1 /* alloca.c -- allocate automatically reclaimed memory
2 (Mostly) portable public-domain implementation -- D A Gwyn
4 This implementation of the PWB library alloca function,
5 which is used to allocate space off the run-time stack so
6 that it is automatically reclaimed upon procedure exit,
7 was inspired by discussions with J. Q. Johnson of Cornell.
8 J.Otto Tennant <jot@cray.com> contributed the Cray support.
10 There are some preprocessor constants that can
11 be defined when compiling for your specific system, for
12 improved efficiency; however, the defaults should be okay.
14 The general concept of this implementation is to keep
15 track of all alloca-allocated blocks, and reclaim any
16 that are found to be deeper in the stack than the current
17 invocation. This heuristic does not reclaim storage as
18 soon as it becomes invalid, but it will do so eventually.
20 As a special case, alloca(0) reclaims storage without
21 allocating any. It is a good idea to use alloca(0) in
22 your main control loop, etc. to force garbage collection. */
26 @deftypefn Replacement void* alloca (size_t @var{size})
28 This function allocates memory which will be automatically reclaimed
29 after the procedure exits. The @libib{} implementation does not free
30 the memory immediately but will do so eventually during subsequent
31 calls to this function. Memory is allocated using @code{xmalloc} under
34 The header file @file{alloca-conf.h} can be used in conjunction with the
35 GNU Autoconf test @code{AC_FUNC_ALLOCA} to test for and properly make
36 available this function. The @code{AC_FUNC_ALLOCA} test requires that
37 client code use a block of preprocessor code to be safe (see the Autoconf
38 manual for more); this header incorporates that logic and more, including
39 the possibility of a GCC built-in function.
49 #include <libiberty.h>
58 /* These variables are used by the ASTRDUP implementation that relies
60 const char *libiberty_optr
;
62 unsigned long libiberty_len
;
64 /* If your stack is a linked list of frames, you have to
65 provide an "address metric" ADDRESS_FUNCTION macro. */
67 #if defined (CRAY) && defined (CRAY_STACKSEG_END)
68 static long i00afunc ();
69 #define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
71 #define ADDRESS_FUNCTION(arg) &(arg)
78 /* Define STACK_DIRECTION if you know the direction of stack
79 growth for your system; otherwise it will be automatically
82 STACK_DIRECTION > 0 => grows toward higher addresses
83 STACK_DIRECTION < 0 => grows toward lower addresses
84 STACK_DIRECTION = 0 => direction of growth unknown */
86 #ifndef STACK_DIRECTION
87 #define STACK_DIRECTION 0 /* Direction unknown. */
90 #if STACK_DIRECTION != 0
92 #define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
94 #else /* STACK_DIRECTION == 0; need run-time code. */
96 static int stack_dir
; /* 1 or -1 once known. */
97 #define STACK_DIR stack_dir
100 find_stack_direction ()
102 static char *addr
= NULL
; /* Address of first `dummy', once known. */
103 auto char dummy
; /* To get stack address. */
106 { /* Initial entry. */
107 addr
= ADDRESS_FUNCTION (dummy
);
109 find_stack_direction (); /* Recurse once. */
114 if (ADDRESS_FUNCTION (dummy
) > addr
)
115 stack_dir
= 1; /* Stack grew upward. */
117 stack_dir
= -1; /* Stack grew downward. */
121 #endif /* STACK_DIRECTION == 0 */
123 /* An "alloca header" is used to:
124 (a) chain together all alloca'ed blocks;
125 (b) keep track of stack depth.
