Solve Exercise 8-7: malloc & free, improved

There wasn't much to check for, but I checked for what I could think of.

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;
+}