how a process know's which child ended?

Question

when a process child is terminated he's sending a SIGCHLD to the parent process. Now, if the parent process have more than one child how the parent process knows which child had sent the signal?

Answer 1

The wait() syscall returns the pid of the child that terminated. You can call wait() in your SIGCHLD handler to determine which child terminated.

From the man page:

       wait(): on success, returns the process ID of the terminated child;
   on error, -1 is returned.

Answer 2

In addition to the wait() family of calls as in @shanet's answer, the SIGCHLD itself carries that information.

The si_pid member of the siginfo_t member passed to a three-argument signal handler (SA_SIGINFO) contains the child's PID.