127 It is very important that sizeof(header) agree with malloc
128 alignment chunk size. The following default should work okay. */
131 #define ALIGN_SIZE sizeof(double)
136 char align
[ALIGN_SIZE
]; /* To force sizeof(header). */
139 union hdr
*next
; /* For chaining headers. */
140 char *deep
; /* For stack depth measure. */
144 static header
*last_alloca_header
= NULL
; /* -> last alloca header. */
146 /* Return a pointer to at least SIZE bytes of storage,
147 which will be automatically reclaimed upon exit from
148 the procedure that called alloca. Originally, this space
149 was supposed to be taken from the current stack frame of the
150 caller, but that method cannot be made to work for some
151 implementations of C, for example under Gould's UTX/32. */
153 /* @undocumented C_alloca */
159 auto char probe
; /* Probes stack depth: */
160 register char *depth
= ADDRESS_FUNCTION (probe
);
162 #if STACK_DIRECTION == 0
163 if (STACK_DIR
== 0) /* Unknown growth direction. */
164 find_stack_direction ();
167 /* Reclaim garbage, defined as all alloca'd storage that
168 was allocated from deeper in the stack than currently. */
171 register header
*hp
; /* Traverses linked list. */
173 for (hp
= last_alloca_header
; hp
!= NULL
;)
174 if ((STACK_DIR
> 0 && hp
->h
.deep
> depth
)
175 || (STACK_DIR
< 0 && hp
->h
.deep
< depth
))
177 register header
*np
= hp
->h
.next
;
179 free ((PTR
) hp
); /* Collect garbage. */
181 hp
= np
; /* -> next header. */
184 break; /* Rest are not deeper. */
186 last_alloca_header
= hp
; /* -> last valid storage. */
190 return NULL
; /* No allocation required. */
192 /* Allocate combined header + user data storage. */
195 register PTR
new = xmalloc (sizeof (header
) + size
);
196 /* Address of header. */
201 ((header
*) new)->h
.next
= last_alloca_header
;
202 ((header
*) new)->h
.deep
= depth
;
204 last_alloca_header
= (header
*) new;
206 /* User storage begins just after header. */
208 return (PTR
) ((char *) new + sizeof (header
));
212 #if defined (CRAY) && defined (CRAY_STACKSEG_END)
214 #ifdef DEBUG_I00AFUNC
221 /* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
222 struct stack_control_header
224 long shgrow
:32; /* Number of times stack has grown. */
225 long shaseg
:32; /* Size of increments to stack. */
226 long shhwm
:32; /* High water mark of stack. */
227 long shsize
:32; /* Current size of stack (all segments). */
230 /* The stack segment linkage control information occurs at
231 the high-address end of a stack segment. (The stack
232 grows from low addresses to high addresses.) The initial
233 part of the stack segment linkage control information is
234 0200 (octal) words. This provides for register storage
235 for the routine which overflows the stack. */
237 struct stack_segment_linkage
239 long ss
[0200]; /* 0200 overflow words. */
240 long sssize
:32; /* Number of words in this segment. */
241 long ssbase
:32; /* Offset to stack base. */
243 long sspseg
:32; /* Offset to linkage control of previous
246 long sstcpt
:32; /* Pointer to task common address block. */
247 long sscsnm
; /* Private control structure number for
249 long ssusr1
; /* Reserved for user. */
250 long ssusr2
; /* Reserved for user. */
251 long sstpid
; /* Process ID for pid based multi-tasking. */
252 long ssgvup
; /* Pointer to multitasking thread giveup. */
253 long sscray
[7]; /* Reserved for Cray Research. */
273 /* The following structure defines the vector of words
274 returned by the STKSTAT library routine. */
277 long now
; /* Current total stack size. */
278 long maxc
; /* Amount of contiguous space which would
279 be required to satisfy the maximum
280 stack demand to date. */
281 long high_water
; /* Stack high-water mark. */
282 long overflows
; /* Number of stack overflow ($STKOFEN) calls. */
283 long hits
; /* Number of internal buffer hits. */
284 long extends
; /* Number of block extensions. */
285 long stko_mallocs
; /* Block allocations by $STKOFEN. */
286 long underflows
; /* Number of stack underflow calls ($STKRETN). */
287 long stko_free
; /* Number of deallocations by $STKRETN. */
288 long stkm_free
; /* Number of deallocations by $STKMRET. */
289 long segments
; /* Current number of stack segments. */
290 long maxs
; /* Maximum number of stack segments so far. */
291 long pad_size
; /* Stack pad size. */
292 long current_address
; /* Current stack segment address. */
293 long current_size
; /* Current stack segment size. This
294 number is actually corrupted by STKSTAT to
295 include the fifteen word trailer area. */
296 long initial_address
; /* Address of initial segment. */
297 long initial_size
; /* Size of initial segment. */
300 /* The following structure describes the data structure which trails
301 any stack segment. I think that the description in 'asdef' is
302 out of date. I only describe the parts that I am sure about. */
306 long this_address
; /* Address of this block. */
307 long this_size
; /* Size of this block (does not include
311 long link
; /* Address of trailer block of previous
326 #endif /* not CRAY_STACK */
329 /* Determine a "stack measure" for an arbitrary ADDRESS.
