From 2bbb384783f667226e117e62992779498bc7e5b8 Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Sun, 23 Jan 2022 17:59:56 -0800
Subject: Solve Exercise 8-6: calloc()

I chose to implement calloc() by calling malloc.

This exercise went well, but I found it difficult to verify the contents
of the headers and buffers within memory. Valgrind showed the correct
number of bytes allocated, and a memory dump pointed to a good
result, but research was not conclusive.
---
 ch8/8-06_calloc.c | 136 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)
 create mode 100644 ch8/8-06_calloc.c

diff --git a/ch8/8-06_calloc.c b/ch8/8-06_calloc.c
new file mode 100644
index 0000000..34f18ab
--- /dev/null
+++ b/ch8/8-06_calloc.c
@@ -0,0 +1,136 @@
+#include <stdio.h>
+#include <string.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 8-6: The standard library function calloc(n, size) returns a
+ * pointer to `n` objects of size `size`, with the storage initialized to zero.
+ * Write `calloc`, by calling malloc or by modifying it.
+ *
+ * Notes: Replacing the standard library functions here can be problematic if
+ * you also intend to use the stdlib to output text or use other functions that
+ * interact with *alloc, morecore, and free.
+ *
+ * I prefixed the functions with 'z' so I could hack on them *and* use the
+ * standard library as needed.
+ *
+ * Ideally, one would use a debugger or other tool to show that memory is
+ * assigned in the free list, and verify the size of the block of memory. We'll
+ * revisit this when I'm good at debuggers. :)
+ */
+
+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) {
+	/* TODO: get the size field updated correctly */
+	Header *p, *prevp;
+	unsigned nunits;
+	nunits = (nbytes + sizeof (Header) - 1) / sizeof (Header) + 1;
+
+	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) {
+				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 (cp == (char*) -1) {
+		return NULL;
+	}
+	up = (Header *) cp;
+	up->s.size = nu;
+	zfree((void *)(up+1));
+	return freep;
+}
+
+void zfree(void *ap) {
+	Header *bp, *p;
+	bp = (Header *)ap - 1;
+	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 += 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 += 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 there!";
+	char *buf = zcalloc(strlen(foo) + 1, sizeof (char));
+	strncpy(buf, foo, 5);
+	printf("%s\n", buf);
+	printf("buf is %d long\n", strlen(buf));
+	printf("foo is %d long\n", strlen(foo));
+	printf("the header says buf's max size is %d\n", freep->s.size);
+	return 0;
+}
-- 
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