
3994 | ||
6f798e5c NC |
3995 | @cindex thumb entry point |
3996 | @cindex entry point, thumb | |
3997 | @kindex --thumb-entry=@var{entry} | |
3998 | The @samp{--thumb-entry} switch is a duplicate of the generic | |
3999 | @samp{--entry} switch, in that it sets the program's starting address. | |
4000 | But it also sets the bottom bit of the address, so that it can be | |
4001 | branched to using a BX instruction, and the program will start | |
4002 | executing in Thumb mode straight away. | |
4003 | ||
74459f0e TW |
4004 | @ifset TICOFF |
4005 | @node TI COFF | |
4006 | @section @code{ld}'s support for various TI COFF versions | |
4007 | @cindex TI COFF versions | |
4008 | @kindex --format=@var{version} | |
4009 | The @samp{--format} switch allows selection of one of the various | |
4010 | TI COFF versions. The latest of this writing is 2; versions 0 and 1 are | |
4011 | also supported. The TI COFF versions also vary in header byte-order | |
4012 | format; @code{ld} will read any version or byte order, but the output | |
4013 | header format depends on the default specified by the specific target. | |
4014 | @end ifset | |
4015 | ||
252b5132 RH |
4016 | @ifclear GENERIC |
4017 | @lowersections | |
4018 | @end ifclear | |
4019 | ||
4020 | @ifclear SingleFormat | |
4021 | @node BFD | |
4022 | @chapter BFD | |
4023 | ||
4024 | @cindex back end | |
4025 | @cindex object file management | |
4026 | @cindex object formats available | |
4027 | @kindex objdump -i | |
4028 | The linker accesses object and archive files using the BFD libraries. | |
4029 | These libraries allow the linker to use the same routines to operate on | |
4030 | object files whatever the object file format. A different object file | |
4031 | format can be supported simply by creating a new BFD back end and adding | |
4032 | it to the library. To conserve runtime memory, however, the linker and | |
4033 | associated tools are usually configured to support only a subset of the | |
4034 | object file formats available. You can use @code{objdump -i} | |
4035 | (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to | |
4036 | list all the formats available for your configuration. | |
4037 | ||
4038 | @cindex BFD requirements | |
4039 | @cindex requirements for BFD | |
4040 | As with most implementations, BFD is a compromise between | |
4041 | several conflicting requirements. The major factor influencing | |
4042 | BFD design was efficiency: any time used converting between | |
4043 | formats is time which would not have been spent had BFD not | |
4044 | been involved. This is partly offset by abstraction payback; since | |
4045 | BFD simplifies applications and back ends, more time and care | |
4046 | may be spent optimizing algorithms for a greater speed. | |
4047 | ||
4048 | One minor artifact of the BFD solution which you should bear in | |
4049 | mind is the potential for information loss. There are two places where | |
4050 | useful information can be lost using the BFD mechanism: during | |
4051 | conversion and during output. @xref{BFD information loss}. | |
4052 | ||
4053 | @menu | |
4054 | * BFD outline:: How it works: an outline of BFD | |
4055 | @end menu | |
4056 | ||
4057 | @node BFD outline | |
4058 | @section How it works: an outline of BFD | |
4059 | @cindex opening object files | |
4060 | @include bfdsumm.texi | |
4061 | @end ifclear | |
4062 | ||
4063 | @node Reporting Bugs | |
4064 | @chapter Reporting Bugs | |
4065 | @cindex bugs in @code{ld} | |
4066 | @cindex reporting bugs in @code{ld} | |
4067 | ||
4068 | Your bug reports play an essential role in making @code{ld} reliable. | |
4069 | ||
4070 | Reporting a bug may help you by bringing a solution to your problem, or | |
4071 | it may not. But in any case the principal function of a bug report is | |
4072 | to help the entire community by making the next version of @code{ld} | |
4073 | work better. Bug reports are your contribution to the maintenance of | |
4074 | @code{ld}. | |
4075 | ||
4076 | In order for a bug report to serve its purpose, you must include the | |
4077 | information that enables us to fix the bug. | |
4078 | ||
4079 | @menu | |
4080 | * Bug Criteria:: Have you found a bug? | |
