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ad787160 C |
1 | /* |
2 | alloca -- (mostly) portable public-domain implementation -- D A Gwyn | |
3 | ||
4 | last edit: 86/05/30 rms | |
5 | include config.h, since on VMS it renames some symbols. | |
6 | Use xmalloc instead of malloc. | |
7 | ||
8 | This implementation of the PWB library alloca() function, | |
9 | which is used to allocate space off the run-time stack so | |
10 | that it is automatically reclaimed upon procedure exit, | |
11 | was inspired by discussions with J. Q. Johnson of Cornell. | |
12 | ||
13 | It should work under any C implementation that uses an | |
14 | actual procedure stack (as opposed to a linked list of | |
15 | frames). There are some preprocessor constants that can | |
16 | be defined when compiling for your specific system, for | |
17 | improved efficiency; however, the defaults should be okay. | |
18 | ||
19 | The general concept of this implementation is to keep | |
20 | track of all alloca()-allocated blocks, and reclaim any | |
21 | that are found to be deeper in the stack than the current | |
22 | invocation. This heuristic does not reclaim storage as | |
23 | soon as it becomes invalid, but it will do so eventually. | |
24 | ||
25 | As a special case, alloca(0) reclaims storage without | |
26 | allocating any. It is a good idea to use alloca(0) in | |
27 | your main control loop, etc. to force garbage collection. | |
28 | */ | |
29 | #define xmalloc malloc | |
30 | ||
31 | #ifndef lint | |
32 | static char SCCSid[] = "@(#)alloca.c 1.1"; /* for the "what" utility */ | |
33 | #endif | |
34 | ||
35 | #ifdef emacs | |
36 | #include "config.h" | |
37 | #ifdef static | |
38 | /* actually, only want this if static is defined as "" | |
39 | -- this is for usg, in which emacs must undefine static | |
40 | in order to make unexec workable | |
41 | */ | |
42 | #ifndef STACK_DIRECTION | |
43 | you | |
44 | lose | |
45 | -- must know STACK_DIRECTION at compile-time | |
46 | #endif /* STACK_DIRECTION undefined */ | |
47 | #endif /* static */ | |
48 | #endif /* emacs */ | |
49 | ||
50 | #ifndef ADDRESS_FUNCTION | |
51 | #define ADDRESS_FUNCTION(arg) &arg | |
52 | #endif | |
53 | ||
54 | #ifndef alloca /* If compiling with GCC, this file's not needed. */ | |
55 | ||
56 | #ifdef __STDC__ | |
57 | typedef void *pointer; /* generic pointer type */ | |
58 | #else | |
59 | typedef char *pointer; /* generic pointer type */ | |
60 | #endif | |
61 | ||
62 | #define NULL 0 /* null pointer constant */ | |
63 | ||
64 | extern void free(); | |
65 | extern pointer xmalloc(); | |
66 | ||
67 | /* | |
68 | Define STACK_DIRECTION if you know the direction of stack | |
69 | growth for your system; otherwise it will be automatically | |
70 | deduced at run-time. | |
71 | ||
72 | STACK_DIRECTION > 0 => grows toward higher addresses | |
73 | STACK_DIRECTION < 0 => grows toward lower addresses | |
74 | STACK_DIRECTION = 0 => direction of growth unknown | |
75 | */ | |
76 | ||
77 | #ifndef STACK_DIRECTION | |
78 | #define STACK_DIRECTION 0 /* direction unknown */ | |
79 | #endif | |
80 | ||
81 | #if STACK_DIRECTION != 0 | |
82 | ||
83 | #define STACK_DIR STACK_DIRECTION /* known at compile-time */ | |
84 | ||
85 | #else /* STACK_DIRECTION == 0; need run-time code */ | |
86 | ||
87 | static int stack_dir; /* 1 or -1 once known */ | |
88 | #define STACK_DIR stack_dir | |
89 | ||
90 | static void | |
91 | find_stack_direction (/* void */) | |
92 | { | |
93 | static char *addr = NULL; /* address of first | |
