CITS2002 - Lecture 10

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CITS2002 - Lecture 9
Forking Processes - Slides

Creating Process with fork()

• fork() creates a new process (child) from the calling process (parent)
  i.e. it makes a copy of the current process
  • Both parent and child continue running same code, starting after the fork() line
• fork() can return different values:
  • 0 $-$ in the child process (because 0 is not valid PID)
  • >0 (child’s PID) $-$ in the parent process
  • -1 $-$ error (failed to create process)

Thus, you can write code that behaves differently in parent vs child:

int pid = fork();
 
if (pid == -1) {
    printf("fork failed\n");
} else if (pid == 0) {
    // child
    printf("I am the child process!\n");
} else {
    // parent
    printf("I am the parent process, my child has PID %d\n", pid);
}

Memory in Parent vs Child Process

• When fork() is used, both the parent and child process get an exact copy of the same memory
  • Variables, stack and heap are duplicated
• After fork(), the two processes won’t see changes to the memory that the other makes

Modern OSes don’t copy everything immediately and instead wait till one of the programs changes something → this saves time and memory

Running New Programs with exec()

• Sometimes we don’t want child program to run same code as parent
• Using exec() we can replace the child program with another program
  • PID remains the same
  • Note: does not always needed to be called in child program

int main(void) {
    int pid = fork();
 
    if (pid == 0) {  
        // child process
        char *args[] = {"ls", "-l", NULL};
        execvp("ls", args);   // replace child with "ls -l"
    } else {
        // parent process
        wait(NULL);           // wait for child to finish
        printf("Child finished!\n");
    }
    return 0;
}

Above, fork() makes child program, execvp() replaces child program with ls -l which lists files in a directory

• There are many different variants of exec()
  • l = “list” (you pass arguments one by one)
  • v = “vector” (you pass arguments as an array/vector)
  • p = search your $PATH for program instead of giving full path

Above, "ls" is the program name and args is an array of argument passed to the program

Waiting for Child Process

• Using wait(&status), the parent process can pause until the child process has finished executing
  • Once the child has finished running, the exit value is passed through status
• When child calls exit(value), the value it returns represents its status:
  • exit(0) → success
  • exit(1) (or other non-zero) → failure

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CITS2002 - Lecture 11