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分类: C/C++

2010-07-18 23:12:10

学习本质的利器:FreeBSD的早期实现版本的源代码
From:

/*
 * Copyright (c) 1983, 1993
 *    The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    This product includes software developed by the University of
 *    California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)malloc.c    8.1 (Berkeley) 6/4/93";
#endif /* LIBC_SCCS and not lint */

/*
 * malloc.c (Caltech) 2/21/82
 * Chris Kingsley, kingsley@cit-20.
 *
 * This is a very fast storage allocator.  It allocates blocks of a small
 * number of different sizes, and keeps free lists of each size.  Blocks that
 * don't exactly fit are passed up to the next larger size.  In this
 * implementation, the available sizes are 2^n-4 (or 2^n-10) bytes long.
 * This is designed for use in a virtual memory environment.
 */

#include
#include
#include
#include

#define    NULL 0

static void morecore();
static int findbucket();

/*
 * The overhead on a block is at least 4 bytes.  When free, this space
 * contains a pointer to the next free block, and the bottom two bits must
 * be zero.  When in use, the first byte is set to MAGIC, and the second
 * byte is the size index.  The remaining bytes are for alignment.
 * If range checking is enabled then a second word holds the size of the
 * requested block, less 1, rounded up to a multiple of sizeof(RMAGIC).
 * The order of elements is critical: ov_magic must overlay the low order
 * bits of ov_next, and ov_magic can not be a valid ov_next bit pattern.
 */
union    overhead {
    union    overhead *ov_next;    /* when free */
    struct {
        u_char    ovu_magic;    /* magic number */
        u_char    ovu_index;    /* bucket # */
#ifdef RCHECK
        u_short    ovu_rmagic;    /* range magic number */
        u_int    ovu_size;    /* actual block size */
#endif
    } ovu;
#define    ov_magic    ovu.ovu_magic
#define    ov_index    ovu.ovu_index
#define    ov_rmagic    ovu.ovu_rmagic
#define    ov_size        ovu.ovu_size
};

#define    MAGIC        0xef        /* magic # on accounting info */
#define RMAGIC        0x5555        /* magic # on range info */

#ifdef RCHECK
#define    RSLOP        sizeof (u_short)
#else
#define    RSLOP        0
#endif

/*
 * nextf[i] is the pointer to the next free block of size 2^(i+3).  The
 * smallest allocatable block is 8 bytes.  The overhead information
 * precedes the data area returned to the user.
 */
#define    NBUCKETS 30
static    union overhead *nextf[NBUCKETS];
extern    char *sbrk();

static    int pagesz;            /* page size */
static    int pagebucket;            /* page size bucket */

#ifdef MSTATS
/*
 * nmalloc[i] is the difference between the number of mallocs and frees
 * for a given block size.
 */
static    u_int nmalloc[NBUCKETS];
#include
#endif

#if defined(DEBUG) || defined(RCHECK)
#define    ASSERT(p)   if (!(p)) botch("p")
#include
static
botch(s)
    char *s;
{
    fprintf(stderr, "\r\nassertion botched: %s\r\n", s);
     (void) fflush(stderr);        /* just in case user buffered it */
    abort();
}
#else
#define    ASSERT(p)
#endif

void *
malloc(nbytes)
    size_t nbytes;
{
      register union overhead *op;
      register int bucket, n;
    register unsigned amt;

