waitpid returns pid=0 and WIFEXITED=1 how to get pid?

3
votes

Steps:

Fork and start process in a different program group
Stop process with SIGTSTP
Restart process with SIGCONT
Process ends

Problem: The SIGCHLD handler has:

waitpid(-1, &status, WNOHANG | WUNTRACED);

upon return pid=0 and WIFEXITED=1 so, the process exited, but I can't get the pid? I need the pid.

From the man page: "if WNOHANG was specified and one or more child(ren) specified by pid exist, but have not yet changed state, then 0 is returned"
But it seems the status has changed to exited.

2
clinuxpidwaitpid

2 Answers

4
votes

The status is meaningless if the pid returned was 0. Think about it. A return of 0 means you have one or more children that have yet to change state. What would the state of a child that has yet to change state be? If there were multiple children, which child is the status code referencing?

This is analogous to checking errno on a successful call. Anything from a previous call can be in errno but it has nothing to do with the most recent successful call because errno is usually not set on success.

1
votes

The return value of waitpid is the PID of the child that was waited for.