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alloca.c

/*
      alloca -- (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.

      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"
#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

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

#define     NULL  0                 /* null pointer constant */

extern void free();
extern pointer    malloc();

/*
      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
    {
      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    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));
  }
}

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