4081 | * Bug Reporting:: How to report bugs | |
4082 | @end menu | |
4083 | ||
4084 | @node Bug Criteria | |
4085 | @section Have you found a bug? | |
4086 | @cindex bug criteria | |
4087 | ||
4088 | If you are not sure whether you have found a bug, here are some guidelines: | |
4089 | ||
4090 | @itemize @bullet | |
4091 | @cindex fatal signal | |
4092 | @cindex linker crash | |
4093 | @cindex crash of linker | |
4094 | @item | |
4095 | If the linker gets a fatal signal, for any input whatever, that is a | |
4096 | @code{ld} bug. Reliable linkers never crash. | |
4097 | ||
4098 | @cindex error on valid input | |
4099 | @item | |
4100 | If @code{ld} produces an error message for valid input, that is a bug. | |
4101 | ||
4102 | @cindex invalid input | |
4103 | @item | |
4104 | If @code{ld} does not produce an error message for invalid input, that | |
4105 | may be a bug. In the general case, the linker can not verify that | |
4106 | object files are correct. | |
4107 | ||
4108 | @item | |
4109 | If you are an experienced user of linkers, your suggestions for | |
4110 | improvement of @code{ld} are welcome in any case. | |
4111 | @end itemize | |
4112 | ||
4113 | @node Bug Reporting | |
4114 | @section How to report bugs | |
4115 | @cindex bug reports | |
4116 | @cindex @code{ld} bugs, reporting | |
4117 | ||
4118 | A number of companies and individuals offer support for @sc{gnu} | |
4119 | products. If you obtained @code{ld} from a support organization, we | |
4120 | recommend you contact that organization first. | |
4121 | ||
4122 | You can find contact information for many support companies and | |
4123 | individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs | |
4124 | distribution. | |
4125 | ||
4126 | Otherwise, send bug reports for @code{ld} to | |
4127 | @samp{bug-gnu-utils@@gnu.org}. | |
4128 | ||
4129 | The fundamental principle of reporting bugs usefully is this: | |
4130 | @strong{report all the facts}. If you are not sure whether to state a | |
4131 | fact or leave it out, state it! | |
4132 | ||
4133 | Often people omit facts because they think they know what causes the | |
4134 | problem and assume that some details do not matter. Thus, you might | |
4135 | assume that the name of a symbol you use in an example does not matter. | |
4136 | Well, probably it does not, but one cannot be sure. Perhaps the bug is | |
4137 | a stray memory reference which happens to fetch from the location where | |
4138 | that name is stored in memory; perhaps, if the name were different, the | |
4139 | contents of that location would fool the linker into doing the right | |
4140 | thing despite the bug. Play it safe and give a specific, complete | |
4141 | example. That is the easiest thing for you to do, and the most helpful. | |
4142 | ||
4143 | Keep in mind that the purpose of a bug report is to enable us to fix the bug if | |
4144 | it is new to us. Therefore, always write your bug reports on the assumption | |
4145 | that the bug has not been reported previously. | |
4146 | ||
4147 | Sometimes people give a few sketchy facts and ask, ``Does this ring a | |
4148 | bell?'' Those bug reports are useless, and we urge everyone to | |
4149 | @emph{refuse to respond to them} except to chide the sender to report | |
4150 | bugs properly. | |
4151 | ||
4152 | To enable us to fix the bug, you should include all these things: | |
4153 | ||
4154 | @itemize @bullet | |
4155 | @item | |
4156 | The version of @code{ld}. @code{ld} announces it if you start it with | |
4157 | the @samp{--version} argument. | |
4158 | ||
4159 | Without this, we will not know whether there is any point in looking for | |
4160 | the bug in the current version of @code{ld}. | |
4161 | ||
4162 | @item | |
4163 | Any patches you may have applied to the @code{ld} source, including any | |
4164 | patches made to the @code{BFD} library. | |
4165 | ||
4166 | @item | |
4167 | The type of machine you are using, and the operating system name and | |
4168 | version number. | |
4169 | ||
4170 | @item | |
4171 | What compiler (and its version) was used to compile @code{ld}---e.g. | |
4172 | ``@code{gcc-2.7}''. | |
4173 | ||
4174 | @item | |
4175 | The command arguments you gave the linker to link your example and | |
