When and why should you use WNOHANG with waitpid()?

linux waitpid
user3305321 picture user3305321 · Mar 3, 2015 · Viewed 7.6k times · Source

I'm currently in a systems programming class and we went over the wait system call functions today. I was reading over the section on waitpid() system call and in the options section it lists one called WNOHANG.

pid_t waitpid*(pid_t pid, int *status, int options);

WNOHANG: If no child specified by pid (from the parameters) has yet changed state, then return immediately, instead of blocking. In this case, the return value of waitpid() is 0. If the calling process has no children that match the specification in pid, waitpid() fails with the error ECHILD.

I understand waitpid() was implemented to solve the limitations in wait(); however, I'm not really sure about why you would use the WNOHANG option flag.

If I were to render a guess it would be so that the parent process can preform other tasks and perhaps keep checking on its children to see if any of them have terminated. Sort of how a demon process sits in the background and waits for requests.

Any situational examples or regular examples would help as well.

Thanks in advance!

Answer

Tomasz Myrta picture Tomasz Myrta · Mar 3, 2015

You don't need to keep checking on children. It is job of SIGCHLD signal handler. Every time this handler is fired, you check terminated children:

pid_t pid;
int status;
while ((pid=waitpid(-1,&status,WNOHANG)) > 0)
{
  //process terminated child
}

Related questions

waitpid with execl used in child returns -1 with ECHILD?

When do I need to use waitpid if I am using execl in a child process which may take time to finish? When I use waitpid in the parent, it tells me that the child is running as the return …

Read More
Return code when OOM killer kills a process

I am running a multiprogrammed workload (based on SPEC CPU2006 benchmarks) on a POWER7 system using SUSE SLES 11. Sometimes, each application in the workload consumes a significant amount of memory and the total memory footprint exceeds the available memory installed …

Read More
wait() function in Ubuntu

I am learning Processes and their behavior in Ubuntu, but I am a bit confused in wait(). So my questions are: How the statment while(wait(NULL)>0); is working? What is the purpose of NULL in wait()? I have …

Read More