@subtitle The GNU linker
@sp 1
@subtitle @code{ld} version 2
-@subtitle January 1994
+@subtitle April 1998
@author Steve Chamberlain
-@author Cygnus Support
+@author Ian Lance Taylor
+@author Cygnus Solutions
@page
@tex
{\parskip=0pt
-\hfill Cygnus Support\par
-\hfill steve\@cygnus.com, doc\@cygnus.com\par
+\hfill Cygnus Solutions\par
+\hfill ian\@cygnus.com, doc\@cygnus.com\par
\hfill {\it Using LD, the GNU linker}\par
\hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par
}
default linker script.
You may also use linker scripts implicitly by naming them as input files
-to the linker, as though they were files to be linked. If the linker
-opens a file which it can not recognize as an object file or as an
-archive file, it will try to read it as a linker script. If the file
-can not be parsed as a linker script, the linker will report an error.
-
-An implicit linker script will not replace the default linker script.
-Typically an implicit linker script would contain only the @code{INPUT},
-@code{GROUP}, or @code{VERSION} commands.
+to the linker, as though they were files to be linked. @xref{Implicit
+Linker Scripts}.
@menu
* Basic Script Concepts:: Basic Linker Script Concepts
* PHDRS:: PHDRS Command
* VERSION:: VERSION Command
* Expressions:: Expressions in Linker Scripts
+* Implicit Linker Scripts:: Implicit Linker Scripts
@end menu
@node Basic Script Concepts
Every loadable or allocatable output section has two addresses. The
first is the @dfn{VMA}, or virtual memory address. This is the address
-the sectin will have when the output file is run. The second is the
+the section will have when the output file is run. The second is the
@dfn{LMA}, or load memory address. This is the address at which the
section will be loaded. In most cases the two addresses will be the
same. An example of when they might be different is when a data section
address is set from the current value of the location counter. The
location counter is then incremented by the size of the output section.
+The first line inside the @samp{SECTIONS} command of the above example
+sets the value of the special symbol @samp{.}, which is the location
+counter. If you do not specify the address of an output section in some
+other way (other ways are described later), the address is set from the
+current value of the location counter. The location counter is then
+incremented by the size of the output section. At the start of the
+@samp{SECTIONS} command, the location counter has the value @samp{0}.
+
The second line defines an output section, @samp{.text}. The colon is
required syntax which may be ignored for now. Within the curly braces
after the output section name, you list the names of the input sections
@samp{.text} section in the output file to be @samp{0x10000}.
The remaining lines define the @samp{.data} and @samp{.bss} sections in
-the output file. The @samp{.data} output section will be at address
-@samp{0x8000000}. When the @samp{.bss} output section is defined, the
-value of the location counter will be @samp{0x8000000} plus the size of
-the @samp{.data} output section. The effect is that the @samp{.bss}
-output section will follow immediately after the @samp{.data} output
-section in memory.
+the output file. The linker will place the @samp{.data} output section
+at address @samp{0x8000000}. After the linker places the @samp{.data}
+output section, the value of the location counter will be
+@samp{0x8000000} plus the size of the @samp{.data} output section. The
+effect is that the linker will place the @samp{.bss} output section
+immediately after the @samp{.data} output section in memory
+
+The linker will ensure that each output section has the required
+alignment, by increasing the location counter if necessary. In this
+example, the specified addresses for the @samp{.text} and @samp{.data}
+sections will probably satisfy any alignment constraints, but the linker
+may have to create a small gap between the @samp{.data} and @samp{.bss}
+sections.
That's it! That's a simple and complete linker script.
@table @code
@item FORCE_COMMON_ALLOCATION
@kindex FORCE_COMMON_ALLOCATION
-@cindex common allocation
+@cindex common allocation in linker script
This command has the same effect as the @samp{-d} command-line option:
to make @code{ld} assign space to common symbols even if a relocatable
output file is specified (@samp{-r}).
@}
@end smallexample
-In this example, if the program defines @samp{_etext}, the linker will
-give a multiple definition error. If, on the other hand, the program
-defines @samp{etext}, the linker will silently use the definition in the
-program. If the program references @samp{etext} but does not define it,
-the linker will use the definition in the linker script.
+In this example, if the program defines @samp{_etext} (with a leading
+underscore), the linker will give a multiple definition error. If, on
+the other hand, the program defines @samp{etext} (with no leading
+underscore), the linker will silently use the definition in the program.
+If the program references @samp{etext} but does not define it, the
+linker will use the definition in the linker script.
@node SECTIONS
@section SECTIONS command
using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
one or more segments, then all subsequent allocated sections will be
assigned to those segments as well, unless they use an explicitly
-@code{:@var{phdr}} modifier. To prevent a section from being assigned
-to a segment when it would normally default to one, use @code{:NONE}.
+@code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
+linker to not put the section in any segment at all.
Here is a simple example:
@smallexample
@end group
@end smallexample
-If you have defined a memory region named @samp{mem}, you can direct the
-linker to place specific output sections into that memory region by
-using the @samp{>@var{region}} output section attribute. @xref{Output
-Section Region}. If no address was specified for the output section,
-the linker will set the address to the next available address within the
-memory region. If the combined output sections directed to a memory
-region are too large for the region, the linker will issue an error
-message.
+Once you define a memory region, you can direct the linker to place
+specific output sections into that memory region by using the
+@samp{>@var{region}} output section attribute. For example, if you have
+a memory region named @samp{mem}, you would use @samp{>mem} in the
+output section definition. @xref{Output Section Region}. If no address
+was specified for the output section, the linker will set the address to
+the next available address within the memory region. If the combined
+output sections directed to a memory region are too large for the
+region, the linker will issue an error message.
@node PHDRS
@section PHDRS Command
then the linker will place all subsequent allocatable sections which do
not specify @samp{:@var{phdr}} in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
-placed in a single segment. To prevent a section from being assigned to
-a segment when it would normally default to one, use @code{:NONE}.
+placed in a single segment. You can use @code{:NONE} to override the
+default segment and tell the linker to not put the section in any
+segment at all.
@kindex FILEHDR
@kindex PHDRS
command (@pxref{PHDRS}).
@end table
+@node Implicit Linker Scripts
+@section Implicit Linker Scripts
+@cindex implicit linker scripts
+If you specify a linker input file which the linker can not recognize as
+an object file or an archive file, it will try to read the file as a
+linker script. If the file can not be parsed as a linker script, the
+linker will report an error.
+
+An implicit linker script will not replace the default linker script.
+
+Typically an implicit linker script would contain only symbol
+assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
+commands.
+
+Any input files read because of an implicit linker script will be read
+at the position in the command line where the implicit linker script was
+read. This can affect archive searching.
+
@ifset GENERIC
@node Machine Dependent
@chapter Machine Dependent Features
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