db_salloc.c

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/*-
 * See the file LICENSE for redistribution information.
 *
 * Copyright (c) 1996, 1997, 1998, 1999, 2000
 *      Sleepycat Software.  All rights reserved.
 */

#include "config.h"

#ifndef lint
static const char revid[] = "$Id: db__salloc_8c-source.html,v 1.1 2008/06/08 10:18:16 sebdiaz Exp $";
#endif /* not lint */

#ifndef NO_SYSTEM_INCLUDES
#include <sys/types.h>

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

#include "db_int.h"

/*
 * Implement shared memory region allocation, using simple first-fit algorithm.
 * The model is that we take a "chunk" of shared memory store and begin carving
 * it up into areas, similarly to how malloc works.  We do coalescing on free.
 *
 * The "len" field in the __data struct contains the length of the free region
 * (less the size_t bytes that holds the length).  We use the address provided
 * by the caller to find this length, which allows us to free a chunk without
 * requiring that the caller pass in the length of the chunk they're freeing.
 */
SH_LIST_HEAD(__head);
struct __data {
        size_t len;
        SH_LIST_ENTRY links;
};

/*
 * CDB___db_shalloc_init --
 *      Initialize the area as one large chunk.
 *
 * PUBLIC: void CDB___db_shalloc_init __P((void *, size_t));
 */
void
CDB___db_shalloc_init(area, size)
        void *area;
        size_t size;
{
        struct __data *elp;
        struct __head *hp;

        hp = area;
        SH_LIST_INIT(hp);

        elp = (struct __data *)(hp + 1);
        elp->len = size - sizeof(struct __head) - sizeof(elp->len);
        SH_LIST_INSERT_HEAD(hp, elp, links, __data);
}

/*
 * CDB___db_shalloc --
 *      Allocate some space from the shared region.
 *
 * PUBLIC: int CDB___db_shalloc __P((void *, size_t, size_t, void *));
 */
int
CDB___db_shalloc(p, len, align, retp)
        void *p, *retp;
        size_t len, align;
{
        struct __data *elp;
        size_t *sp;
        void *rp;

        /* Never allocate less than the size of a struct __data. */
        if (len < sizeof(struct __data))
                len = sizeof(struct __data);

#ifdef DIAGNOSTIC
        /* Add room for a guard byte. */
        ++len;
#endif

        /* Never align to less than a db_align_t boundary. */
        if (align <= sizeof(db_align_t))
                align = sizeof(db_align_t);

        /* Walk the list, looking for a slot. */
        for (elp = SH_LIST_FIRST((struct __head *)p, __data);
            elp != NULL;
            elp = SH_LIST_NEXT(elp, links, __data)) {
                /*
                 * Calculate the value of the returned pointer if we were to
                 * use this chunk.
                 *      + Find the end of the chunk.
                 *      + Subtract the memory the user wants.
                 *      + Find the closest previous correctly-aligned address.
                 */
                rp = (u_int8_t *)elp + sizeof(size_t) + elp->len;
                rp = (u_int8_t *)rp - len;
                rp = (u_int8_t *)((db_alignp_t)rp & ~(align - 1));

                /*
                 * Rp may now point before elp->links, in which case the chunk
                 * was too small, and we have to try again.
                 */
                if ((u_int8_t *)rp < (u_int8_t *)&elp->links)
                        continue;

                *(void **)retp = rp;
#ifdef DIAGNOSTIC
                /*
                 * At this point, whether or not we still need to split up a
                 * chunk, retp is the address of the region we are returning,
                 * and (u_int8_t *)elp + sizeof(size_t) + elp->len gives us
                 * the address of the first byte after the end of the chunk.
                 * Make the byte immediately before that the guard byte.
                 */
                *((u_int8_t *)elp + sizeof(size_t) + elp->len - 1) = GUARD_BYTE;
#endif

#define SHALLOC_FRAGMENT        32
                /*
                 * If there are at least SHALLOC_FRAGMENT additional bytes of
                 * memory, divide the chunk into two chunks.
                 */
                if ((u_int8_t *)rp >=
                    (u_int8_t *)&elp->links + SHALLOC_FRAGMENT) {
                        sp = rp;
                        *--sp = elp->len -
                            ((u_int8_t *)rp - (u_int8_t *)&elp->links);
                        elp->len -= *sp + sizeof(size_t);
                        return (0);
                }

                /*
                 * Otherwise, we return the entire chunk, wasting some amount
                 * of space to keep the list compact.  However, because the
                 * address we're returning to the user may not be the address
                 * of the start of the region for alignment reasons, set the
                 * size_t length fields back to the "real" length field to a
                 * flag value, so that we can find the real length during free.
                 */
#define ILLEGAL_SIZE    1
                SH_LIST_REMOVE(elp, links, __data);
                for (sp = rp; (u_int8_t *)--sp >= (u_int8_t *)&elp->links;)
                        *sp = ILLEGAL_SIZE;
                return (0);
        }

        return (ENOMEM);
}

/*
 * CDB___db_shalloc_free --
 *      Free a shared memory allocation.
 *
 * PUBLIC: void CDB___db_shalloc_free __P((void *, void *));
 */
void
CDB___db_shalloc_free(regionp, ptr)
        void *regionp, *ptr;
{
        struct __data *elp, *lastp, *newp;
        struct __head *hp;
        size_t free_size, *sp;
        int merged;

