我很困惑在这个使用fork（）克隆进程的示例函数中如何处理execvp（）

Question

I have the following function from a book titled "Advanced Linux Programming".

int spawn (char* program, char** arg_list)
{
    pid_t child_pid;

    /* Duplicate this process. */
    child_pid = fork ();

    if (child_pid != 0)
    /* This is the parent process. */
    return child_pid;

    else {
    /* Now execute PROGRAM, searching for it in the path. */
    execvp (program, arg_list);

    /* The execvp function returns only if an error occurs. */
    fprintf (stderr, “an error occurred in execvp\n”);
    abort ();
    }
}

But I'm confused that, in cases where ls is executed successfully, the error is not printed, but in case it fails, it prints the error which is put in the line following it.

My Question This line fprintf (stderr, “an error occurred in execvp\\n”); is after the execvp() function, and it is expected to be executed after the execution of execvp() finishes, but it is not the case, and it is executed only if execvp() encounters an error. It seems the function spawn() finishes as soon as it executes execvp() successfully. Am I right?

Answer 1

You can have a look at the manpage for execvp , it says:

The exec() family of functions replaces the current process image with a new process image.

So, what does that mean? It means, if execvp succeeds, your program wont be in memory anymore, thus it wont ever reach the error message. Your program in memory will be replaced by the new program (in your case ls if i understood it correctly).

So, if your program is able to reach the error message printout, then the execvp function will have failed. Otherwise the other program starts execution.

The reason why your programm will be still running is the fork command, which creates a copy of the process image, so you will be having two same processes running of which only one will be replaced by the command you try to execute. This is achieved by the if clause if (child_pid != 0) , as the fork command will duplicate the process and return the new Process ID ( PID ). If this is set to 0 (see man 3 fork ), then its the new child process, if its != 0 its the parent process. Your function there only executes execvp if its the child process, the parent process encounters an early return.