e91b87a3 |
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 | #ifndef lint |
30 | static char SCCSid[] = "@(#)alloca.c 1.1"; /* for the "what" utility */ |
31 | #endif |
32 | |
33 | #ifdef emacs |
34 | #include "config.h" |
35 | #ifdef static |
36 | /* actually, only want this if static is defined as "" |
37 | -- this is for usg, in which emacs must undefine static |
38 | in order to make unexec workable |
39 | */ |
40 | #ifndef STACK_DIRECTION |
41 | you |
42 | lose |
43 | -- must know STACK_DIRECTION at compile-time |
44 | #endif /* STACK_DIRECTION undefined */ |
45 | #endif static |
46 | #endif emacs |
47 | |
48 | #ifdef X3J11 |
49 | typedef void *pointer; /* generic pointer type */ |
50 | #else |
51 | typedef char *pointer; /* generic pointer type */ |
52 | #endif |
53 | |
54 | #define NULL 0 /* null pointer constant */ |
55 | |
56 | extern void free(); |
57 | extern pointer xmalloc(); |
58 | |
59 | /* |
60 | Define STACK_DIRECTION if you know the direction of stack |
61 | growth for your system; otherwise it will be automatically |
62 | deduced at run-time. |
63 | |
64 | STACK_DIRECTION > 0 => grows toward higher addresses |
65 | STACK_DIRECTION < 0 => grows toward lower addresses |
66 | STACK_DIRECTION = 0 => direction of growth unknown |
67 | */ |
68 | |
69 | #ifndef STACK_DIRECTION |
70 | #define STACK_DIRECTION 0 /* direction unknown */ |
71 | #endif |
72 | |
73 | #if STACK_DIRECTION != 0 |
74 | |
75 | #define STACK_DIR STACK_DIRECTION /* known at compile-time */ |
76 | |
77 | #else /* STACK_DIRECTION == 0; need run-time code */ |
78 | |
79 | static int stack_dir; /* 1 or -1 once known */ |
80 | #define STACK_DIR stack_dir |
81 | |
82 | static void |
83 | find_stack_direction (/* void */) |
84 | { |
85 | static char *addr = NULL; /* address of first |
86 | `dummy', once known */ |
87 | auto char dummy; /* to get stack address */ |
88 | |
89 | if (addr == NULL) |
90 | { /* initial entry */ |
91 | addr = &dummy; |
92 | |
93 | find_stack_direction (); /* recurse once */ |
94 | } |
95 | else /* second entry */ |
96 | if (&dummy > addr) |
97 | stack_dir = 1; /* stack grew upward */ |
98 | else |
99 | stack_dir = -1; /* stack grew downward */ |
100 | } |
101 | |
102 | #endif /* STACK_DIRECTION == 0 */ |
103 | |
104 | /* |
105 | An "alloca header" is used to: |
106 | (a) chain together all alloca()ed blocks; |
107 | (b) keep track of stack depth. |
108 | |
109 | It is very important that sizeof(header) agree with malloc() |
110 | alignment chunk size. The following default should work okay. |
111 | */ |
112 | |
113 | #ifndef ALIGN_SIZE |
114 | #define ALIGN_SIZE sizeof(double) |
115 | #endif |
116 | |
117 | typedef union hdr |
118 | { |
119 | char align[ALIGN_SIZE]; /* to force sizeof(header) */ |
120 | struct |
121 | { |
122 | union hdr *next; /* for chaining headers */ |
123 | char *deep; /* for stack depth measure */ |
124 | } h; |
125 | } header; |
126 | |
127 | /* |
128 | alloca( size ) returns a pointer to at least `size' bytes of |
129 | storage which will be automatically reclaimed upon exit from |
130 | the procedure that called alloca(). Originally, this space |
131 | was supposed to be taken from the current stack frame of the |
132 | caller, but that method cannot be made to work for some |
133 | implementations of C, for example under Gould's UTX/32. |
134 | */ |
135 | |
136 | static header *last_alloca_header = NULL; /* -> last alloca header */ |
137 | |
138 | pointer |
139 | alloca (size) /* returns pointer to storage */ |
140 | unsigned size; /* # bytes to allocate */ |
141 | { |
142 | auto char probe; /* probes stack depth: */ |
143 | register char *depth = &probe; |
144 | |
145 | #if STACK_DIRECTION == 0 |
146 | if (STACK_DIR == 0) /* unknown growth direction */ |
147 | find_stack_direction (); |
148 | #endif |
149 | |
150 | /* Reclaim garbage, defined as all alloca()ed storage that |
151 | was allocated from deeper in the stack than currently. */ |
152 | |
153 | { |
154 | register header *hp; /* traverses linked list */ |
155 | |
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) |
159 | { |
160 | register header *np = hp->h.next; |
161 | |
162 | free ((pointer) hp); /* collect garbage */ |
163 | |
164 | hp = np; /* -> next header */ |
165 | } |
166 | else |
167 | break; /* rest are not deeper */ |
168 | |
169 | last_alloca_header = hp; /* -> last valid storage */ |
170 | } |
171 | |
172 | if (size == 0) |
173 | return NULL; /* no allocation required */ |
174 | |
175 | /* Allocate combined header + user data storage. */ |
176 | |
177 | { |
178 | register pointer new = xmalloc (sizeof (header) + size); |
179 | /* address of header */ |
180 | |
181 | ((header *)new)->h.next = last_alloca_header; |
182 | ((header *)new)->h.deep = depth; |
183 | |
184 | last_alloca_header = (header *)new; |
185 | |
186 | /* User storage begins just after header. */ |
187 | |
188 | return (pointer)((char *)new + sizeof(header)); |
189 | } |
190 | } |
191 | |