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.

You're all doing it wrong.

OP, you can usually find the size of malloc'd memory 4 bytes before its start, so subtract 4 from the pointer and read that as an int. If it appears negative, just change its sign.

If you're compiling for 64-bit, you might have to change change 4 to 8.