malloc has a very nice and simple interface: you ask for a certain amount of memory, and you’re given a void pointer to your block of contiguous bytes that’s been magicked into existence. Once you’re done with it, you call free with the same pointer and the memory disappears into the ether.
But what’s actually happening here? Obviously, as we’re running as a userspace program, malloc must be asking the kernel for the memory through syscalls. However, we may be allocating many small blocks, and syscalls are expensive (575 ns for an empty one, as measured below), so some bookkeeping is done in the background. We’ll demonstrate this on GNU libc (glibc) with the program snippet below, which calls malloc(16) a thousand times.
