ARM Club 1/ARM_CLUB_CD.iso/contents/apps/clib/progs/utilslib/c/Alloca

Text File | 1991-09-02 | 4.9 KB | 176 lines

/*
    alloca -- (mostly) portable public-domain implementation -- D A Gwyn

    Arc port:    90/01/28    pfm
       Convert to work on Archimedes. Note that the assumption
       of a simple procedure stack is NOT guaranteed under the
       Arm procedure call standard. C Version 3.00 may well
       violate it (but the only result should be a delay in
       reclaiming some storage... I hope).

    ** Further note: In the face of stack extension, where the
    ** new stack chunk may be at a higher or lower address than
    ** the old chunk (no guarantees!), this may fail horribly.
    ** Nothing I can do about this - sorry! I would recommend not
    ** using alloca() in production quality code.
    ** On reflection, you could - I suppose - trace back through
    ** the stack chunk chain. But it's too much work for me to
    ** bother with for now.

    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.
*/

#include <stdlib.h>
#include <stddef.h>
#include "utils.h"

/*
    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

/* known at compile-time */
#define    STACK_DIR    STACK_DIRECTION

#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;
        /* recurse once */
        find_stack_direction ();
    }
    else if (&dummy > addr)        /* second entry */
        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 */

void *alloca (size_t size)
{
    /* variable 'probe' probes stack depth: */
    auto char    probe;
    register char    *depth = &probe;
    register header *hp;
    register header *new;

#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. */

    for (hp = last_alloca_header; hp != NULL;)
    {
        if (STACK_DIR > 0 && hp->h.deep > depth
         || STACK_DIR < 0 && hp->h.deep < depth)
        {
            /* collect garbage */
            register header *np = hp->h.next;
            free (hp);
            hp = np;
        }
        else /* rest are not deeper */
            break;
    }

    last_alloca_header = hp;    /* -> last valid storage */

    if (size == 0)
        return NULL;    /* no allocation required */

    /* Allocate combined header + user data storage. */

    new = emalloc (sizeof (header) + size);
    new->h.next = last_alloca_header;
    new->h.deep = depth;

    last_alloca_header = new;

    /* User storage begins just after header. */
    return (void *)((char *)new + sizeof(header));
}