Child process takes over parent process when running fork()

Question

I have a problem where I am trying to run multiple commands through execvp(). To do this, I have a while loop doing through each command, parsing it, then calling a function to use fork + exec.

The issue is, I will run fork + exec, and while I wait for the exec to go through, the parent runs and continues on to the second loop, ie. the second command. But what happens, from my understanding, is the child process from the previous loop takes over and all of a sudden, the child process is the current process.

How do you run a process in the child process, but maintain control in the parent? All the examples I've seen on SO have been for parent processes what wait for the child process to terminate before continuing, but I don't have a choice in this assignment - I'm supposed to keep the child processes running and check later to see if they're still running.

Here's some pseudocode of my thought process:

funct forkprocess() {
   //call fork
   //call execvp within child process, let process run and return control to parent
       //if execvp failed, kill child process
   //return child pid
}

int main() {
   while(not end of file containing list of commands) :
      //parse command for execvp call
      //call forkprocess() to run process
      //if childpid is returned, report it and store pid
      //else, report failure
   }
}

My output has been close to the following:

\\parent PID is printed
\\any code outside the if-else ladder for fork is printed
\\it jumps back to main and prints any statements there

[jumps to next iteration in loop, ie. the next command]

\\child PID is printed
\\parent PID is printed
\\any code outside the if-else ladder for fork is printed
\\it jumps back to main and prints any statements there
\\child PID is printed

Answer 1

You need to make sure that exec is only called in the child, not the parent. When fork() returns, it does so twice (once in the child, once in the parent), and it returns the following values:

• non-zero (the PID of the child) if we are in the parent. We can use this PID to wait for the child to terminate using waitpid.
• zero if we are in the child.
• negative if the fork failed (in which case it only returns once, in the parent process, since a child process could not be created)

You may want to consider using the following structure:

int pid = fork();
if(pid < 0) {
    perror("fork"); // remember to error check!
} else if (pid == 0) {
    // We are in the child
    // ...
    if(execvp(...) < 0) {
        perror("exec");
    }
} else {
    // We are in the parent; wait for the child to terminate using waitpid if desired.
    waitpid(...);
}

I include the waitpid call here for simplicity; since your assignment requires you to keep the child processes running you can check their status later by using either that function, or by handling SIGCHLD.

Answer 2

int main(int argc, char ** argv) {
    int status;

    if((waitpid(fork(), &status, 0)) == -1) {
       execvp(...);
       /* print error */
       exit(1);
    }
    return 0;
}

Thanks for the question, if fork is passed straight into waitpid the parent will wait, meanwhile the child will execute exec, which will exit the thread on success or return on failure and continue to print the error and exit. This is really helpful for executing multiple functions at once ...

int main(int argc, char ** argv) {
    int *status = malloc(sizeof(int) * (argc - 1));
    int i = 0;

    while(((waitpid(fork(), &status[i++], 0)) == -1) || i < argc) {
       execvp(argv[i], &argv[i + 1]);
       /* print error */
       exit(1);
    }
    return 0;
}