Shell Problem: How to manage terminated or stopped child process.

In both cases, a SIGCHLD is sent to the parent.

Signal handler for SIGCHLD is the one place to handle both events.

What needs to be done in this signal handler for the two cases?

• If a child process terminated, it should be removed from the job table.
• If a child was stopped, its state should be changed in the job table.

When the SIGCHLD signal is received, the signal handler needs to know what child changed state and whether it terminated or stopped.

The handler can call waitpid with the right parameters to get this information.

Since signals are not queued, the handler must call waitpid for all processes that have already terminated or stopped, but must not block!

What are the correct parameters to use?

waitpid parameters	wait set	blocks if	return value
waitpid(-1, &status, 0)	terminated children	children still exist, but none have terminated	pid of a terminated child or -1 if wait set empty
waitpid(-1, &status, WUNTRACED)	terminated OR stopped children	children still exist, but none have terminated OR stopped	pid of a child in the wait set or -1 if no more children
waitpid(-1, &status, WNOHANG)	terminated children	never blocks	pid of a child in the wait set or 0 if children still exist, but none have terminated or -1 no more children
waitpid(-1, &status, WNOHANG | WUNTRACED)	terminated or stopped children	never blocks	pid of a child in the wait set or 0 if children still exist, but none are terminated or stopped or -1 if no more children exist