allocating memory

have this pointer
Char *ptr
i allocate memory as
ptr = (char *) malloc (sizeof(char) * 100);
say later on in the program i may or may not need to store more characters so i'll use realloc to assign more memory. How can i find out how much memory is currently assigned to ptr so i cant find if i need to resize it or not.

OP -- if you want to write sepples, you write

std::vector<char> x(100);
char * ptr = &x[0]; // if you have a 'C' based API that needs a raw pointer

remembering to catch std::bad_alloc, and resetting ptr each time you change the size of the vector.