4176 | observe the bug. To guarantee you will not omit something important, | |
4177 | list them all. A copy of the Makefile (or the output from make) is | |
4178 | sufficient. | |
4179 | ||
4180 | If we were to try to guess the arguments, we would probably guess wrong | |
4181 | and then we might not encounter the bug. | |
4182 | ||
4183 | @item | |
4184 | A complete input file, or set of input files, that will reproduce the | |
4185 | bug. It is generally most helpful to send the actual object files, | |
4186 | uuencoded if necessary to get them through the mail system. Making them | |
4187 | available for anonymous FTP is not as good, but may be the only | |
4188 | reasonable choice for large object files. | |
4189 | ||
4190 | If the source files were assembled using @code{gas} or compiled using | |
4191 | @code{gcc}, then it may be OK to send the source files rather than the | |
4192 | object files. In this case, be sure to say exactly what version of | |
4193 | @code{gas} or @code{gcc} was used to produce the object files. Also say | |
4194 | how @code{gas} or @code{gcc} were configured. | |
4195 | ||
4196 | @item | |
4197 | A description of what behavior you observe that you believe is | |
4198 | incorrect. For example, ``It gets a fatal signal.'' | |
4199 | ||
4200 | Of course, if the bug is that @code{ld} gets a fatal signal, then we | |
4201 | will certainly notice it. But if the bug is incorrect output, we might | |
4202 | not notice unless it is glaringly wrong. You might as well not give us | |
4203 | a chance to make a mistake. | |
4204 | ||
4205 | Even if the problem you experience is a fatal signal, you should still | |
4206 | say so explicitly. Suppose something strange is going on, such as, your | |
4207 | copy of @code{ld} is out of synch, or you have encountered a bug in the | |
4208 | C library on your system. (This has happened!) Your copy might crash | |
4209 | and ours would not. If you told us to expect a crash, then when ours | |
4210 | fails to crash, we would know that the bug was not happening for us. If | |
4211 | you had not told us to expect a crash, then we would not be able to draw | |
4212 | any conclusion from our observations. | |
4213 | ||
4214 | @item | |
4215 | If you wish to suggest changes to the @code{ld} source, send us context | |
4216 | diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or | |
4217 | @samp{-p} option. Always send diffs from the old file to the new file. | |
4218 | If you even discuss something in the @code{ld} source, refer to it by | |
4219 | context, not by line number. | |
4220 | ||
4221 | The line numbers in our development sources will not match those in your | |
4222 | sources. Your line numbers would convey no useful information to us. | |
4223 | @end itemize | |
4224 | ||
4225 | Here are some things that are not necessary: | |
4226 | ||
4227 | @itemize @bullet | |
4228 | @item | |
4229 | A description of the envelope of the bug. | |
4230 | ||
4231 | Often people who encounter a bug spend a lot of time investigating | |
4232 | which changes to the input file will make the bug go away and which | |
4233 | changes will not affect it. | |
4234 | ||
4235 | This is often time consuming and not very useful, because the way we | |
4236 | will find the bug is by running a single example under the debugger | |
4237 | with breakpoints, not by pure deduction from a series of examples. | |
4238 | We recommend that you save your time for something else. | |
4239 | ||
4240 | Of course, if you can find a simpler example to report @emph{instead} | |
4241 | of the original one, that is a convenience for us. Errors in the | |
4242 | output will be easier to spot, running under the debugger will take | |
4243 | less time, and so on. | |
4244 | ||
4245 | However, simplification is not vital; if you do not want to do this, | |
4246 | report the bug anyway and send us the entire test case you used. | |
4247 | ||
4248 | @item | |
4249 | A patch for the bug. | |
4250 | ||
4251 | A patch for the bug does help us if it is a good one. But do not omit | |
4252 | the necessary information, such as the test case, on the assumption that | |
4253 | a patch is all we need. We might see problems with your patch and decide | |
