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/* alloca -- (mostly) portable public-domain implementation -- D A Gwyn
last edit: 86/05/30 rms
include config.h, since on VMS it renames some symbols.
Use xmalloc instead of malloc.
This implementation of the PWB library alloca() function,
which is used to allocate space off the run-time stack so
that it is automatically reclaimed upon procedure exit,
was inspired by discussions with J. Q. Johnson of Cornell.
It should work under any C implementation that uses an
actual procedure stack (as opposed to a linked list of
frames). There are some preprocessor constants that can
be defined when compiling for your specific system, for
improved efficiency; however, the defaults should be okay.
The general concept of this implementation is to keep
track of all alloca()-allocated blocks, and reclaim any
that are found to be deeper in the stack than the current
invocation. This heuristic does not reclaim storage as
soon as it becomes invalid, but it will do so eventually.
As a special case, alloca(0) reclaims storage without
allocating any. It is a good idea to use alloca(0) in
your main control loop, etc. to force garbage collection.
#ifndef lint
static char SCCSid[] = "@(#)alloca.c 1.1"; /* for the "what" utility */
#include <sys/types.h>
#include "config.h"
#ifdef emacs
#ifdef static
/* actually, only want this if static is defined as ""
-- this is for usg, in which emacs must undefine static
in order to make unexec workable
-- must know STACK_DIRECTION at compile-time
#endif /* STACK_DIRECTION undefined */
#endif /* static */
#endif /* emacs */
typedef void *pointer; /* generic pointer type */
#define NULL 0 /* null pointer constant */
#ifdef RUBY_LIB
#define xmalloc ruby_xmalloc
#define xfree ruby_xfree
extern void xfree();
extern pointer xmalloc();
Define STACK_DIRECTION if you know the direction of stack
growth for your system; otherwise it will be automatically
deduced at run-time.
STACK_DIRECTION > 0 => grows toward higher addresses
STACK_DIRECTION < 0 => grows toward lower addresses
STACK_DIRECTION = 0 => direction of growth unknown
#define STACK_DIRECTION 0 /* direction unknown */
#define STACK_DIR STACK_DIRECTION /* known at compile-time */
#else /* STACK_DIRECTION == 0; need run-time code */
static int stack_dir; /* 1 or -1 once known */
#define STACK_DIR stack_dir
static void
find_stack_direction (/* void */)
static char *addr = NULL; /* address of first
`dummy', once known */
auto char dummy; /* to get stack address */
if (addr == NULL)
{ /* initial entry */
addr = &dummy;
find_stack_direction (); /* recurse once */
else /* second entry */
if (&dummy > addr)
stack_dir = 1; /* stack grew upward */
stack_dir = -1; /* stack grew downward */
#endif /* STACK_DIRECTION == 0 */
An "alloca header" is used to:
(a) chain together all alloca()ed blocks;
(b) keep track of stack depth.
It is very important that sizeof(header) agree with malloc()
alignment chunk size. The following default should work okay.
#ifndef ALIGN_SIZE
#define ALIGN_SIZE sizeof(double)
typedef union hdr
char align[ALIGN_SIZE]; /* to force sizeof(header) */
union hdr *next; /* for chaining headers */
char *deep; /* for stack depth measure */
} h;
} header;
alloca( size ) returns a pointer to at least `size' bytes of
storage which will be automatically reclaimed upon exit from
the procedure that called alloca(). Originally, this space
was supposed to be taken from the current stack frame of the
caller, but that method cannot be made to work for some
implementations of C, for example under Gould's UTX/32.
static header *last_alloca_header = NULL; /* -> last alloca header */
alloca (size_t size) /* returns pointer to storage */
auto char probe; /* probes stack depth: */
register char *depth = &probe;
if (STACK_DIR == 0) /* unknown growth direction */
find_stack_direction ();
/* Reclaim garbage, defined as all alloca()ed storage that
was allocated from deeper in the stack than currently. */
register header *hp; /* traverses linked list */
for (hp = last_alloca_header; hp != NULL;)
if (STACK_DIR > 0 && hp->h.deep > depth
|| STACK_DIR < 0 && hp->h.deep < depth)
register header *np = hp->;
xfree ((pointer) hp); /* collect garbage */
hp = np; /* -> next header */
break; /* rest are not deeper */
last_alloca_header = hp; /* -> last valid storage */
if (size == 0)
return NULL; /* no allocation required */
/* Allocate combined header + user data storage. */
register pointer new = xmalloc (sizeof (header) + size);
/* address of header */
((header *)new)-> = last_alloca_header;
((header *)new)->h.deep = depth;
last_alloca_header = (header *)new;
/* User storage begins just after header. */
return (pointer)((char *)new + sizeof(header));
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