gnulib/lib/alloca.c

/* alloca.c -- allocate automatically reclaimed memory
   This file is in the public domain.  */

/* (Mostly) portable 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.  */

#include <config.h>

#include <alloca.h>

#include <string.h>
#include <stdlib.h>

/* If compiling with GCC or clang, this file is not needed.  */
#if !(defined __GNUC__ || defined __clang__)

/* If someone has defined alloca as a macro,
   there must be some other way alloca is supposed to work.  */
# ifndef alloca

/* 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 int
find_stack_direction (int *addr, int depth)
{
  int dir, dummy = 0;
  if (! addr)
    addr = &dummy;
  *addr = addr < &dummy ? 1 : addr == &dummy ? 0 : -1;
  dir = depth ? find_stack_direction (addr, depth - 1) : 0;
  return dir + dummy;
}

#  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.  */

void *
alloca (size_t size)
{
  auto char probe;              /* Probes stack depth: */
  register char *depth = &probe;

#  if STACK_DIRECTION == 0
  if (STACK_DIR == 0)           /* Unknown growth direction.  */
    STACK_DIR = find_stack_direction (NULL, (size & 1) + 20);
#  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 (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.  */

  {
    /* Address of header.  */
    register header *new;

    size_t combined_size = sizeof (header) + size;
    if (combined_size < sizeof (header))
      memory_full ();

    new = malloc (combined_size);

    if (! new)
      memory_full ();

    new->h.next = last_alloca_header;
    new->h.deep = depth;

    last_alloca_header = new;

    /* User storage begins just after header.  */

    return (void *) (new + 1);
  }
}

# endif /* no alloca */
#endif /* not GCC || clang */