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#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <limits.h>
/* The C Programming Language: 2nd Edition
*
* Exercise 8-7: `malloc` accepts a size request without checking its
* plausibility; `free` believes that the block it is asked to free contains a
* valid size field. Improve these routines so that they take more pains with
* error checking.
*
* Notes: There isn't much to check for, since a lot of boundary checking is
* already done. The primary things to watch out for in functions that accept
* pointers is the existence of a NULL pointer, which has undefined behavior
* when dereferenced or otherwise worked with. We also check for the size value
* of a header not exceeding the system's integer limits. Fairly simple error
* checking, but it works.
*/
typedef long Align;
union header {
struct {
union header *ptr;
unsigned size;
} s;
/* unused, just for alignment */
Align x;
} hdr;
typedef union header Header;
static Header base;
static Header *freep = NULL;
static Header *zmorecore(unsigned);
void *zmalloc(unsigned);
void zfree(void *);
void *zmalloc(unsigned nbytes) {
if (nbytes == 0 || nbytes > (UINT_MAX - sizeof (Header))) {
/* cannot ask for zero memory */
errno = EINVAL;
return NULL;
}
printf("Requesting %d bytes.\n", nbytes);
Header *p, *prevp;
unsigned nunits;
/* Set the number to something more sane */
nunits = (nbytes + sizeof (Header));
/* is the K&R wrong here...? */
/* nunits = (nbytes + sizeof (Header) - 1) / sizeof (Header) + 1; */
printf("%d units need allocated.\n", nunits);
if ((prevp = freep) == NULL) {
base.s.ptr = freep = prevp = &base;
base.s.size = 0;
}
for (p = prevp->s.ptr; ; prevp = p, p = p->s.ptr) {
/* larger than requested units */
if (p->s.size >= nunits) {
if (p->s.size == nunits) {
prevp->s.ptr = p->s.ptr;
} else {
p->s.size -= nunits;
p += p->s.size;
p->s.size = nunits;
}
freep = prevp;
return (void *)(p+1);
}
if (p == freep) {
if ((p = zmorecore(nunits)) == NULL) {
errno = ENOMEM;
return NULL;
}
}
}
}
#define NALLOC 1024
static Header *zmorecore(unsigned nu) {
char *cp, *sbrk(int);
Header *up;
if (nu < NALLOC) {
nu = NALLOC;
}
cp = sbrk(nu * sizeof (Header));
if (errno == ENOMEM) {
return NULL;
}
up = (Header *) cp;
up->s.size = nu;
zfree((void *)(up+1));
return freep;
}
void zfree(void *ap) {
if (ap == NULL) {
printf("Cannot free null pointer.\n");
errno = EINVAL;
return;
}
Header *bp, *p;
bp = (Header *)ap - 1;
if (bp->s.size == 0) {
printf("Cannot free block of size 0.\n");
errno = EINVAL;
return;
}
for (p = freep; !(bp > p && bp < p->s.ptr); p = p->s.ptr) {
if (p >= p->s.ptr && (bp > p || bp < p->s.ptr)) {
break;
}
}
if (bp + bp->s.size == p->s.ptr && bp->s.size < (UINT_MAX - p->s.ptr->s.size)) {
bp->s.size += p->s.ptr->s.size;
bp->s.ptr = p->s.ptr->s.ptr;
} else {
bp->s.ptr = p->s.ptr;
}
if (p + p->s.size == bp && p->s.size < (UINT_MAX - bp->s.size)) {
p->s.size += bp->s.size;
p->s.ptr = bp->s.ptr;
} else {
p->s.ptr = bp;
}
freep = p;
}
void * zcalloc(int count, unsigned size) {
char *p;
int i;
p = zmalloc(count * size);
if (p != NULL) {
for (i = 0; i < (count * size); i++) {
p[i] = '\0';
}
return p;
} else {
return (int *)(-1);
}
}
int main(int argc, char *argv[]) {
char *foo = "Hello world.";
char *baz = zcalloc(strlen(foo) + 1, sizeof (char));
strncpy(baz, foo, strlen(foo)+1);
printf("%s\n", baz);
printf("baz is %d long\n", strlen(baz));
printf("\nHEADER INFO:\n");
Header *i;
int c = 1;
for (i = freep; ; i = i->s.ptr, c++) {
if (c > 1 && i == &base ||
i->s.ptr == i) {
break;
}
printf("Header %d:\n", c);
printf("Address: %p\n", i);
printf("Size: %d\n", i->s.size);
printf("Next: %p\n", i->s.ptr);
printf("Data:\n%s\n", i+i->s.size+1);
}
printf("baz's address is %p\n", baz);
zfree(baz);
printf("\nLet's try allocating zero.\n");
char *beep = zcalloc(0, sizeof (char));
if (beep = (char *)(-1)) {
printf("Nope.\n");
}
printf("\nLet's try allocating UINT_MAX + 1.\n");
char *borp = zcalloc(UINT_MAX + 1, sizeof (char));
if (borp = (char *)(-1)) {
printf("Nope.\n");
}
return 0;
}
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