What does waitpid() do?

Question

What is the use of waitpid()?

Answer 1

It's used generally to wait until a specific process finishes (or otherwise changes state if you're using special flags), based on its process ID (otherwise known as a pid).

It can also be used to wait for any of a group of child processes, either one from a specific process group or any child of the current process.

See here for the gory details.