move core memalign code from aligned_alloc to __memalign

there are two motivations for this change. one is to avoid
gratuitously depending on a C11 symbol for implementing a POSIX
function. the other pertains to the documented semantics. C11 does not
define any behavior for aligned_alloc when the length argument is not
a multiple of the alignment argument. posix_memalign on the other hand
places no requirements on the length argument. using __memalign as the
implementation of both, rather than trying to implement one in terms
of the other when their documented contracts differ, eliminates this
confusion.

src/malloc/aligned_alloc.c

 #include <stdlib.h>
-#include <stdint.h>
-#include <errno.h>
 
-/* This function should work with most dlmalloc-like chunk bookkeeping
- * systems, but it's only guaranteed to work with the native implementation
- * used in this library. */
+void *__memalign(size_t, size_t);
 
 void *aligned_alloc(size_t align, size_t len)
 {
-	unsigned char *mem, *new, *end;
-	size_t header, footer;
-
-	if ((align & -align) != align) {
-		errno = EINVAL;
-		return NULL;
-	}
-
-	if (len > SIZE_MAX - align) {
-		errno = ENOMEM;
-		return NULL;
-	}
-
-	if (align <= 4*sizeof(size_t)) {
-		if (!(mem = malloc(len)))
-			return NULL;
-		return mem;
-	}
-
-	if (!(mem = malloc(len + align-1)))
-		return NULL;
-
-	header = ((size_t *)mem)[-1];
-	new = (void *)((uintptr_t)mem + align-1 & -align);
-
-	if (!(header & 7)) {
-		((size_t *)new)[-2] = ((size_t *)mem)[-2] + (new-mem);
-		((size_t *)new)[-1] = ((size_t *)mem)[-1] - (new-mem);
-		return new;
-	}
-
-	end = mem + (header & -8);
-	footer = ((size_t *)end)[-2];
-
-	((size_t *)mem)[-1] = header&7 | new-mem;
-	((size_t *)new)[-2] = footer&7 | new-mem;
-	((size_t *)new)[-1] = header&7 | end-new;
-	((size_t *)end)[-2] = footer&7 | end-new;
-
-	if (new != mem) free(mem);
-	return new;
+	return __memalign(align, len);
 }

src/malloc/memalign.c

 #include <stdlib.h>
+#include <stdint.h>
 #include <errno.h>
+#include "libc.h"
 
-void *memalign(size_t align, size_t len)
+/* This function should work with most dlmalloc-like chunk bookkeeping
+ * systems, but it's only guaranteed to work with the native implementation
+ * used in this library. */
+
+void *__memalign(size_t align, size_t len)
 {
-	return aligned_alloc(align, len);
+	unsigned char *mem, *new, *end;
+	size_t header, footer;
+
+	if ((align & -align) != align) {
+		errno = EINVAL;
+		return NULL;
+	}
+
+	if (len > SIZE_MAX - align) {
+		errno = ENOMEM;
+		return NULL;
+	}
+
+	if (align <= 4*sizeof(size_t)) {
+		if (!(mem = malloc(len)))
+			return NULL;
+		return mem;
+	}
+
+	if (!(mem = malloc(len + align-1)))
+		return NULL;
+
+	header = ((size_t *)mem)[-1];
+	new = (void *)((uintptr_t)mem + align-1 & -align);
+
+	if (!(header & 7)) {
+		((size_t *)new)[-2] = ((size_t *)mem)[-2] + (new-mem);
+		((size_t *)new)[-1] = ((size_t *)mem)[-1] - (new-mem);
+		return new;
+	}
+
+	end = mem + (header & -8);
+	footer = ((size_t *)end)[-2];
+
+	((size_t *)mem)[-1] = header&7 | new-mem;
+	((size_t *)new)[-2] = footer&7 | new-mem;
+	((size_t *)new)[-1] = header&7 | end-new;
+	((size_t *)end)[-2] = footer&7 | end-new;
+
+	if (new != mem) free(mem);
+	return new;
 }
+
+weak_alias(__memalign, memalign);

src/malloc/posix_memalign.c

 #include <stdlib.h>
 #include <errno.h>
 
+void *__memalign(size_t, size_t);
+
 int posix_memalign(void **res, size_t align, size_t len)
 {
 	if (align < sizeof(void *)) return EINVAL;
-	void *mem = aligned_alloc(align, len);
+	void *mem = __memalign(align, len);
 	if (!mem) return errno;
 	*res = mem;
 	return 0;