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|
/* alloca.c -- allocate automatically reclaimed memory
(Mostly) portable public-domain implementation -- D A Gwyn
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.
J.Otto Tennant <jot@cray.com> contributed the Cray support.
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. */
/*
@deftypefn Replacement void* alloca (size_t @var{size})
This function allocates memory which will be automatically reclaimed
after the procedure exits. The @libib{} implementation does not free
the memory immediately but will do so eventually during subsequent
calls to this function. Memory is allocated using @code{xmalloc} under
normal circumstances.
The header file @file{alloca-conf.h} can be used in conjunction with the
GNU Autoconf test @code{AC_FUNC_ALLOCA} to test for and properly make
available this function. The @code{AC_FUNC_ALLOCA} test requires that
client code use a block of preprocessor code to be safe (see the Autoconf
manual for more); this header incorporates that logic and more, including
the possibility of a GCC built-in function.
@end deftypefn
*/
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
#include <libiberty.h>
#ifdef HAVE_STRING_H
#include <string.h>
#endif
#ifdef HAVE_STDLIB_H
#include <stdlib.h>
#endif
/* These variables are used by the ASTRDUP implementation that relies
on C_alloca. */
#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */
const char *libiberty_optr;
char *libiberty_nptr;
unsigned long libiberty_len;
#ifdef __cplusplus
}
#endif /* __cplusplus */
/* If your stack is a linked list of frames, you have to
provide an "address metric" ADDRESS_FUNCTION macro. */
#if defined (CRAY) && defined (CRAY_STACKSEG_END)
static long i00afunc ();
#define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
#else
#define ADDRESS_FUNCTION(arg) &(arg)
#endif
#ifndef NULL
#define NULL 0
#endif
/* 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 */
#ifndef STACK_DIRECTION
#define STACK_DIRECTION 0 /* Direction unknown. */
#endif
#if STACK_DIRECTION != 0
#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 = ADDRESS_FUNCTION (dummy);
find_stack_direction (); /* Recurse once. */
}
else
{
/* Second entry. */
if (ADDRESS_FUNCTION (dummy) > addr)
stack_dir = 1; /* Stack grew upward. */
else
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)
#endif
typedef union hdr
{
char align[ALIGN_SIZE]; /* To force sizeof(header). */
struct
{
union hdr *next; /* For chaining headers. */
char *deep; /* For stack depth measure. */
} h;
} header;
static header *last_alloca_header = NULL; /* -> last alloca header. */
/* Return 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. */
/* @undocumented C_alloca */
PTR
C_alloca (size_t size)
{
auto char probe; /* Probes stack depth: */
register char *depth = ADDRESS_FUNCTION (probe);
#if STACK_DIRECTION == 0
if (STACK_DIR == 0) /* Unknown growth direction. */
find_stack_direction ();
#endif
/* Reclaim garbage, defined as all alloca'd 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->h.next;
free ((PTR) hp); /* Collect garbage. */
hp = np; /* -> next header. */
}
else
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. */
{
|