4254 | to fix the problem another way, or we might not understand it at all. | |
4255 | ||
4256 | Sometimes with a program as complicated as @code{ld} it is very hard to | |
4257 | construct an example that will make the program follow a certain path | |
4258 | through the code. If you do not send us the example, we will not be | |
4259 | able to construct one, so we will not be able to verify that the bug is | |
4260 | fixed. | |
4261 | ||
4262 | And if we cannot understand what bug you are trying to fix, or why your | |
4263 | patch should be an improvement, we will not install it. A test case will | |
4264 | help us to understand. | |
4265 | ||
4266 | @item | |
4267 | A guess about what the bug is or what it depends on. | |
4268 | ||
4269 | Such guesses are usually wrong. Even we cannot guess right about such | |
4270 | things without first using the debugger to find the facts. | |
4271 | @end itemize | |
4272 | ||
4273 | @node MRI | |
4274 | @appendix MRI Compatible Script Files | |
4275 | @cindex MRI compatibility | |
4276 | To aid users making the transition to @sc{gnu} @code{ld} from the MRI | |
4277 | linker, @code{ld} can use MRI compatible linker scripts as an | |
4278 | alternative to the more general-purpose linker scripting language | |
4279 | described in @ref{Scripts}. MRI compatible linker scripts have a much | |
4280 | simpler command set than the scripting language otherwise used with | |
4281 | @code{ld}. @sc{gnu} @code{ld} supports the most commonly used MRI | |
4282 | linker commands; these commands are described here. | |
4283 | ||
4284 | In general, MRI scripts aren't of much use with the @code{a.out} object | |
4285 | file format, since it only has three sections and MRI scripts lack some | |
4286 | features to make use of them. | |
4287 | ||
4288 | You can specify a file containing an MRI-compatible script using the | |
4289 | @samp{-c} command-line option. | |
4290 | ||
4291 | Each command in an MRI-compatible script occupies its own line; each | |
4292 | command line starts with the keyword that identifies the command (though | |
4293 | blank lines are also allowed for punctuation). If a line of an | |
4294 | MRI-compatible script begins with an unrecognized keyword, @code{ld} | |
4295 | issues a warning message, but continues processing the script. | |
4296 | ||
4297 | Lines beginning with @samp{*} are comments. | |
4298 | ||
4299 | You can write these commands using all upper-case letters, or all | |
4300 | lower case; for example, @samp{chip} is the same as @samp{CHIP}. | |
4301 | The following list shows only the upper-case form of each command. | |
4302 | ||
4303 | @table @code | |
4304 | @cindex @code{ABSOLUTE} (MRI) | |
4305 | @item ABSOLUTE @var{secname} | |
4306 | @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname} | |
4307 | Normally, @code{ld} includes in the output file all sections from all | |
4308 | the input files. However, in an MRI-compatible script, you can use the | |
4309 | @code{ABSOLUTE} command to restrict the sections that will be present in | |
4310 | your output program. If the @code{ABSOLUTE} command is used at all in a | |
4311 | script, then only the sections named explicitly in @code{ABSOLUTE} | |
4312 | commands will appear in the linker output. You can still use other | |
4313 | input sections (whatever you select on the command line, or using | |
4314 | @code{LOAD}) to resolve addresses in the output file. | |
4315 | ||
4316 | @cindex @code{ALIAS} (MRI) | |
4317 | @item ALIAS @var{out-secname}, @var{in-secname} | |
4318 | Use this command to place the data from input section @var{in-secname} | |
4319 | in a section called @var{out-secname} in the linker output file. | |
4320 | ||
4321 | @var{in-secname} may be an integer. | |
4322 | ||
4323 | @cindex @code{ALIGN} (MRI) | |
4324 | @item ALIGN @var{secname} = @var{expression} | |
4325 | Align the section called @var{secname} to @var{expression}. The | |
4326 | @var{expression} should be a power of two. | |
4327 | ||
4328 | @cindex @code{BASE} (MRI) | |
4329 | @item BASE @var{expression} | |
4330 | Use the value of @var{expression} as the lowest address (other than | |
4331 | absolute addresses) in the output file. | |
