If free() knows the length of my array, why can't I ask for it in my own code?

Question

I know that it\'s a common convention to pass the length of dynamically allocated arrays to functions that manipulate them:

void initializeAndFree(int* anArr         


        

Answer 1

After reading Klatchko's answer, I myself tried it and ptr[-1] indeed stores the actual memory (usually more than the memory we asked for probably to save against segmentation fault).

{
  char *a = malloc(1);
  printf("%u\n", ((size_t *)a)[-1]);   //prints 17
  free(a);
  exit(0);
}

Trying with different sizes, GCC allocates the memory as follows:

Initially memory allocated is 17 bytes.
The allocated memory is atleast 5 bytes more than requested size, if more is requested, it allocates 8 bytes more.

• If size is [0,12], memory allocated is 17.
• If size is [13], memory allocated is 25.
• If size is [20], memory allocated is 25.
• If size is [21], memory allocated is 33.