202 lines
5.5 KiB
C
202 lines
5.5 KiB
C
/* alloca.c -- allocate automatically reclaimed memory
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This file is in the public domain. */
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/* (Mostly) portable implementation -- D A Gwyn
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This implementation of the PWB library alloca function,
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which is used to allocate space off the run-time stack so
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that it is automatically reclaimed upon procedure exit,
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was inspired by discussions with J. Q. Johnson of Cornell.
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J.Otto Tennant <jot@cray.com> contributed the Cray support.
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There are some preprocessor constants that can
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be defined when compiling for your specific system, for
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improved efficiency; however, the defaults should be okay.
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The general concept of this implementation is to keep
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track of all alloca-allocated blocks, and reclaim any
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that are found to be deeper in the stack than the current
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invocation. This heuristic does not reclaim storage as
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soon as it becomes invalid, but it will do so eventually.
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As a special case, alloca(0) reclaims storage without
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allocating any. It is a good idea to use alloca(0) in
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your main control loop, etc. to force garbage collection. */
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#include <config.h>
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#include <alloca.h>
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#include <string.h>
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#include <stdlib.h>
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#ifdef emacs
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# include "lisp.h"
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# include "blockinput.h"
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# ifdef EMACS_FREE
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# undef free
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# define free EMACS_FREE
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# endif
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#else
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# define memory_full() abort ()
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#endif
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/* If compiling with GCC or clang, this file is not needed. */
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#if !(defined __GNUC__ || defined __clang__)
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/* If someone has defined alloca as a macro,
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there must be some other way alloca is supposed to work. */
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# ifndef alloca
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# ifdef emacs
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# ifdef static
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/* actually, only want this if static is defined as ""
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-- this is for usg, in which emacs must undefine static
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in order to make unexec workable
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*/
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# ifndef STACK_DIRECTION
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you
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lose
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-- must know STACK_DIRECTION at compile-time
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/* Using #error here is not wise since this file should work for
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old and obscure compilers. */
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# endif /* STACK_DIRECTION undefined */
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# endif /* static */
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# endif /* emacs */
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/* Define STACK_DIRECTION if you know the direction of stack
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growth for your system; otherwise it will be automatically
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deduced at run-time.
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STACK_DIRECTION > 0 => grows toward higher addresses
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STACK_DIRECTION < 0 => grows toward lower addresses
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STACK_DIRECTION = 0 => direction of growth unknown */
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# ifndef STACK_DIRECTION
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# define STACK_DIRECTION 0 /* Direction unknown. */
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# endif
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# if STACK_DIRECTION != 0
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# define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
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# else /* STACK_DIRECTION == 0; need run-time code. */
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static int stack_dir; /* 1 or -1 once known. */
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# define STACK_DIR stack_dir
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static int
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find_stack_direction (int *addr, int depth)
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{
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int dir, dummy = 0;
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if (! addr)
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addr = &dummy;
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*addr = addr < &dummy ? 1 : addr == &dummy ? 0 : -1;
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dir = depth ? find_stack_direction (addr, depth - 1) : 0;
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return dir + dummy;
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}
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# endif /* STACK_DIRECTION == 0 */
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/* An "alloca header" is used to:
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(a) chain together all alloca'ed blocks;
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(b) keep track of stack depth.
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It is very important that sizeof(header) agree with malloc
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alignment chunk size. The following default should work okay. */
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# ifndef ALIGN_SIZE
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# define ALIGN_SIZE sizeof(double)
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# endif
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typedef union hdr
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{
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char align[ALIGN_SIZE]; /* To force sizeof(header). */
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struct
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{
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union hdr *next; /* For chaining headers. */
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char *deep; /* For stack depth measure. */
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} h;
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} header;
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static header *last_alloca_header = NULL; /* -> last alloca header. */
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/* Return a pointer to at least SIZE bytes of storage,
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which will be automatically reclaimed upon exit from
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the procedure that called alloca. Originally, this space
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was supposed to be taken from the current stack frame of the
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caller, but that method cannot be made to work for some
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implementations of C, for example under Gould's UTX/32. */
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void *
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alloca (size_t size)
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{
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auto char probe; /* Probes stack depth: */
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register char *depth = &probe;
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# if STACK_DIRECTION == 0
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if (STACK_DIR == 0) /* Unknown growth direction. */
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STACK_DIR = find_stack_direction (NULL, (size & 1) + 20);
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# endif
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/* Reclaim garbage, defined as all alloca'd storage that
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was allocated from deeper in the stack than currently. */
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{
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register header *hp; /* Traverses linked list. */
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# ifdef emacs
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BLOCK_INPUT;
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# endif
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for (hp = last_alloca_header; hp != NULL;)
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if ((STACK_DIR > 0 && hp->h.deep > depth)
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|| (STACK_DIR < 0 && hp->h.deep < depth))
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{
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register header *np = hp->h.next;
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free (hp); /* Collect garbage. */
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hp = np; /* -> next header. */
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}
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else
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break; /* Rest are not deeper. */
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last_alloca_header = hp; /* -> last valid storage. */
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# ifdef emacs
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UNBLOCK_INPUT;
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# endif
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}
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if (size == 0)
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return NULL; /* No allocation required. */
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/* Allocate combined header + user data storage. */
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{
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/* Address of header. */
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register header *new;
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size_t combined_size = sizeof (header) + size;
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if (combined_size < sizeof (header))
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memory_full ();
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new = malloc (combined_size);
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if (! new)
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memory_full ();
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new->h.next = last_alloca_header;
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new->h.deep = depth;
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last_alloca_header = new;
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/* User storage begins just after header. */
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return (void *) (new + 1);
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}
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}
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# endif /* no alloca */
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#endif /* not GCC || clang */
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