| 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 | |