        /*
         * Step back over flagged length fields to find the beginning of
         * the object and its real size.
         */
        for (sp = (size_t *)ptr; sp[-1] == ILLEGAL_SIZE; --sp)
                ;
        ptr = sp;

        newp = (struct __data *)((u_int8_t *)ptr - sizeof(size_t));
        free_size = newp->len;

#ifdef DIAGNOSTIC
        /*
         * The "real size" includes the guard byte;  it's just the last
         * byte in the chunk, and the caller never knew it existed.
         *
         * Check it to make sure it hasn't been stomped.
         */
        if (*((u_int8_t *)ptr + free_size - 1) != GUARD_BYTE) {
                /*
                 * Eventually, once we push a DB_ENV handle down to these
                 * routines, we should use the standard output channels.
                 */
                fprintf(stderr,
                    "Guard byte incorrect during shared memory free.\n");
                abort();
                /* NOTREACHED */
        }

        /* Trash the returned memory (including guard byte). */
        memset(ptr, CLEAR_BYTE, free_size);
#endif

        /*
         * Walk the list, looking for where this entry goes.
         *
         * We keep the free list sorted by address so that coalescing is
         * trivial.
         *
         * XXX
         * Probably worth profiling this to see how expensive it is.
         */
        hp = (struct __head *)regionp;
        for (elp = SH_LIST_FIRST(hp, __data), lastp = NULL;
            elp != NULL && (void *)elp < (void *)ptr;
            lastp = elp, elp = SH_LIST_NEXT(elp, links, __data))
                ;

        /*
         * Elp is either NULL (we reached the end of the list), or the slot
         * after the one that's being returned.  Lastp is either NULL (we're
         * returning the first element of the list) or the element before the
         * one being returned.
         *
         * Check for coalescing with the next element.
         */
        merged = 0;
        if ((u_int8_t *)ptr + free_size == (u_int8_t *)elp) {
                newp->len += elp->len + sizeof(size_t);
                SH_LIST_REMOVE(elp, links, __data);
                if (lastp != NULL)
                        SH_LIST_INSERT_AFTER(lastp, newp, links, __data);
                else
                        SH_LIST_INSERT_HEAD(hp, newp, links, __data);
                merged = 1;
        }

        /* Check for coalescing with the previous element. */
        if (lastp != NULL && (u_int8_t *)lastp +
            lastp->len + sizeof(size_t) == (u_int8_t *)newp) {
                lastp->len += newp->len + sizeof(size_t);

                /*
                 * If we have already put the new element into the list take
                 * it back off again because it's just been merged with the
                 * previous element.
                 */
                if (merged)
                        SH_LIST_REMOVE(newp, links, __data);
                merged = 1;
        }

        if (!merged) {
                if (lastp == NULL)
                        SH_LIST_INSERT_HEAD(hp, newp, links, __data);
                else
                        SH_LIST_INSERT_AFTER(lastp, newp, links, __data);
        }
}

/*
 * CDB___db_shalloc_count --
 *      Return the amount of memory on the free list.
 *
 * PUBLIC: size_t CDB___db_shalloc_count __P((void *));
 */
size_t
CDB___db_shalloc_count(addr)
        void *addr;
{
        struct __data *elp;
        size_t count;

        count = 0;
        for (elp = SH_LIST_FIRST((struct __head *)addr, __data);
            elp != NULL;
            elp = SH_LIST_NEXT(elp, links, __data))
                count += elp->len;

        return (count);
}

/*
 * CDB___db_shsizeof --
 *      Return the size of a shalloc'd piece of memory.
 *
 * !!!
 * Note that this is from an internal standpoint -- it includes not only
 * the size of the memory being used, but also the extra alignment bytes
 * in front and, #ifdef DIAGNOSTIC, the guard byte at the end.
 *
 * PUBLIC: size_t CDB___db_shsizeof __P((void *));
 */
size_t
CDB___db_shsizeof(ptr)
        void *ptr;
{
        struct __data *elp;
        size_t *sp;

        /*
         * Step back over flagged length fields to find the beginning of
         * the object and its real size.
         */
        for (sp = (size_t *)ptr; sp[-1] == ILLEGAL_SIZE; --sp)
                ;

        elp = (struct __data *)((u_int8_t *)sp - sizeof(size_t));
        return (elp->len);
}

/*
 * CDB___db_shalloc_dump --
 *
 * PUBLIC: void CDB___db_shalloc_dump __P((void *, FILE *));
 */
void
CDB___db_shalloc_dump(addr, fp)
        void *addr;
        FILE *fp;
{
        struct __data *elp;

        /* Make it easy to call from the debugger. */
        if (fp == NULL)
                fp = stderr;

        fprintf(fp, "%s\nMemory free list\n", DB_LINE);

        for (elp = SH_LIST_FIRST((struct __head *)addr, __data);
            elp != NULL;
            elp = SH_LIST_NEXT(elp, links, __data))
                fprintf(fp, "%#lx: %lu\t", (u_long)elp, (u_long)elp->len);
        fprintf(fp, "\n");
}

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