malloc: add malloc_usable_size()

1 files changed, 14 insertions, 0 deletions

diff --git a/libc/stdlib/malloc-standard/malloc_usable_size.c b/libc/stdlib/malloc-standard/malloc_usable_size.c
new file mode 100644
index 000000000..c78c6a867
--- /dev/null
+++ b/libc/stdlib/malloc-standard/malloc_usable_size.c
@@ -0,0 +1,14 @@
+/*
+  malloc_usable_size - fully inspired by musl implementation
+*/
+
+#include "malloc.h"
+
+/* for malloc_usable_size  */
+#define OVERHEAD (2*sizeof(size_t))
+#define CHUNK_SIZE(c) ((c)->size & -2)
+#define MEM_TO_CHUNK(p) (struct malloc_chunk *)((char *)(p) - OVERHEAD)
+
+size_t malloc_usable_size(void *p) {
+	return p ? CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD : 0;
+}