You can't truly do it in "standard" C, because the size of an int and of a char aren't fixed. Let's say you are using a compiler under Windows or Linux on an intel PC...
int i = 5; char a = ((char*)&i)[0]; char b = ((char*)&i)[1];
Remember of endianness of your machine! And that int are "normally" 32 bits, so 4 chars!
But you probably meant "i want to stringify a number", so ignore this response :-)