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libc: Fix _brk() in Darwin

Darwin lacks a brk() syscall, and the system code was implement
a very basic version using static memory. The code was using the
Linux prototype, while we moved to use the SUSv2 interface, that
just returns 0 or -1 in case of error. The implementation didn't
set the value of errno in case of error, that could generate
strange error diagnosis.

--- a/src/libc/arch/amd64/darwin/_getheap.c

+++ /dev/null

@@ -1,18 +1,0 @@

-static char heap[16384];

-

-void *

-_getheap(void)

-{

-	return heap;

-}

-

-void *

-_brk(void *addr)

-{

-	static char *cur = heap;

-	char *p = addr;

-

-	if (p < heap || p > &heap[sizeof(heap) - 1])

-		return (void *)-1;

-	return cur = p;

-}

--- /dev/null

+++ b/src/libc/arch/darwin/Makefile

@@ -1,0 +1,9 @@

+.POSIX:

+PROJECTDIR = ../../../..

+include $(PROJECTDIR)/scripts/rules.mk

+include ../../rules.mk

+

+OBJS=\

+	_getheap.$O\

+

+all: $(OBJS)

--- /dev/null

+++ b/src/libc/arch/darwin/_getheap.c

@@ -1,0 +1,31 @@

+#include <errno.h>

+#include <stddef.h>

+

+#include "../../syscall.h"

+

+static char heap[16384];

+

+/*

+ * TODO: Temporary solution until we implement mmap in Darwin

+ *       because it does not support any form of brk().

+ */

+void *

+_getheap(void)

+{

+	return heap;

+}

+

+int

+_brk(void *addr)

+{

+	static char *cur = heap;

+	char *p = addr;

+

+	if (p < heap || p > &heap[sizeof(heap) - 1]) {

+		errno = ENOMEM;

+		return -1;

+	}

+	cur = p;

+

+	return 0;

+}