What can cause exec to fail? What happens next?

Question

What are the reasons that an exec (execl,execlp, etc.) can fail? If you make a call to exec and it returns, are there any best practices other than just panicking and calling exit?

Answer 1

The problem with handling exec failure is that usually exec is performed in a child process, and you want to do the error handling in the parent process. But you can't just exit(errno) because (1) you don't know if error codes fit in an exit code, and (2), you can't distinguish between failure to exec and failure exit codes from the new program you exec.

The best solution I know is using pipes to communicate the success or failure of exec:

1. Before forking, open a pipe in the parent process.
2. After forking, the parent closes the writing end of the pipe and reads from the reading end.
3. The child closes the reading end and sets the close-on-exec flag for the writing end.
4. The child calls exec.
5. If exec fails, the child writes the error code back to the parent using the pipe, then exits.
6. The parent reads eof (a zero-length read) if the child successfully performed exec, since close-on-exec made successful exec close the writing end of the pipe. Or, if exec failed, the parent reads the error code and can proceed accordingly. Either way, the parent blocks until the child calls exec.
7. The parent closes the reading end of the pipe.