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 */
45 /* If your stack is a linked list of frames, you have to
46 provide an "address metric" ADDRESS_FUNCTION macro. */
50 #define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
52 #define ADDRESS_FUNCTION(arg) &(arg)
56 typedef void *pointer;
58 typedef char *pointer;
63 /* Define STACK_DIRECTION if you know the direction of stack
64 growth for your system; otherwise it will be automatically
67 STACK_DIRECTION > 0 => grows toward higher addresses
68 STACK_DIRECTION < 0 => grows toward lower addresses
69 STACK_DIRECTION = 0 => direction of growth unknown */
71 #ifndef STACK_DIRECTION
72 #define STACK_DIRECTION 0 /* Direction unknown. */
75 #if STACK_DIRECTION != 0
77 #define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
79 #else /* STACK_DIRECTION == 0; need run-time code. */
81 static int stack_dir; /* 1 or -1 once known. */
82 #define STACK_DIR stack_dir
85 find_stack_direction ()
87 static char *addr = NULL; /* Address of first `dummy', once known. */
88 auto char dummy; /* To get stack address. */
91 { /* Initial entry. */
92 addr = ADDRESS_FUNCTION (dummy);
94 find_stack_direction (); /* Recurse once. */
99 if (ADDRESS_FUNCTION (dummy) > addr)
100 stack_dir = 1; /* Stack grew upward. */
102 stack_dir = -1; /* Stack grew downward. */
106 #endif /* STACK_DIRECTION == 0 */
108 /* An "alloca header" is used to:
109 (a) chain together all alloca'ed blocks;
110 (b) keep track of stack depth.
112 It is very important that sizeof(header) agree with malloc
113 alignment chunk size. The following default should work okay. */
116 #define ALIGN_SIZE sizeof(double)
121 char align[ALIGN_SIZE]; /* To force sizeof(header). */
124 union hdr *next; /* For chaining headers. */
125 char *deep; /* For stack depth measure. */
129 static header *last_alloca_header = NULL; /* -> last alloca header. */
131 /* Return a pointer to at least SIZE bytes of storage,
132 which will be automatically reclaimed upon exit from
133 the procedure that called alloca. Originally, this space
134 was supposed to be taken from the current stack frame of the
135 caller, but that method cannot be made to work for some
136 implementations of C, for example under Gould's UTX/32. */
142 auto char probe; /* Probes stack depth: */
143 register char *depth = ADDRESS_FUNCTION (probe);
145 #if STACK_DIRECTION == 0
146 if (STACK_DIR == 0) /* Unknown growth direction. */
147 find_stack_direction ();
150 /* Reclaim garbage, defined as all alloca'd storage that
151 was allocated from deeper in the stack than currently. */
154 register header *hp; /* Traverses linked list. */
156 for (hp = last_alloca_header; hp != NULL;)
157 if ((STACK_DIR > 0 && hp->h.deep > depth)
158 || (STACK_DIR < 0 && hp->h.deep < depth))
160 register header *np = hp->h.next;
162 free ((pointer) hp); /* Collect garbage. */
164 hp = np; /* -> next header. */
167 break; /* Rest are not deeper. */
169 last_alloca_header = hp; /* -> last valid storage. */
173 return NULL; /* No allocation required. */
175 /* Allocate combined header + user data storage. */
178 register pointer new = malloc (sizeof (header) + size);
179 /* Address of header. */
181 ((header *) new)->h.next = last_alloca_header;
182 ((header *) new)->h.deep = depth;
184 last_alloca_header = (header *) new;
186 /* User storage begins just after header. */
188 return (pointer) ((char *) new + sizeof (header));
194 #ifdef DEBUG_I00AFUNC
201 /* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
202 struct stack_control_header
204 long shgrow:32; /* Number of times stack has grown. */
205 long shaseg:32; /* Size of increments to stack. */
206 long shhwm:32; /* High water mark of stack. */
207 long shsize:32; /* Current size of stack (all segments). */
210 /* The stack segment linkage control information occurs at
211 the high-address end of a stack segment. (The stack
212 grows from low addresses to high addresses.) The initial
213 part of the stack segment linkage control information is
214 0200 (octal) words. This provides for register storage
215 for the routine which overflows the stack. */
217 struct stack_segment_linkage
219 long ss[0200]; /* 0200 overflow words. */
220 long sssize:32; /* Number of words in this segment. */
221 long ssbase:32; /* Offset to stack base. */
223 long sspseg:32; /* Offset to linkage control of previous
226 long sstcpt:32; /* Pointer to task common address block. */
227 long sscsnm; /* Private control structure number for
229 long ssusr1; /* Reserved for user. */
230 long ssusr2; /* Reserved for user. */
231 long sstpid; /* Process ID for pid based multi-tasking. */
232 long ssgvup; /* Pointer to multitasking thread giveup. */
233 long sscray[7]; /* Reserved for Cray Research. */
253 /* The following structure defines the vector of words
254 returned by the STKSTAT library routine. */
257 long now; /* Current total stack size. */
