c - How to dynamically calculate the size of a dynamically allocated memory -

considering code follows:

int i, a_size, s_size, n; char **a;  a_size = 100; // examples s_size = 10;  = malloc(a_size * sizeof(char*)); (int = 0; < a_size; i++)     a[i] = malloc((s_size) * sizeof(char));  

now, calculate how many elements there inside array dynamically (thus, ignoring a_size). proper way that?

in general can't, , should take care of kind of book-keeping yourself, 1 possibility store additional row pointer set null (aka sentinel):

a = malloc((a_size + 1) * sizeof(char*));   // allocate additional row pointer (int = 0; < a_size; i++)            // allocate rows     a[i] = malloc(s_size);  a[a_size] = null;                           // set sentinel row null 

then can determine size iterating through row pointers until find null row. note quite inefficient if often, or if number of rows large.