    /*
     * First time malloc is called, setup page size and
     * align break pointer so all data will be page aligned.
     */
    if (pagesz == 0) {
        pagesz = n = getpagesize();
        op = (union overhead *)sbrk(0);
          n = n - sizeof (*op) - ((int)op & (n - 1));
        if (n < 0)
            n += pagesz;
          if (n) {
              if (sbrk(n) == (char *)-1)
                return (NULL);
        }
        bucket = 0;
        amt = 8;
        while (pagesz > amt) {
            amt <<= 1;
            bucket++;
        }
        pagebucket = bucket;
    }
    /*
     * Convert amount of memory requested into closest block size
     * stored in hash buckets which satisfies request.
     * Account for space used per block for accounting.
     */
    if (nbytes <= (n = pagesz - sizeof (*op) - RSLOP)) {
#ifndef RCHECK
        amt = 8;    /* size of first bucket */
        bucket = 0;
#else
        amt = 16;    /* size of first bucket */
        bucket = 1;
#endif
        n = -(sizeof (*op) + RSLOP);
    } else {
        amt = pagesz;
        bucket = pagebucket;
    }
    while (nbytes > amt + n) {
        amt <<= 1;
        if (amt == 0)
            return (NULL);
        bucket++;
    }
    /*
     * If nothing in hash bucket right now,
     * request more memory from the system.
     */
      if ((op = nextf[bucket]) == NULL) {
          morecore(bucket);
          if ((op = nextf[bucket]) == NULL)
              return (NULL);
    }
    /* remove from linked list */
      nextf[bucket] = op->ov_next;
    op->ov_magic = MAGIC;
    op->ov_index = bucket;
#ifdef MSTATS
      nmalloc[bucket]++;
#endif
#ifdef RCHECK
    /*
     * Record allocated size of block and
     * bound space with magic numbers.
     */
    op->ov_size = (nbytes + RSLOP - 1) & ~(RSLOP - 1);
    op->ov_rmagic = RMAGIC;
      *(u_short *)((caddr_t)(op + 1) + op->ov_size) = RMAGIC;
#endif
      return ((char *)(op + 1));
}

/*
 * Allocate more memory to the indicated bucket.
 */
static void
morecore(bucket)
    int bucket;
{
      register union overhead *op;
    register int sz;        /* size of desired block */
      int amt;            /* amount to allocate */
      int nblks;            /* how many blocks we get */

    /*
     * sbrk_size <= 0 only for big, FLUFFY, requests (about
     * 2^30 bytes on a VAX, I think) or for a negative arg.
     */
    sz = 1 << (bucket + 3);
#ifdef DEBUG
    ASSERT(sz > 0);
#else
    if (sz <= 0)
        return;
#endif
    if (sz < pagesz) {
        amt = pagesz;
          nblks = amt / sz;
    } else {
        amt = sz + pagesz;
        nblks = 1;
    }
    op = (union overhead *)sbrk(amt);
    /* no more room! */
      if ((int)op == -1)
          return;
    /*
     * Add new memory allocated to that on
     * free list for this hash bucket.
     */
      nextf[bucket] = op;
      while (--nblks > 0) {
        op->ov_next = (union overhead *)((caddr_t)op + sz);
        op = (union overhead *)((caddr_t)op + sz);
      }
}

void
free(cp)
    void *cp;
{   
      register int size;
    register union overhead *op;

      if (cp == NULL)
          return;
    op = (union overhead *)((caddr_t)cp - sizeof (union overhead));
#ifdef DEBUG
      ASSERT(op->ov_magic == MAGIC);        /* make sure it was in use */
#else
    if (op->ov_magic != MAGIC)
        return;                /* sanity */
#endif
#ifdef RCHECK
      ASSERT(op->ov_rmagic == RMAGIC);
    ASSERT(*(u_short *)((caddr_t)(op + 1) + op->ov_size) == RMAGIC);
#endif
      size = op->ov_index;
      ASSERT(size < NBUCKETS);
    op->ov_next = nextf[size];    /* also clobbers ov_magic */
      nextf[size] = op;
#ifdef MSTATS
      nmalloc[size]--;
#endif
}

