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 2, this file's not needed. */
29 #if !defined (__GNUC__) || __GNUC__ < 2
31 /* If someone has defined alloca as a macro,
32 there must be some other way alloca is supposed to work. */
37 /* actually, only want this if static is defined as ""
38 -- this is for usg, in which emacs must undefine static
39 in order to make unexec workable
41 #ifndef STACK_DIRECTION
44 -- must know STACK_DIRECTION at compile-time
45 #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. */
52 #if defined (CRAY) && defined (CRAY_STACKSEG_END)
54 #define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
56 #define ADDRESS_FUNCTION(arg) &(arg)
60 typedef void *pointer;
62 typedef char *pointer;
67 /* Different portions of Emacs need to call different versions of
68 malloc. The Emacs executable needs alloca to call xmalloc, because
69 ordinary malloc isn't protected from input signals. On the other
70 hand, the utilities in lib-src need alloca to call malloc; some of
71 them are very simple, and don't have an xmalloc routine.
73 Non-Emacs programs expect this to call use xmalloc.
75 Callers below should use malloc. */
77 extern pointer malloc ();
79 /* Define STACK_DIRECTION if you know the direction of stack
80 growth for your system; otherwise it will be automatically
83 STACK_DIRECTION > 0 => grows toward higher addresses
84 STACK_DIRECTION < 0 => grows toward lower addresses
85 STACK_DIRECTION = 0 => direction of growth unknown */
87 #ifndef STACK_DIRECTION
88 #define STACK_DIRECTION 0 /* Direction unknown. */
91 #if STACK_DIRECTION != 0
93 #define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
95 #else /* STACK_DIRECTION == 0; need run-time code. */
97 static int stack_dir; /* 1 or -1 once known. */
98 #define STACK_DIR stack_dir
101 find_stack_direction ()
103 static char *addr = NULL; /* Address of first `dummy', once known. */
104 auto char dummy; /* To get stack address. */
107 { /* Initial entry. */
108 addr = ADDRESS_FUNCTION (dummy);
110 find_stack_direction (); /* Recurse once. */
115 if (ADDRESS_FUNCTION (dummy) > addr)
116 stack_dir = 1; /* Stack grew upward. */
118 stack_dir = -1; /* Stack grew downward. */
122 #endif /* STACK_DIRECTION == 0 */
124 /* An "alloca header" is used to:
125 (a) chain together all alloca'ed blocks;
126 (b) keep track of stack depth.
128 It is very important that sizeof(header) agree with malloc
129 alignment chunk size. The following default should work okay. */
132 #define ALIGN_SIZE SIZEOF(double)
137 char align[ALIGN_SIZE]; /* To force sizeof(header). */
140 union hdr *next; /* For chaining headers. */
141 char *deep; /* For stack depth measure. */
145 static header *last_alloca_header = NULL; /* -> last alloca header. */
147 /* Return a pointer to at least SIZE bytes of storage,
148 which will be automatically reclaimed upon exit from
149 the procedure that called alloca. Originally, this space
150 was supposed to be taken from the current stack frame of the
151 caller, but that method cannot be made to work for some
152 implementations of C, for example under Gould's UTX/32. */
158 auto char probe; /* Probes stack depth: */
159 register char *depth = ADDRESS_FUNCTION (probe);
161 #if STACK_DIRECTION == 0
162 if (STACK_DIR == 0) /* Unknown growth direction. */
163 find_stack_direction ();
166 /* Reclaim garbage, defined as all alloca'd storage that
167 was allocated from deeper in the stack than currently. */
170 register header *hp; /* Traverses linked list. */
172 for (hp = last_alloca_header; hp != NULL;)
173 if ((STACK_DIR > 0 && hp->h.deep > depth)
174 || (STACK_DIR < 0 && hp->h.deep < depth))
176 register header *np = hp->h.next;
178 free ((pointer) hp); /* Collect garbage. */
180 hp = np; /* -> next header. */
183 break; /* Rest are not deeper. */
185 last_alloca_header = hp; /* -> last valid storage. */
189 return NULL; /* No allocation required. */
191 /* Allocate combined header + user data storage. */
194 register pointer new = malloc (SIZEOF (header) + size);
195 /* Address of header. */
197 ((header *) new)->h.next = last_alloca_header;
198 ((header *) new)->h.deep = depth;
200 last_alloca_header = (header *) new;
202 /* User storage begins just after header. */
204 return (pointer) ((char *) new + SIZEOF (header));
208 #if defined (CRAY) && defined (CRAY_STACKSEG_END)
210 #ifdef DEBUG_I00AFUNC
217 /* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
218 struct stack_control_header
220 long shgrow:32; /* Number of times stack has grown. */
221 long shaseg:32; /* Size of increments to stack. */
222 long shhwm:32; /* High water mark of stack. */
223 long shsize:32; /* Current size of stack (all segments). */
226 /* The stack segment linkage control information occurs at
227 the high-address end of a stack segment. (The stack
228 grows from low addresses to high addresses.) The initial
229 part of the stack segment linkage control information is
230 0200 (octal) words. This provides for register storage
231 for the routine which overflows the stack. */
233 struct stack_segment_linkage
235 long ss[0200]; /* 0200 overflow words. */
236 long sssize:32; /* Number of words in this segment. */
237 long ssbase:32; /* Offset to stack base. */
239 long sspseg:32; /* Offset to linkage control of previous
242 long sstcpt:32; /* Pointer to task common address block. */
243 long sscsnm; /* Private control structure number for
