学习本质的利器：FreeBSD的早期实现版本的源代码
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/*
* 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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