4332 | ||
4333 | @cindex @code{CHIP} (MRI) | |
4334 | @item CHIP @var{expression} | |
4335 | @itemx CHIP @var{expression}, @var{expression} | |
4336 | This command does nothing; it is accepted only for compatibility. | |
4337 | ||
4338 | @cindex @code{END} (MRI) | |
4339 | @item END | |
4340 | This command does nothing whatever; it's only accepted for compatibility. | |
4341 | ||
4342 | @cindex @code{FORMAT} (MRI) | |
4343 | @item FORMAT @var{output-format} | |
4344 | Similar to the @code{OUTPUT_FORMAT} command in the more general linker | |
4345 | language, but restricted to one of these output formats: | |
4346 | ||
4347 | @enumerate | |
4348 | @item | |
4349 | S-records, if @var{output-format} is @samp{S} | |
4350 | ||
4351 | @item | |
4352 | IEEE, if @var{output-format} is @samp{IEEE} | |
4353 | ||
4354 | @item | |
4355 | COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is | |
4356 | @samp{COFF} | |
4357 | @end enumerate | |
4358 | ||
4359 | @cindex @code{LIST} (MRI) | |
4360 | @item LIST @var{anything}@dots{} | |
4361 | Print (to the standard output file) a link map, as produced by the | |
4362 | @code{ld} command-line option @samp{-M}. | |
4363 | ||
4364 | The keyword @code{LIST} may be followed by anything on the | |
4365 | same line, with no change in its effect. | |
4366 | ||
4367 | @cindex @code{LOAD} (MRI) | |
4368 | @item LOAD @var{filename} | |
4369 | @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename} | |
4370 | Include one or more object file @var{filename} in the link; this has the | |
4371 | same effect as specifying @var{filename} directly on the @code{ld} | |
4372 | command line. | |
4373 | ||
4374 | @cindex @code{NAME} (MRI) | |
4375 | @item NAME @var{output-name} | |
4376 | @var{output-name} is the name for the program produced by @code{ld}; the | |
4377 | MRI-compatible command @code{NAME} is equivalent to the command-line | |
4378 | option @samp{-o} or the general script language command @code{OUTPUT}. | |
4379 | ||
4380 | @cindex @code{ORDER} (MRI) | |
4381 | @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname} | |
4382 | @itemx ORDER @var{secname} @var{secname} @var{secname} | |
4383 | Normally, @code{ld} orders the sections in its output file in the | |
4384 | order in which they first appear in the input files. In an MRI-compatible | |
4385 | script, you can override this ordering with the @code{ORDER} command. The | |
4386 | sections you list with @code{ORDER} will appear first in your output | |
4387 | file, in the order specified. | |
4388 | ||
4389 | @cindex @code{PUBLIC} (MRI) | |
4390 | @item PUBLIC @var{name}=@var{expression} | |
4391 | @itemx PUBLIC @var{name},@var{expression} | |
4392 | @itemx PUBLIC @var{name} @var{expression} | |
4393 | Supply a value (@var{expression}) for external symbol | |
4394 | @var{name} used in the linker input files. | |
4395 | ||
4396 | @cindex @code{SECT} (MRI) | |
4397 | @item SECT @var{secname}, @var{expression} | |
4398 | @itemx SECT @var{secname}=@var{expression} | |
4399 | @itemx SECT @var{secname} @var{expression} | |
4400 | You can use any of these three forms of the @code{SECT} command to | |
4401 | specify the start address (@var{expression}) for section @var{secname}. | |
4402 | If you have more than one @code{SECT} statement for the same | |
4403 | @var{secname}, only the @emph{first} sets the start address. | |
4404 | @end table | |
4405 | ||
4406 | @node Index | |
4407 | @unnumbered Index | |
4408 | ||
4409 | @printindex cp | |
4410 | ||
4411 | @tex | |
4412 | % I think something like @colophon should be in texinfo. In the | |
4413 | % meantime: | |
4414 | \long\def\colophon{\hbox to0pt{}\vfill | |
4415 | \centerline{The body of this manual is set in} | |
4416 | \centerline{\fontname\tenrm,} | |
4417 | \centerline{with headings in {\bf\fontname\tenbf}} | |
4418 | \centerline{and examples in {\tt\fontname\tentt}.} | |
4419 | \centerline{{\it\fontname\tenit\/} and} | |
4420 | \centerline{{\sl\fontname\tensl\/}} | |
4421 | \centerline{are used for emphasis.}\vfill} | |
4422 | \page\colophon | |
4423 | % Blame: doc@cygnus.com, 28mar91. | |
4424 | @end tex | |
4425 | ||
4426 | ||
4427 | @contents | |
4428 | @bye | |
4429 | ||
4430 |