330 I doubt that "lint" will like this much. */
333 i00afunc (long *address
)
335 struct stk_stat status
;
336 struct stk_trailer
*trailer
;
340 /* We want to iterate through all of the segments. The first
341 step is to get the stack status structure. We could do this
342 more quickly and more directly, perhaps, by referencing the
343 $LM00 common block, but I know that this works. */
347 /* Set up the iteration. */
349 trailer
= (struct stk_trailer
*) (status
.current_address
350 + status
.current_size
353 /* There must be at least one stack segment. Therefore it is
354 a fatal error if "trailer" is null. */
359 /* Discard segments that do not contain our argument address. */
363 block
= (long *) trailer
->this_address
;
364 size
= trailer
->this_size
;
365 if (block
== 0 || size
== 0)
367 trailer
= (struct stk_trailer
*) trailer
->link
;
368 if ((block
<= address
) && (address
< (block
+ size
)))
372 /* Set the result to the offset in this segment and add the sizes
373 of all predecessor segments. */
375 result
= address
- block
;
384 if (trailer
->this_size
<= 0)
386 result
+= trailer
->this_size
;
387 trailer
= (struct stk_trailer
*) trailer
->link
;
389 while (trailer
!= 0);
391 /* We are done. Note that if you present a bogus address (one
392 not in any segment), you will get a different number back, formed
393 from subtracting the address of the first block. This is probably
394 not what you want. */
399 #else /* not CRAY2 */
400 /* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
401 Determine the number of the cell within the stack,
402 given the address of the cell. The purpose of this
403 routine is to linearize, in some sense, stack addresses
407 i00afunc (long address
)
411 long size
, pseg
, this_segment
, stack
;
414 struct stack_segment_linkage
*ssptr
;
416 /* Register B67 contains the address of the end of the
417 current stack segment. If you (as a subprogram) store
418 your registers on the stack and find that you are past
419 the contents of B67, you have overflowed the segment.
421 B67 also points to the stack segment linkage control
422 area, which is what we are really interested in. */
424 stkl
= CRAY_STACKSEG_END ();
425 ssptr
= (struct stack_segment_linkage
*) stkl
;
427 /* If one subtracts 'size' from the end of the segment,
428 one has the address of the first word of the segment.
430 If this is not the first segment, 'pseg' will be
433 pseg
= ssptr
->sspseg
;
434 size
= ssptr
->sssize
;
436 this_segment
= stkl
- size
;
438 /* It is possible that calling this routine itself caused
439 a stack overflow. Discard stack segments which do not
440 contain the target address. */
442 while (!(this_segment
<= address
&& address
<= stkl
))
444 #ifdef DEBUG_I00AFUNC
445 fprintf (stderr
, "%011o %011o %011o\n", this_segment
, address
, stkl
);
450 ssptr
= (struct stack_segment_linkage
*) stkl
;
451 size
= ssptr
->sssize
;
452 pseg
= ssptr
->sspseg
;
453 this_segment
= stkl
- size
;
456 result
= address
- this_segment
;
458 /* If you subtract pseg from the current end of the stack,
459 you get the address of the previous stack segment's end.
460 This seems a little convoluted to me, but I'll bet you save
461 a cycle somewhere. */
465 #ifdef DEBUG_I00AFUNC
466 fprintf (stderr
, "%011o %011o\n", pseg
, size
);
469 ssptr
= (struct stack_segment_linkage
*) stkl
;
470 size
= ssptr
->sssize
;
471 pseg
= ssptr
->sspseg
;
477 #endif /* not CRAY2 */
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