245 long ssusr1; /* Reserved for user. */
246 long ssusr2; /* Reserved for user. */
247 long sstpid; /* Process ID for pid based multi-tasking. */
248 long ssgvup; /* Pointer to multitasking thread giveup. */
249 long sscray[7]; /* Reserved for Cray Research. */
269 /* The following structure defines the vector of words
270 returned by the STKSTAT library routine. */
273 long now; /* Current total stack size. */
274 long maxc; /* Amount of contiguous space which would
275 be required to satisfy the maximum
276 stack demand to date. */
277 long high_water; /* Stack high-water mark. */
278 long overflows; /* Number of stack overflow ($STKOFEN) calls. */
279 long hits; /* Number of internal buffer hits. */
280 long extends; /* Number of block extensions. */
281 long stko_mallocs; /* Block allocations by $STKOFEN. */
282 long underflows; /* Number of stack underflow calls ($STKRETN). */
283 long stko_free; /* Number of deallocations by $STKRETN. */
284 long stkm_free; /* Number of deallocations by $STKMRET. */
285 long segments; /* Current number of stack segments. */
286 long maxs; /* Maximum number of stack segments so far. */
287 long pad_size; /* Stack pad size. */
288 long current_address; /* Current stack segment address. */
289 long current_size; /* Current stack segment size. This
290 number is actually corrupted by STKSTAT to
291 include the fifteen word trailer area. */
292 long initial_address; /* Address of initial segment. */
293 long initial_size; /* Size of initial segment. */
296 /* The following structure describes the data structure which trails
297 any stack segment. I think that the description in 'asdef' is
298 out of date. I only describe the parts that I am sure about. */
302 long this_address; /* Address of this block. */
303 long this_size; /* Size of this block (does not include
307 long link; /* Address of trailer block of previous
322 #endif /* not CRAY_STACK */
325 /* Determine a "stack measure" for an arbitrary ADDRESS.
326 I doubt that "lint" will like this much. */
329 i00afunc (long *address)
331 struct stk_stat status;
332 struct stk_trailer *trailer;
336 /* We want to iterate through all of the segments. The first
337 step is to get the stack status structure. We could do this
338 more quickly and more directly, perhaps, by referencing the
339 $LM00 common block, but I know that this works. */
343 /* Set up the iteration. */
345 trailer = (struct stk_trailer *) (status.current_address
346 + status.current_size
349 /* There must be at least one stack segment. Therefore it is
350 a fatal error if "trailer" is null. */
355 /* Discard segments that do not contain our argument address. */
359 block = (long *) trailer->this_address;
360 size = trailer->this_size;
361 if (block == 0 || size == 0)
363 trailer = (struct stk_trailer *) trailer->link;
364 if ((block <= address) && (address < (block + size)))
368 /* Set the result to the offset in this segment and add the sizes
369 of all predecessor segments. */
371 result = address - block;
380 if (trailer->this_size <= 0)
382 result += trailer->this_size;
383 trailer = (struct stk_trailer *) trailer->link;
385 while (trailer != 0);
387 /* We are done. Note that if you present a bogus address (one
388 not in any segment), you will get a different number back, formed
389 from subtracting the address of the first block. This is probably
390 not what you want. */
395 #else /* not CRAY2 */
396 /* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
397 Determine the number of the cell within the stack,
398 given the address of the cell. The purpose of this
399 routine is to linearize, in some sense, stack addresses
403 i00afunc (long address)
407 long size, pseg, this_segment, stack;
410 struct stack_segment_linkage *ssptr;
412 /* Register B67 contains the address of the end of the
413 current stack segment. If you (as a subprogram) store
414 your registers on the stack and find that you are past
415 the contents of B67, you have overflowed the segment.
417 B67 also points to the stack segment linkage control
418 area, which is what we are really interested in. */
420 stkl = CRAY_STACKSEG_END ();
421 ssptr = (struct stack_segment_linkage *) stkl;
423 /* If one subtracts 'size' from the end of the segment,
424 one has the address of the first word of the segment.
426 If this is not the first segment, 'pseg' will be
429 pseg = ssptr->sspseg;
430 size = ssptr->sssize;
432 this_segment = stkl - size;
434 /* It is possible that calling this routine itself caused
435 a stack overflow. Discard stack segments which do not
436 contain the target address. */
438 while (!(this_segment <= address && address <= stkl))
440 #ifdef DEBUG_I00AFUNC
441 fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl);
446 ssptr = (struct stack_segment_linkage *) stkl;
447 size = ssptr->sssize;
448 pseg = ssptr->sspseg;
449 this_segment = stkl - size;
452 result = address - this_segment;
454 /* If you subtract pseg from the current end of the stack,
455 you get the address of the previous stack segment's end.
456 This seems a little convoluted to me, but I'll bet you save
457 a cycle somewhere. */
461 #ifdef DEBUG_I00AFUNC
462 fprintf (stderr, "%011o %011o\n", pseg, size);
465 ssptr = (struct stack_segment_linkage *) stkl;
466 size = ssptr->sssize;
467 pseg = ssptr->sspseg;
473 #endif /* not CRAY2 */
476 #endif /* no alloca */
477 #endif /* not GCC version 2 */
479 #endif /* ifndef HAVE_ALLOCA */