fusl/src/malloc/malloc_usable_size.c

#include <malloc.h>

void* (*const __realloc_dep)(void*, size_t) = realloc;

struct chunk {
  size_t psize, csize;
  struct chunk *next, *prev;
};

#define OVERHEAD (2 * sizeof(size_t))
#define CHUNK_SIZE(c) ((c)->csize & -2)
#define MEM_TO_CHUNK(p) (struct chunk*)((char*)(p)-OVERHEAD)

size_t malloc_usable_size(void* p) {
  return p ? CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD : 0;
}