讨论内存分配函数malloc，内存释放函数free,如何实现？

[lypp]

请给出实现思路，最好有代码。大家一起讨论。

[xiaoke26]

可以参阅赵X的《linux源代码注释》里面有详细解释，www.oldlinux.org可以下到该书

[lovefreex]

没有注释看不懂~~`

[ma100]

单步调试,看看汇编

[sankt]

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Permission to use, copy, modify, and distribute this software
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Bellcore be liable for any special, indirect or consequential
damages or any damages whatsoever resulting from loss of use,
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other tortious action, arising out of or in connection with the
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****************************************************************/

#ifndef CRAY
#define STACKMIN 512
#define MINBLK (2*sizeof(struct mem) + 16)
#define F _malloc_free_
#define SBGULP 8192
#include "string.h"/* for memcpy */

#ifdef KR_headers
#define Char char
#define Unsigned unsigned
#define Int /*int*/
#else
#define Char void
#define Unsigned size_t
#define Int int
#endif

typedef struct mem {
struct mem *next;
Unsigned len;
} mem;

mem *F;

Char *
#ifdef KR_headers
malloc(size)
register Unsigned size;
#else
malloc(register Unsigned size)
#endif
{
register mem *p, *q, *r, *s;
unsigned register k, m;
extern Char *sbrk(Int);
char *top, *top1;

size = (size+7) & ~7;
r = (mem *) &F;
for (p = F, q = 0; p; r = p, p = p->next) {
if ((k = p->len) >= size && (!q || m > k)) {
m = k;
q = p;
s = r;
}
}
if (q) {
if (q->len - size >= MINBLK) { /* split block */
p = (mem *) (((char *) (q+1)) + size);
p->next = q->next;
p->len = q->len - size - sizeof(mem);
s->next = p;
q->len = size;
}
else
s->next = q->next;
}
else {
top = (Char *)(((long)sbrk(0) + 7) & ~7);
if (F && (char *)(F+1) + F->len == top) {
q = F;
F = F->next;
}
else
q = (mem *) top;
top1 = (char *)(q+1) + size;
if (sbrk((int)(top1-top+SBGULP)) == (Char *) -1)
return 0;
r = (mem *)top1;
r->len = SBGULP - sizeof(mem);
r->next = F;
F = r;
q->len = size;
}
return (Char *) (q+1);
}

void
#ifdef KR_headers
free(f)
Char *f;
#else
free(Char *f)
#endif
{
mem *p, *q, *r;
char *pn, *qn;

if (!f) return;
q = (mem *) ((char *)f - sizeof(mem));
qn = (char *)f + q->len;
for (p = F, r = (mem *) &F; ; r = p, p = p->next) {
if (qn == (Char *) p) {
q->len += p->len + sizeof(mem);
p = p->next;
}
pn = p ? ((char *) (p+1)) + p->len : 0;
if (pn == (Char *) q) {
p->len += sizeof(mem) + q->len;
q->len = 0;
q->next = p;
r->next = p;
break;
}
if (pn < (char *) q) {
r->next = q;
q->next = p;
break;
}
}
}

Char *
#ifdef KR_headers
realloc(f, size)
Char *f;
Unsigned size;
#else
realloc(Char *f, Unsigned size)
#endif
{
mem *p;
Char *q, *f1;
Unsigned s1;

if (!f) return malloc(size);
p = (mem *) ((char *)f - sizeof(mem));
s1 = p->len;
free(f);
if (s1 > size)
s1 = size + 7 & ~7;
if (!p->len) {
f1 = (Char *)(p->next + 1);
memcpy(f1, f, s1);
f = f1;
}
q = malloc(size);
if (q && q != f)
memcpy(q, f, s1);
return q;
}

/* The following (calloc) should really be in a separate file, */
/* but defining it here sometimes avoids confusion on systems */
/* that do not provide calloc in its own file. */

Char *
#ifdef KR_headers
calloc(n, m) Unsigned m, n;
#else
calloc(Unsigned n, Unsigned m)
#endif
{
Char *rv;
rv = malloc(n *= m);
if (n && rv)
memset(rv, 0, n);
return rv;
}
#endif