source: vendor/bash/3.1/lib/malloc/alloca.c

/* 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 <[email protected]> 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.  */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

/* If compiling with GCC 2, this file's not needed.  */
#if !defined (__GNUC__) || __GNUC__ < 2

#include <bashtypes.h>          /* for size_t */

/* If alloca is defined somewhere, this file is not needed. */
#ifndef alloca

#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
   */
#ifndef STACK_DIRECTION
you
lose
-- must know STACK_DIRECTION at compile-time
#endif /* STACK_DIRECTION undefined */
#endif /* static */
#endif /* emacs */

/* 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)
long i00afunc ();
#define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
#else
#define ADDRESS_FUNCTION(arg) &(arg)
#endif /* CRAY && CRAY_STACKSEG_END */

#if __STDC__
typedef void *pointer;
#else
typedef char *pointer;
#endif

#define NULL    0

/* Different portions of Emacs need to call different versions of
   malloc.  The Emacs executable needs alloca to call xmalloc, because
   ordinary malloc isn't protected from input signals.  On the other
   hand, the utilities in lib-src need alloca to call malloc; some of
   them are very simple, and don't have an xmalloc routine.

   Non-Emacs programs expect this to call use xmalloc.

   Callers below should use malloc.  */

#ifndef emacs
#define malloc xmalloc
extern pointer xmalloc ();
#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 ()
{
  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.  */

pointer
alloca (size)
     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 ((pointer) 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.  */

  {
    register pointer new = malloc (sizeof (header) + size);
    /* Address of header.  */

    ((header *) new)->h.next = 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));
  }
}

#if defined (CRAY) && defined (CRAY_STACKSEG_END)

#ifdef DEBUG_I00AFUNC
#include <stdio.h>
#endif

#ifndef CRAY_STACK