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authorzlg <zlg@zlg.space>2022-01-23 21:20:25 -0800
committerzlg <zlg@zlg.space>2022-01-23 21:20:25 -0800
commit06988a01833c5f669f83b16b35f0b3f70c36f807 (patch)
tree1d63c9b2792558f44099d099f69471291f4b7f63 /ch8
parentSolve Exercise 8-6: calloc() (diff)
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Solve Exercise 8-7: malloc & free, improved
There wasn't much to check for, but I checked for what I could think of.
Diffstat (limited to 'ch8')
-rw-r--r--ch8/8-07_malloc-improved.c179
1 files changed, 179 insertions, 0 deletions
diff --git a/ch8/8-07_malloc-improved.c b/ch8/8-07_malloc-improved.c
new file mode 100644
index 0000000..48e835b
--- /dev/null
+++ b/ch8/8-07_malloc-improved.c
@@ -0,0 +1,179 @@
+#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;
+}