Note that it is easy to implement that function easily even if your system does not support it; write a wrapper for malloc/realloc/free that adds space to store the size. Example:
```c
void*my_malloc(int x) {
int*h;
if(!x) return 0;
h=malloc(x+sizeof(int));
if(!h) return 0;
*h=x;
return h+1;
}
void my_free(void*x) {
int*o=x;
if(x) free(o-1);
}
```
(Note: This code is untested)