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malloc.c
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C/C++ Source or Header
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1980-02-17
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4KB
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191 lines
/* Copyright (c) 1979 Regents of the University of California */
#ifdef debug
#define ASSERT(p) if(!(p))botch("p");else
botch(s)
char *s;
{
printf("assertion botched: %s\n",s);
abort();
}
#else
#define ASSERT(p)
#endif
/* avoid break bug */
#ifdef pdp11
#define GRANULE 64
#else
#define GRANULE 0
#endif
/* C storage allocator
* circular first-fit strategy
* works with noncontiguous, but monotonically linked, arena
* each block is preceded by a ptr to the (pointer of)
* the next following block
* blocks are exact number of words long
* aligned to the data type requirements of ALIGN
* pointers to blocks must have BUSY bit 0
* bit in ptr is 1 for busy, 0 for idle
* gaps in arena are merely noted as busy blocks
* last block of arena (pointed to by alloct) is empty and
* has a pointer to first
* idle blocks are coalesced during space search
*
* a different implementation may need to redefine
* ALIGN, NALIGN, BLOCK, BUSY, INT
* where INT is integer type to which a pointer can be cast
*/
#define INT int
#define ALIGN int
#define NALIGN 1
#define WORD sizeof(union store)
#define BLOCK 1024 /* a multiple of WORD*/
#define BUSY 1
#define NULL 0
#define testbusy(p) ((INT)(p)&BUSY)
#define setbusy(p) (union store *)((INT)(p)|BUSY)
#define clearbusy(p) (union store *)((INT)(p)&~BUSY)
union store { union store *ptr;
ALIGN dummy[NALIGN];
int calloc; /*calloc clears an array of integers*/
};
static union store allocs[2]; /*initial arena*/
static union store *allocp; /*search ptr*/
static union store *alloct; /*arena top*/
static union store *allocx; /*for benefit of realloc*/
char *sbrk();
char *
malloc(nbytes)
unsigned nbytes;
{
register union store *p, *q;
register nw;
static temp; /*coroutines assume no auto*/
if(allocs[0].ptr==0) { /*first time*/
allocs[0].ptr = setbusy(&allocs[1]);
allocs[1].ptr = setbusy(&allocs[0]);
alloct = &allocs[1];
allocp = &allocs[0];
}
nw = (nbytes+WORD+WORD-1)/WORD;
ASSERT(allocp>=allocs && allocp<=alloct);
ASSERT(allock());
for(p=allocp; ; ) {
for(temp=0; ; ) {
if(!testbusy(p->ptr)) {
while(!testbusy((q=p->ptr)->ptr)) {
ASSERT(q>p&&q<alloct);
p->ptr = q->ptr;
}
if(q>=p+nw && p+nw>=p)
goto found;
}
q = p;
p = clearbusy(p->ptr);
if(p>q)
ASSERT(p<=alloct);
else if(q!=alloct || p!=allocs) {
ASSERT(q==alloct&&p==allocs);
return(NULL);
} else if(++temp>1)
break;
}
temp = ((nw+BLOCK/WORD)/(BLOCK/WORD))*(BLOCK/WORD);
q = (union store *)sbrk(0);
if(q+temp+GRANULE < q) {
return(NULL);
}
q = (union store *)sbrk(temp*WORD);
if((INT)q == -1) {
return(NULL);
}
ASSERT(q>alloct);
alloct->ptr = q;
if(q!=alloct+1)
alloct->ptr = setbusy(alloct->ptr);
alloct = q->ptr = q+temp-1;
alloct->ptr = setbusy(allocs);
}
found:
allocp = p + nw;
ASSERT(allocp<=alloct);
if(q>allocp) {
allocx = allocp->ptr;
allocp->ptr = p->ptr;
}
p->ptr = setbusy(allocp);
return((char *)(p+1));
}
/* freeing strategy tuned for LIFO allocation
*/
free(ap)
register char *ap;
{
register union store *p = (union store *)ap;
ASSERT(p>clearbusy(allocs[1].ptr)&&p<=alloct);
ASSERT(allock());
allocp = --p;
ASSERT(testbusy(p->ptr));
p->ptr = clearbusy(p->ptr);
ASSERT(p->ptr > allocp && p->ptr <= alloct);
}
/* realloc(p, nbytes) reallocates a block obtained from malloc()
* and freed since last call of malloc()
* to have new size nbytes, and old content
* returns new location, or 0 on failure
*/
char *
realloc(p, nbytes)
register union store *p;
unsigned nbytes;
{
register union store *q;
union store *s, *t;
register unsigned nw;
unsigned onw;
if(testbusy(p[-1].ptr))
free((char *)p);
onw = p[-1].ptr - p;
q = (union store *)malloc(nbytes);
if(q==NULL || q==p)
return((char *)q);
s = p;
t = q;
nw = (nbytes+WORD-1)/WORD;
if(nw<onw)
onw = nw;
while(onw--!=0)
*t++ = *s++;
if(q<p && q+nw>=p)
(q+(q+nw-p))->ptr = allocx;
return((char *)q);
}
#ifdef debug
allock()
{
#ifdef longdebug
register union store *p;
int x;
x = 0;
for(p= &allocs[0]; clearbusy(p->ptr) > p; p=clearbusy(p->ptr)) {
if(p==allocp)
x++;
}
ASSERT(p==alloct);
return(x==1|p==allocp);
#else
return(1);
#endif
}
#endif