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. */
28 /* If compiling with GCC, this file's not needed. */
33 /* actually, only want this if static is defined as ""
34 -- this is for usg, in which emacs must undefine static
35 in order to make unexec workable
37 #ifndef STACK_DIRECTION
40 -- must know STACK_DIRECTION at compile-time
41 #endif /* STACK_DIRECTION undefined */
49 /* If your stack is a linked list of frames, you have to
50 provide an "address metric" ADDRESS_FUNCTION macro. */
54 #define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
56 #define ADDRESS_FUNCTION(arg) &(arg)
60 typedef void *pointer;
62 typedef char *pointer;
67 extern pointer (*_mp_allocate_func) ();
68 extern void (*_mp_free_func) ();
70 /* Define STACK_DIRECTION if you know the direction of stack
71 growth for your system; otherwise it will be automatically
74 STACK_DIRECTION > 0 => grows toward higher addresses
75 STACK_DIRECTION < 0 => grows toward lower addresses
76 STACK_DIRECTION = 0 => direction of growth unknown */
78 #ifndef STACK_DIRECTION
79 #define STACK_DIRECTION 0 /* Direction unknown. */
82 #if STACK_DIRECTION != 0
84 #define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
86 #else /* STACK_DIRECTION == 0; need run-time code. */
88 static int stack_dir; /* 1 or -1 once known. */
89 #define STACK_DIR stack_dir
92 find_stack_direction ()
94 static char *addr = NULL; /* Address of first `dummy', once known. */
95 auto char dummy; /* To get stack address. */
98 { /* Initial entry. */
99 addr = ADDRESS_FUNCTION (dummy);
101 find_stack_direction (); /* Recurse once. */
106 if (ADDRESS_FUNCTION (dummy) > addr)
107 stack_dir = 1; /* Stack grew upward. */
109 stack_dir = -1; /* Stack grew downward. */
113 #endif /* STACK_DIRECTION == 0 */
115 /* An "alloca header" is used to:
116 (a) chain together all alloca'ed blocks;
117 (b) keep track of stack depth.
119 It is very important that sizeof(header) agree with malloc
120 alignment chunk size. The following default should work okay. */
123 #define ALIGN_SIZE sizeof(double)
128 char align[ALIGN_SIZE]; /* To force sizeof(header). */
131 union hdr *next; /* For chaining headers. */
132 char *deep; /* For stack depth measure. */
136 static header *last_alloca_header = NULL; /* -> last alloca header. */
138 /* Return a pointer to at least SIZE bytes of storage,
139 which will be automatically reclaimed upon exit from
140 the procedure that called alloca. Originally, this space
141 was supposed to be taken from the current stack frame of the
142 caller, but that method cannot be made to work for some
143 implementations of C, for example under Gould's UTX/32. */
149 auto char probe; /* Probes stack depth: */
150 register char *depth = ADDRESS_FUNCTION (probe);
152 #if STACK_DIRECTION == 0
153 if (STACK_DIR == 0) /* Unknown growth direction. */
154 find_stack_direction ();
157 /* Reclaim garbage, defined as all alloca'd storage that
158 was allocated from deeper in the stack than currently. */
161 register header *hp; /* Traverses linked list. */
163 for (hp = last_alloca_header; hp != NULL;)
164 if ((STACK_DIR > 0 && hp->h.deep > depth)
165 || (STACK_DIR < 0 && hp->h.deep < depth))
167 register header *np = hp->h.next;
169 (*_mp_free_func) ((pointer) hp); /* Collect garbage. */
171 hp = np; /* -> next header. */
174 break; /* Rest are not deeper. */
176 last_alloca_header = hp; /* -> last valid storage. */
180 return NULL; /* No allocation required. */
182 /* Allocate combined header + user data storage. */
185 register pointer new = (*_mp_allocate_func) (sizeof (header) + size);
186 /* Address of header. */
188 ((header *) new)->h.next = last_alloca_header;
189 ((header *) new)->h.deep = depth;
191 last_alloca_header = (header *) new;
193 /* User storage begins just after header. */
195 return (pointer) ((char *) new + sizeof (header));
201 #ifdef DEBUG_I00AFUNC
208 /* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
209 struct stack_control_header
211 long shgrow:32; /* Number of times stack has grown. */
212 long shaseg:32; /* Size of increments to stack. */
213 long shhwm:32; /* High water mark of stack. */
214 long shsize:32; /* Current size of stack (all segments). */
217 /* The stack segment linkage control information occurs at
218 the high-address end of a stack segment. (The stack
219 grows from low addresses to high addresses.) The initial
220 part of the stack segment linkage control information is
221 0200 (octal) words. This provides for register storage
222 for the routine which overflows the stack. */
224 struct stack_segment_linkage
226 long ss[0200]; /* 0200 overflow words. */
227 long sssize:32; /* Number of words in this segment. */
228 long ssbase:32; /* Offset to stack base. */
230 long sspseg:32; /* Offset to linkage control of previous
233 long sstcpt:32; /* Pointer to task common address block. */
234 long sscsnm; /* Private control structure number for
236 long ssusr1; /* Reserved for user. */
237 long ssusr2; /* Reserved for user. */
238 long sstpid; /* Process ID for pid based multi-tasking. */
239 long ssgvup; /* Pointer to multitasking thread giveup. */