/*
 * When a program attempts "storage compaction" as mentioned in the
 * old malloc man page, it realloc's an already freed block.  Usually
 * this is the last block it freed; occasionally it might be farther
 * back.  We have to search all the free lists for the block in order
 * to determine its bucket: 1st we make one pass thru the lists
 * checking only the first block in each; if that fails we search
 * ``__realloc_srchlen'' blocks in each list for a match (the variable
 * is extern so the caller can modify it).  If that fails we just copy
 * however many bytes was given to realloc() and hope it's not huge.
 */
int __realloc_srchlen = 4;    /* 4 should be plenty, -1 =>'s whole list */

void *
realloc(cp, nbytes)
    void *cp;
    size_t nbytes;
{   
      register u_int onb;
    register int i;
    union overhead *op;
      char *res;
    int was_alloced = 0;

      if (cp == NULL)
          return (malloc(nbytes));
    op = (union overhead *)((caddr_t)cp - sizeof (union overhead));
    if (op->ov_magic == MAGIC) {
        was_alloced++;
        i = op->ov_index;
    } else {
        /*
         * Already free, doing "compaction".
         *
         * Search for the old block of memory on the
         * free list.  First, check the most common
         * case (last element free'd), then (this failing)
         * the last ``__realloc_srchlen'' items free'd.
         * If all lookups fail, then assume the size of
         * the memory block being realloc'd is the
         * largest possible (so that all "nbytes" of new
         * memory are copied into).  Note that this could cause
         * a memory fault if the old area was tiny, and the moon
         * is gibbous.  However, that is very unlikely.
         */
        if ((i = findbucket(op, 1)) < 0 &&
            (i = findbucket(op, __realloc_srchlen)) < 0)
            i = NBUCKETS;
    }
    onb = 1 << (i + 3);
    if (onb < pagesz)
        onb -= sizeof (*op) + RSLOP;
    else
        onb += pagesz - sizeof (*op) - RSLOP;
    /* avoid the copy if same size block */
    if (was_alloced) {
        if (i) {
            i = 1 << (i + 2);
            if (i < pagesz)
                i -= sizeof (*op) + RSLOP;
            else
                i += pagesz - sizeof (*op) - RSLOP;
        }
        if (nbytes <= onb && nbytes > i) {
#ifdef RCHECK
            op->ov_size = (nbytes + RSLOP - 1) & ~(RSLOP - 1);
            *(u_short *)((caddr_t)(op + 1) + op->ov_size) = RMAGIC;
#endif
            return(cp);
        } else
            free(cp);
    }
      if ((res = malloc(nbytes)) == NULL)
          return (NULL);
      if (cp != res)        /* common optimization if "compacting" */
        bcopy(cp, res, (nbytes < onb) ? nbytes : onb);
      return (res);
}

/*
 * Search ``srchlen'' elements of each free list for a block whose
 * header starts at ``freep''.  If srchlen is -1 search the whole list.
 * Return bucket number, or -1 if not found.
 */
static
findbucket(freep, srchlen)
    union overhead *freep;
    int srchlen;
{
    register union overhead *p;
    register int i, j;

    for (i = 0; i < NBUCKETS; i++) {
        j = 0;
        for (p = nextf[i]; p && j != srchlen; p = p->ov_next) {
            if (p == freep)
                return (i);
            j++;
        }
    }
    return (-1);
}

#ifdef MSTATS
/*
 * mstats - print out statistics about malloc
 *
 * Prints two lines of numbers, one showing the length of the free list
 * for each size category, the second showing the number of mallocs -
 * frees for each size category.
 */
mstats(s)
    char *s;
{
      register int i, j;
      register union overhead *p;
      int totfree = 0,
      totused = 0;

      fprintf(stderr, "Memory allocation statistics %s\nfree:\t", s);
      for (i = 0; i < NBUCKETS; i++) {
          for (j = 0, p = nextf[i]; p; p = p->ov_next, j++)
              ;
          fprintf(stderr, " %d", j);
          totfree += j * (1 << (i + 3));
      }
      fprintf(stderr, "\nused:\t");
      for (i = 0; i < NBUCKETS; i++) {
          fprintf(stderr, " %d", nmalloc[i]);
          totused += nmalloc[i] * (1 << (i + 3));
      }
      fprintf(stderr, "\n\tTotal in use: %d, total free: %d\n",
        totused, totfree);
}
#endif

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chinaunix网友2010-07-28 13:45:18

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