Waitpid for a child process that execv an unexecutable script

Question

I am doing the following steps in my code:

• fork()
• execv in the child process to run an external script
• in the parent process:

---

 While( waitpid(..., WNOHANG) == 0)
 {
     //Send signal that script has started with no error
      ..
 }

The problem I am facing is that if the script is not executable I still end up in the above while loop and I send the signal that script has started with no errors; which is not true.

Obviously I can run a sys call and determine the permission of the script before hand, but is there a better solution?

Answer 1

Catching execve errors is tricky for exactly the reason you've found: the error is seen in the child process but you want to transfer it to the parent process. There's a nice trick to make it possible:

1. Before forking, open a pipe and set the close-on-exec flags (with pipe2 and O_CLOEXEC , or fcntl if the former is not available on your system).

2. After forking, have the parent close the writing side of the pipe and read from the reading end.

3. In the child, close the reading end of the pipe, and attempt execve . If it fails, write errno to the pipe and _exit .

4. In the parent, if read returns 0, execve was successful. This is because a successful execve closes the pipe (because it's set for close-on-exec) and causes EOF on the reading end. Otherwise the value read is the error code.

Answer 2

Have the child do exit(2) or some other value if it is still there after it execs the script, and test the exit value inside your parent loop. But why the loop? Just get rid of WNOHANG.