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[libc] Add implementation of sbrk()

This interface can be built over _brk() because the linker knows
which is the last address used by the program. In UNIX systems
this value is represented by the global value end[], and we are
going to follow this convention.

--- a/libc/src/malloc.c

+++ b/libc/src/malloc.c

@@ -1,15 +1,19 @@

 /* See LICENSE file for copyright and license details. */

+#include <stdint.h>

 #include <stdlib.h>

 #include <string.h>

-#include <stdint.h>

 #include "malloc.h"

+#include "syscall.h"

-extern void *_sbrk(intptr_t increment);

+#define MAXADDR ((char *)-1)

+#define ERRADDR ((char *)-1)

+extern char end[];

 static Header base = { .h.next = &base };

 static Header *freep = &base;

+static char *heap = end;

 /*

  * Run over the free list looking for the nearest previous

@@ -66,6 +70,22 @@

 	freep = prev;

 }

+static void *

+sbrk(uintptr_t inc)

+{

+	char *new, *old = heap;

+

+	if (old >= MAXADDR - inc)

+		return ERRADDR;

+

+	new = old + inc;

+	if (_brk(new) < 0)

+		return ERRADDR;

+	heap = new;

+

+	return old;

+}

+

 static Header *

 morecore(size_t nunits)

 {

@@ -75,8 +95,8 @@

 	if (nunits < NALLOC)

 		nunits = NALLOC;

-	rawmem = _sbrk(nunits * sizeof(Header));

-	if (rawmem == (void *)-1)

+	rawmem = sbrk(nunits * sizeof(Header));

+	if (rawmem == ERRADDR)

 		return NULL;

 	hp = (Header*)rawmem;