Liren Large Software Subsidy 7/07.iso/c/c100/4.ddi/COM.ZIP/ALLOCA.CPP

C/C++ Source or Header | 1990-08-02 | 5.5 KB | 201 lines

/*
    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 */
#endif

#ifdef emacs
#include "config.h"
#if defined(static)        /* THINK_C barfs on "#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 */        /* THINK_C barfs on "#endif static" */
#endif emacs

#ifdef X3J11
 typedef void    *pointer;        /* generic pointer type */
#else
 typedef char    *pointer;        /* generic pointer type */
#endif

#ifdef BORLAND
 #include <stdlib.h>
#else
 #ifdef ZORTECH
  #include <stdlib.h>
 #else
  extern void free(void *);
 #endif
#endif
extern pointer    xmalloc(unsigned int);

#ifndef NULL
#define    NULL    0            /* null pointer constant */
#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 = &dummy;

      find_stack_direction ();    /* recurse once */
    }
  else                /* second entry */
    if (&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 h
    {
      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 */

pointer
alloca (size)            /* returns pointer to storage */
     unsigned    size;        /* # bytes to allocate */
{
  auto char    probe;        /* probes stack depth: */
  register char    *depth = &probe;

#if STACK_DIRECTION == 0
  if (STACK_DIR == 0)        /* unknown growth direction */
    find_stack_direction ();
#endif

                /* 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->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    newp = xmalloc (sizeof (header) + size);
    /* address of header */

    ((header *)newp)->h.next = last_alloca_header;
    ((header *)newp)->h.deep = depth;

    last_alloca_header = (header *)newp;

    /* User storage begins just after header. */

    return (pointer)((char *)newp + sizeof(header));
  }
}