1 /* alloca -- (mostly) portable public-domain implementation -- D A Gwyn
2
3 last edit: 86/05/30 rms
4 include config.h, since on VMS it renames some symbols.
5 Use xmalloc instead of malloc.
6
7 This implementation of the PWB library alloca() function,
8 which is used to allocate space off the run-time stack so
9 that it is automatically reclaimed upon procedure exit,
10 was inspired by discussions with J. Q. Johnson of Cornell.
11
12 It should work under any C implementation that uses an
13 actual procedure stack (as opposed to a linked list of
14 frames). There are some preprocessor constants that can
15 be defined when compiling for your specific system, for
16 improved efficiency; however, the defaults should be okay.
17
18 The general concept of this implementation is to keep
19 track of all alloca()-allocated blocks, and reclaim any
20 that are found to be deeper in the stack than the current
21 invocation. This heuristic does not reclaim storage as
22 soon as it becomes invalid, but it will do so eventually.
23
24 As a special case, alloca(0) reclaims storage without
25 allocating any. It is a good idea to use alloca(0) in
26 your main control loop, etc. to force garbage collection.
27 */
28 #ifndef lint
29 static char SCCSid[] = "@(#)alloca.c 1.1"; /* for the "what" utility */
30 #endif
31
32 #include "config.h"
33 #ifdef emacs
34 #ifdef static
35 /* actually, only want this if static is defined as ""
36 -- this is for usg, in which emacs must undefine static
37 in order to make unexec workable
38 */
39 #ifndef STACK_DIRECTION
40 you
41 lose
42 -- must know STACK_DIRECTION at compile-time
43 #endif /* STACK_DIRECTION undefined */
44 #endif /* static */
45 #endif /* emacs */
46
47 #ifdef X3J11
48 typedef void *pointer; /* generic pointer type */
49 #else
50 typedef char *pointer; /* generic pointer type */
51 #endif /* X3J11 */
52
53 #define NULL 0 /* null pointer constant */
54
55 #ifdef RUBY_LIB
56 #define xmalloc ruby_xmalloc
57 #define xfree ruby_xfree
58 #endif
59
60 extern void xfree();
61 extern pointer xmalloc();
62
63 /*
64 Define STACK_DIRECTION if you know the direction of stack
65 growth for your system; otherwise it will be automatically
66 deduced at run-time.
67
68 STACK_DIRECTION > 0 => grows toward higher addresses
69 STACK_DIRECTION < 0 => grows toward lower addresses
70 STACK_DIRECTION = 0 => direction of growth unknown
71 */
72
73 #ifndef STACK_DIRECTION
74 #define STACK_DIRECTION 0 /* direction unknown */
75 #endif
76
77 #if STACK_DIRECTION != 0
78
79 #define STACK_DIR STACK_DIRECTION /* known at compile-time */
80
81 #else /* STACK_DIRECTION == 0; need run-time code */
82
83 static int stack_dir; /* 1 or -1 once known */
84 #define STACK_DIR stack_dir
85
86 static void
87 find_stack_direction (/* void */)
88 {
89 static char *addr = NULL; /* address of first
90 `dummy', once known */
91 auto char dummy; /* to get stack address */
92
93 if (addr == NULL)
94 { /* initial entry */
95 addr = &dummy;
96
97 find_stack_direction (); /* recurse once */
98 }
99 else /* second entry */
100 if (&dummy > addr)
101 stack_dir = 1; /* stack grew upward */
102 else
103 stack_dir = -1; /* stack grew downward */
104 }
105
106 #endif /* STACK_DIRECTION == 0 */
107
108 /*
109 An "alloca header" is used to:
110 (a) chain together all alloca()ed blocks;
111 (b) keep track of stack depth.
112
113 It is very important that sizeof(header) agree with malloc()
114 alignment chunk size. The following default should work okay.
115 */
116
117 #ifndef ALIGN_SIZE
118 #define ALIGN_SIZE sizeof(double)
119 #endif
120
121 typedef union hdr
122 {
123 char align[ALIGN_SIZE]; /* to force sizeof(header) */
124 struct
125 {
126 union hdr *next; /* for chaining headers */
127 char *deep; /* for stack depth measure */
128 } h;
129 } header;
130
131 /*
132 alloca( size ) returns a pointer to at least `size' bytes of
133 storage which will be automatically reclaimed upon exit from
134 the procedure that called alloca(). Originally, this space
135 was supposed to be taken from the current stack frame of the
136 caller, but that method cannot be made to work for some
137 implementations of C, for example under Gould's UTX/32.
138 */
139
140 static header *last_alloca_header = NULL; /* -> last alloca header */
141
142 pointer
143 alloca (size) /* returns pointer to storage */
144 unsigned size; /* # bytes to allocate */
145 {
146 auto char probe; /* probes stack depth: */
147 register char *depth = &probe;
148
149 #if STACK_DIRECTION == 0
150 if (STACK_DIR == 0) /* unknown growth direction */
151 find_stack_direction ();
152 #endif
153
154 /* Reclaim garbage, defined as all alloca()ed storage that
155 was allocated from deeper in the stack than currently. */
156 {
157 register header *hp; /* traverses linked list */
158
159 for (hp = last_alloca_header; hp != NULL;)
160 if (STACK_DIR > 0 && hp->h.deep > depth
161 || STACK_DIR < 0 && hp->h.deep < depth)
162 {
163 register header *np = hp->h.next;
164
165 xfree ((pointer) hp); /* collect garbage */
166
167 hp = np; /* -> next header */
168 }
169 else
170 break; /* rest are not deeper */
171
172 last_alloca_header = hp; /* -> last valid storage */
173 }
174
175 if (size == 0)
176 return NULL; /* no allocation required */
177
178 /* Allocate combined header + user data storage. */
179
180 {
181 register pointer new = xmalloc (sizeof (header) + size);
182 /* address of header */
183
184 ((header *)new)->h.next = last_alloca_header;
185 ((header *)new)->h.deep = depth;
186
187 last_alloca_header = (header *)new;
188
189 /* User storage begins just after header. */
190
191 return (pointer)((char *)new + sizeof(header));
192 }
193 }
194