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I made a program using fork() and exec*(). The problem is I can't determine success or failure of exec() from parent process because it's on separated child process. I think kind of signaling can be used to check this state, but I have no idea about this.

  1. What's the recommended/standard/widely-used way to check this?
  2. And what's the pitfalls that I have to care about while doing this?

Question Detail Update (Sorry for omission of important detail)

I want to keep both processes are running so I can't just wait exiting of child process. In other words, I want to be notified about the child process' exec success or failure.

Hubert Kario
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eonil
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  • You do know that you can query the child status without blocking by using `waitpid` and the `WNOHANG` option ? Depending on your system you might also have `wait3` and `wait4` which also understand this flag. Or do you want to signal something to the parent while keeping the child running (like with IPC semaphores) ? – DarkDust Sep 25 '11 at 16:31
  • @DarkDust I want to keep both processes are running and I want to be notified the child process' `exec` state. (success or failure) Oh you cleared my question. Thanks :) – eonil Sep 25 '11 at 16:33
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    possible duplicate of [What can cause exec to fail? What happens next?](http://stackoverflow.com/questions/3703013/what-can-cause-exec-to-fail-what-happens-next) (In particular, see [the answer from "R."](http://stackoverflow.com/questions/3703013/#3703179)) – Nemo Sep 25 '11 at 16:46

1 Answers1

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Your parent process can use the pid of the child process to detect that it is alive or has exited (and can disambiguate the error code, and died-because-of-signal errors, see waitpid). You can use certain error codes or signals to notify the parent about specific error cases (e.g., in the forked child before the exec), but for a completely generic child, you may not be able to reserve any exit codes or signals (since the parent won't be able to tell if the exec succeeded and then the child exited with those values).

Another approach often used is to create a pipe fd pair (see the 'pipe' syscall), and pass one end to the child (generally the write end) and the other to the parent. The child can use this to send specific error codes to the parent. And the parent can detect premature termination if the pipe is closed without getting any data. There are some pitfalls: SIGPIPE will be sent to the parent if it reads on a pipe with no active writers, and using up an fd (other than stdin/stdout/stderr) in a child process may confuse some badly-written child processes (though close-on-exec can help fix that).

In general, all the code I've seen to make fork+exec robust is pretty hacky.

P.T.
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