94 | `dummy', once known */ | |
95 | auto char dummy; /* to get stack address */ | |
96 | ||
97 | if (addr == NULL) | |
98 | { /* initial entry */ | |
99 | addr = ADDRESS_FUNCTION(dummy); | |
100 | ||
101 | find_stack_direction (); /* recurse once */ | |
102 | } | |
103 | else /* second entry */ | |
104 | if (ADDRESS_FUNCTION(dummy) > addr) | |
105 | stack_dir = 1; /* stack grew upward */ | |
106 | else | |
107 | stack_dir = -1; /* stack grew downward */ | |
108 | } | |
109 | ||
110 | #endif /* STACK_DIRECTION == 0 */ | |
111 | ||
112 | /* | |
113 | An "alloca header" is used to: | |
114 | (a) chain together all alloca()ed blocks; | |
115 | (b) keep track of stack depth. | |
116 | ||
117 | It is very important that sizeof(header) agree with malloc() | |
118 | alignment chunk size. The following default should work okay. | |
119 | */ | |
120 | ||
121 | #ifndef ALIGN_SIZE | |
122 | #define ALIGN_SIZE sizeof(double) | |
123 | #endif | |
124 | ||
125 | typedef union hdr | |
126 | { | |
127 | char align[ALIGN_SIZE]; /* to force sizeof(header) */ | |
128 | struct | |
129 | { | |
130 | union hdr *next; /* for chaining headers */ | |
131 | char *deep; /* for stack depth measure */ | |
132 | } h; | |
133 | } header; | |
134 | ||
135 | /* | |
136 | alloca( size ) returns a pointer to at least `size' bytes of | |
137 | storage which will be automatically reclaimed upon exit from | |
138 | the procedure that called alloca(). Originally, this space | |
139 | was supposed to be taken from the current stack frame of the | |
140 | caller, but that method cannot be made to work for some | |
141 | implementations of C, for example under Gould's UTX/32. | |
142 | */ | |
143 | ||
144 | static header *last_alloca_header = NULL; /* -> last alloca header */ | |
145 | ||
146 | pointer | |
147 | alloca (size) /* returns pointer to storage */ | |
148 | unsigned size; /* # bytes to allocate */ | |
149 | { | |
150 | auto char probe; /* probes stack depth: */ | |
151 | register char *depth = (char *) ADDRESS_FUNCTION(probe); | |
152 | ||
153 | #if STACK_DIRECTION == 0 | |
154 | if (STACK_DIR == 0) /* unknown growth direction */ | |
155 | find_stack_direction (); | |
156 | #endif | |
157 | ||
158 | /* Reclaim garbage, defined as all alloca()ed storage that | |
159 | was allocated from deeper in the stack than currently. */ | |
160 | ||
161 | { | |
162 | register header *hp; /* traverses linked list */ | |
163 | ||
164 | for (hp = last_alloca_header; hp != NULL;) | |
165 | if ( (STACK_DIR > 0 && hp->h.deep > depth) | |
166 | || (STACK_DIR < 0 && hp->h.deep < depth) ) | |
167 | { | |
168 | register header *np = hp->h.next; | |
169 | ||
170 | free ((pointer) hp); /* collect garbage */ | |
171 | ||
172 | hp = np; /* -> next header */ | |
173 | } | |
174 | else | |
175 | break; /* rest are not deeper */ | |
176 | ||
177 | last_alloca_header = hp; /* -> last valid storage */ | |
178 | } | |
179 | ||
180 | if (size == 0) | |
181 | return NULL; /* no allocation required */ | |
182 | ||
183 | /* Allocate combined header + user data storage. */ | |
184 | ||
185 | { | |
186 | register pointer new = xmalloc (sizeof (header) + size); | |
187 | /* address of header */ | |
188 | ||
189 | ((header *)new)->h.next = last_alloca_header; | |
190 | ((header *)new)->h.deep = depth; | |
191 | ||
192 | last_alloca_header = (header *)new; | |
193 | ||
194 | /* User storage begins just after header. */ | |
195 | ||
196 | return (pointer)((char *)new + sizeof(header)); | |
197 | } | |
198 | } | |
199 | ||
200 | #endif /* no alloca */ |