What's the difference between malloc and calloc?

Both allocate heap memory, but they differ in two ways. malloc(size) takes a single byte count and leaves the memory uninitialized, it contains whatever was there before, so reading it before writing yields garbage. calloc(count, size) takes the element count and element size separately, and zero-initializes the whole block. So calloc(n, sizeof(int)) gives you n ints all set to 0. calloc also internally guards against the count * size multiplication overflowing, if it would overflow size_t, calloc fails safely, whereas a hand-written malloc(count * size) could overflow and under-allocate. Use calloc when you need a clean zeroed buffer or when computing the size from untrusted counts.