258 long maxc; /* Amount of contiguous space which would
259 be required to satisfy the maximum
260 stack demand to date. */
261 long high_water; /* Stack high-water mark. */
262 long overflows; /* Number of stack overflow ($STKOFEN) calls. */
263 long hits; /* Number of internal buffer hits. */
264 long extends; /* Number of block extensions. */
265 long stko_mallocs; /* Block allocations by $STKOFEN. */
266 long underflows; /* Number of stack underflow calls ($STKRETN). */
267 long stko_free; /* Number of deallocations by $STKRETN. */
268 long stkm_free; /* Number of deallocations by $STKMRET. */
269 long segments; /* Current number of stack segments. */
270 long maxs; /* Maximum number of stack segments so far. */
271 long pad_size; /* Stack pad size. */
272 long current_address; /* Current stack segment address. */
273 long current_size; /* Current stack segment size. This
274 number is actually corrupted by STKSTAT to
275 include the fifteen word trailer area. */
276 long initial_address; /* Address of initial segment. */
277 long initial_size; /* Size of initial segment. */
280 /* The following structure describes the data structure which trails
281 any stack segment. I think that the description in 'asdef' is
282 out of date. I only describe the parts that I am sure about. */
286 long this_address; /* Address of this block. */
287 long this_size; /* Size of this block (does not include
291 long link; /* Address of trailer block of previous
306 #endif /* not CRAY_STACK */
309 /* Determine a "stack measure" for an arbitrary ADDRESS.
310 I doubt that "lint" will like this much. */
313 i00afunc (long *address)
315 struct stk_stat status;
316 struct stk_trailer *trailer;
320 /* We want to iterate through all of the segments. The first
321 step is to get the stack status structure. We could do this
322 more quickly and more directly, perhaps, by referencing the
323 $LM00 common block, but I know that this works. */
327 /* Set up the iteration. */
329 trailer = (struct stk_trailer *) (status.current_address
330 + status.current_size
333 /* There must be at least one stack segment. Therefore it is
334 a fatal error if "trailer" is null. */
339 /* Discard segments that do not contain our argument address. */
343 block = (long *) trailer->this_address;
344 size = trailer->this_size;
345 if (block == 0 || size == 0)
347 trailer = (struct stk_trailer *) trailer->link;
348 if ((block <= address) && (address < (block + size)))
352 /* Set the result to the offset in this segment and add the sizes
353 of all predecessor segments. */
355 result = address - block;
364 if (trailer->this_size <= 0)
366 result += trailer->this_size;
367 trailer = (struct stk_trailer *) trailer->link;
369 while (trailer != 0);
371 /* We are done. Note that if you present a bogus address (one
372 not in any segment), you will get a different number back, formed
373 from subtracting the address of the first block. This is probably
374 not what you want. */
379 #else /* not CRAY2 */
380 /* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
381 Determine the number of the cell within the stack,
382 given the address of the cell. The purpose of this
383 routine is to linearize, in some sense, stack addresses
387 i00afunc (long address)
391 long size, pseg, this_segment, stack;
394 struct stack_segment_linkage *ssptr;
396 /* Register B67 contains the address of the end of the
397 current stack segment. If you (as a subprogram) store
398 your registers on the stack and find that you are past
399 the contents of B67, you have overflowed the segment.
401 B67 also points to the stack segment linkage control
402 area, which is what we are really interested in. */
404 stkl = CRAY_STACKSEG_END ();
405 ssptr = (struct stack_segment_linkage *) stkl;
407 /* If one subtracts 'size' from the end of the segment,
408 one has the address of the first word of the segment.
410 If this is not the first segment, 'pseg' will be
413 pseg = ssptr->sspseg;
414 size = ssptr->sssize;
416 this_segment = stkl - size;
418 /* It is possible that calling this routine itself caused
419 a stack overflow. Discard stack segments which do not
420 contain the target address. */
422 while (!(this_segment <= address && address <= stkl))
424 #ifdef DEBUG_I00AFUNC
425 fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl);
430 ssptr = (struct stack_segment_linkage *) stkl;
431 size = ssptr->sssize;
432 pseg = ssptr->sspseg;
433 this_segment = stkl - size;
436 result = address - this_segment;
438 /* If you subtract pseg from the current end of the stack,
439 you get the address of the previous stack segment's end.
440 This seems a little convoluted to me, but I'll bet you save
441 a cycle somewhere. */
445 #ifdef DEBUG_I00AFUNC
446 fprintf (stderr, "%011o %011o\n", pseg, size);
449 ssptr = (struct stack_segment_linkage *) stkl;
450 size = ssptr->sssize;
451 pseg = ssptr->sspseg;
457 #endif /* not CRAY2 */
460 #endif /* no alloca */