240 long sscray[7]; /* Reserved for Cray Research. */
260 /* The following structure defines the vector of words
261 returned by the STKSTAT library routine. */
264 long now; /* Current total stack size. */
265 long maxc; /* Amount of contiguous space which would
266 be required to satisfy the maximum
267 stack demand to date. */
268 long high_water; /* Stack high-water mark. */
269 long overflows; /* Number of stack overflow ($STKOFEN) calls. */
270 long hits; /* Number of internal buffer hits. */
271 long extends; /* Number of block extensions. */
272 long stko_mallocs; /* Block allocations by $STKOFEN. */
273 long underflows; /* Number of stack underflow calls ($STKRETN). */
274 long stko_free; /* Number of deallocations by $STKRETN. */
275 long stkm_free; /* Number of deallocations by $STKMRET. */
276 long segments; /* Current number of stack segments. */
277 long maxs; /* Maximum number of stack segments so far. */
278 long pad_size; /* Stack pad size. */
279 long current_address; /* Current stack segment address. */
280 long current_size; /* Current stack segment size. This
281 number is actually corrupted by STKSTAT to
282 include the fifteen word trailer area. */
283 long initial_address; /* Address of initial segment. */
284 long initial_size; /* Size of initial segment. */
287 /* The following structure describes the data structure which trails
288 any stack segment. I think that the description in 'asdef' is
289 out of date. I only describe the parts that I am sure about. */
293 long this_address; /* Address of this block. */
294 long this_size; /* Size of this block (does not include
298 long link; /* Address of trailer block of previous
313 #endif /* not CRAY_STACK */
316 /* Determine a "stack measure" for an arbitrary ADDRESS.
317 I doubt that "lint" will like this much. */
320 i00afunc (long *address)
322 struct stk_stat status;
323 struct stk_trailer *trailer;
327 /* We want to iterate through all of the segments. The first
328 step is to get the stack status structure. We could do this
329 more quickly and more directly, perhaps, by referencing the
330 $LM00 common block, but I know that this works. */
334 /* Set up the iteration. */
336 trailer = (struct stk_trailer *) (status.current_address
337 + status.current_size
340 /* There must be at least one stack segment. Therefore it is
341 a fatal error if "trailer" is null. */
346 /* Discard segments that do not contain our argument address. */
350 block = (long *) trailer->this_address;
351 size = trailer->this_size;
352 if (block == 0 || size == 0)
354 trailer = (struct stk_trailer *) trailer->link;
355 if ((block <= address) && (address < (block + size)))
359 /* Set the result to the offset in this segment and add the sizes
360 of all predecessor segments. */
362 result = address - block;
371 if (trailer->this_size <= 0)
373 result += trailer->this_size;
374 trailer = (struct stk_trailer *) trailer->link;
376 while (trailer != 0);
378 /* We are done. Note that if you present a bogus address (one
379 not in any segment), you will get a different number back, formed
380 from subtracting the address of the first block. This is probably
381 not what you want. */
386 #else /* not CRAY2 */
387 /* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
388 Determine the number of the cell within the stack,
389 given the address of the cell. The purpose of this
390 routine is to linearize, in some sense, stack addresses
394 i00afunc (long address)
398 long size, pseg, this_segment, stack;
401 struct stack_segment_linkage *ssptr;
403 /* Register B67 contains the address of the end of the
404 current stack segment. If you (as a subprogram) store
405 your registers on the stack and find that you are past
406 the contents of B67, you have overflowed the segment.
408 B67 also points to the stack segment linkage control
409 area, which is what we are really interested in. */
411 stkl = CRAY_STACKSEG_END ();
412 ssptr = (struct stack_segment_linkage *) stkl;
414 /* If one subtracts 'size' from the end of the segment,
415 one has the address of the first word of the segment.
417 If this is not the first segment, 'pseg' will be
420 pseg = ssptr->sspseg;
421 size = ssptr->sssize;
423 this_segment = stkl - size;
425 /* It is possible that calling this routine itself caused
426 a stack overflow. Discard stack segments which do not
427 contain the target address. */
429 while (!(this_segment <= address && address <= stkl))
431 #ifdef DEBUG_I00AFUNC
432 fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl);
437 ssptr = (struct stack_segment_linkage *) stkl;
438 size = ssptr->sssize;
439 pseg = ssptr->sspseg;
440 this_segment = stkl - size;
443 result = address - this_segment;
445 /* If you subtract pseg from the current end of the stack,
446 you get the address of the previous stack segment's end.
447 This seems a little convoluted to me, but I'll bet you save
448 a cycle somewhere. */
452 #ifdef DEBUG_I00AFUNC
453 fprintf (stderr, "%011o %011o\n", pseg, size);
456 ssptr = (struct stack_segment_linkage *) stkl;
457 size = ssptr->sssize;
458 pseg = ssptr->sspseg;
464 #endif /* not CRAY2 */
467 #endif /* no alloca */