Wait until all background jobs have terminated, C

2
votes

I know how waitpid(-1...) allows me to wait until all children have finished, such as waitpid(-1, &status). But how can I wait until all background processes are finished? Someone suggested that I can use the same waitpid (in a loop?) to achieve this but I don't see how.

To be clear, I'm implementing a shell, and need to add a new built-in command wait, which waits until all background jobs have terminated before returning to the prompt.

I read somewhere else on SO that "You will also want to call waitpid() with the WNOHANG option regularly - say, immediately before you display the shell prompt. This will allow you to detect when the background process has exited or stopped " But again, child != background. So even that I don't believe.

Edit:

I ended up just doing while(wait(NULL) > 0); and that's it, it worked. But what I'm still confused about is don't I WANT to make a distinction between foreground and background because the wait I'm implementing only waits for the background processes, and all children are equal in the eye of wait() or waitpid().

So again, the children I'm waiting for by using wait() or waitpid() aren't necessarily background processes. Am I wrong?

2
processforkwaitpid
C has no concept of "background processes". Background vs. foreground is a shell job-control thing. So what do you mean by it? – John Bollinger
I am implementing a shell, so you are exactly right. – ajfbiw.s

2 Answers

1
votes

Since you ask in the context of implementing a shell, and evidently your shell supports enough job control to have a concept of background processes, it is reasonable to suppose that your implementation will have a table in which it tracks background jobs. That table can and should track the PID of the process associated with each job.

Having those PIDs in hand, you can waitpid() for specific background jobs until there are no more in the table, or you can waitpid(-1) to collect any and every job, in a loop, until there are no more background jobs in the table.

0
votes

If you want to implement background process (and job control) and catch their termination (at least) you must set a signal handler for SIGCHLD, and call wait(-1) inside it. This will let your shell receive asynchronous notifications of background processes termination. You may have a look at Catching SIGCHLD for example and discussion about this.