Waiting a child process

Question

This is a fragment of my program that is working, but the machine that corrects it tries this:

strace ./watcher echo 2>&1 > /dev/null | grep wait | cut -c1-4

The expected output is:

wait

But my program prints a random number of waits (like this):

wait
wait
wait
wait
wait
wait

This is the code:

    // creates a child process
    pid_t process_id;
    int status;
    process_id = fork();
    switch(process_id)
    {
        case -1: // Error while making the forkFork failed
            fprintf(stderr, "Fork failed.\n");
            break;

        case 0: // Child process.
            execvp(command[0], command); // command here is char**

            // execvp only returns when error.
            Error(argv[0], 1); // Error just prints the error
            exit(1);
            break;

        default: // Parent process.
            while(!waitpid(process_id, &status, WNOHANG));
            finished(status); // It prints how the process has finished.
    }

I think the problem is in that while with the waitpid inside, that is generating a lot of waits. But if I remove it and leave the waitpid alone, I receive this output:

standard input: Input/output error

Is there a way to make only one wait call without getting that error?

Answer 1

You might try to not set WNOHANG on the call to waitpid().

This would make waitpid() block until the child terminates.