2 alloca -- (mostly) portable public-domain implementation -- D A Gwyn
4 edited 8/22/95, 2/28/96, 3/28/96 by BLP for PSPP
7 include config.h, since on VMS it renames some symbols.
8 Use xmalloc instead of malloc.
10 This implementation of the PWB library alloca() function,
11 which is used to allocate space off the run-time stack so
12 that it is automatically reclaimed upon procedure exit,
13 was inspired by discussions with J. Q. Johnson of Cornell.
15 It should work under any C implementation that uses an
16 actual procedure stack (as opposed to a linked list of
17 frames). There are some preprocessor constants that can
18 be defined when compiling for your specific system, for
19 improved efficiency; however, the defaults should be okay.
21 The general concept of this implementation is to keep
22 track of all alloca()-allocated blocks, and reclaim any
23 that are found to be deeper in the stack than the current
24 invocation. This heuristic does not reclaim storage as
25 soon as it becomes invalid, but it will do so eventually.
27 As a special case, alloca(0) reclaims storage without
28 allocating any. It is a good idea to use alloca(0) in
29 your main control loop, etc. to force garbage collection.
38 typedef void *pointer; /* generic pointer type */
39 #define NULL 0 /* null pointer constant */
42 extern pointer xmalloc ();
45 Define STACK_DIRECTION if you know the direction of stack
46 growth for your system; otherwise it will be automatically
49 STACK_DIRECTION > 0 => grows toward higher addresses
50 STACK_DIRECTION < 0 => grows toward lower addresses
51 STACK_DIRECTION = 0 => direction of growth unknown
54 #ifndef STACK_DIRECTION
55 #define STACK_DIRECTION 0 /* direction unknown */
58 #if STACK_DIRECTION != 0
60 #define STACK_DIR STACK_DIRECTION /* known at compile-time */
62 #else /* STACK_DIRECTION == 0; need run-time code */
64 static int stack_dir; /* 1 or -1 once known */
65 #define STACK_DIR stack_dir
68 find_stack_direction (void)
70 static char *addr = NULL; /* address of first
71 `dummy', once known */
72 auto char dummy; /* to get stack address */
78 find_stack_direction (); /* recurse once */
81 /* second entry */ if (&dummy > addr)
82 stack_dir = 1; /* stack grew upward */
84 stack_dir = -1; /* stack grew downward */
87 #endif /* STACK_DIRECTION == 0 */
90 An "alloca header" is used to:
91 (a) chain together all alloca()ed blocks;
92 (b) keep track of stack depth.
94 PORTME: It is very important that sizeof(header) agree with
95 malloc() alignment chunk size. The following default should
99 #define ALIGN_SIZE sizeof(double)
104 char align[ALIGN_SIZE]; /* to force sizeof(header) */
107 union hdr *next; /* for chaining headers */
108 char *deep; /* for stack depth measure */
115 alloca( size ) returns a pointer to at least `size' bytes of
116 storage which will be automatically reclaimed upon exit from
117 the procedure that called alloca(). Originally, this space
118 was supposed to be taken from the current stack frame of the
119 caller, but that method cannot be made to work for some
120 implementations of C, for example under Gould's UTX/32.
123 static header *last_alloca_header = NULL; /* -> last alloca header */
126 alloca (unsigned size) /* returns pointer to storage */
128 auto char probe; /* probes stack depth: */
129 register char *depth = &probe;
131 #if STACK_DIRECTION == 0
132 if (STACK_DIR == 0) /* unknown growth direction */
133 find_stack_direction ();
136 /* Reclaim garbage, defined as all alloca()ed storage that
137 was allocated from deeper in the stack than currently. */
140 register header *hp; /* traverses linked list */
142 for (hp = last_alloca_header; hp != NULL;)
143 if (STACK_DIR > 0 && hp->h.deep > depth
144 || STACK_DIR < 0 && hp->h.deep < depth)
146 register header *np = hp->h.next;
148 free ((pointer) hp); /* collect garbage */
150 hp = np; /* -> next header */
153 break; /* rest are not deeper */
155 last_alloca_header = hp; /* -> last valid storage */
159 return NULL; /* no allocation required */
161 /* Allocate combined header + user data storage. */
164 register pointer new = xmalloc (sizeof (header) + size);
165 /* address of header */
167 ((header *) new)->h.next = last_alloca_header;
168 ((header *) new)->h.deep = depth;
170 last_alloca_header = (header *) new;
172 /* User storage begins just after header. */
174 return (pointer) ((char *) new + sizeof (header));
178 #endif /* !__GNUC__ && !__BORLANDC__ */