fork() System Call

Creates a new process
- The newly-created process has its own copy of the address space, registers, and PC.

Warning: fork() is pretty odd, no matter how unusual your routine-calling patterns are.

A program that calls fork()

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    printf("hello world (pid:%d)\n", (int) getpid());
    int rc = fork();
    if (rc < 0) {
        // fork failed; exit
        fprintf(stderr, "fork failed\n");
        exit(1);
    } else if (rc == 0) {
        // child (new process)
        printf("hello, I am child (pid:%d)\n", (int) getpid());
    } else {
        // parent goes down this path (original process)
        printf("hello, I am parent of %d (pid:%d)\n",
           rc, (int) getpid());
    }
    return 0;
}

Running the fork() example

prompt> ./p1
hello world (pid:29146)
hello, I am parent of 29147 (pid:29146)
hello, I am child (pid:29147)
prompt>

Output: not deterministic
- The CPU scheduler determines which process runs at a given moment in time

The wait() System Call

Sometimes it is useful for a parent to wait for a child process to finish what it has been doing.
wait() system call won’t return until the child has run and exited.

A program that calls wait()

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int
main(int argc, char *argv[])
{
    printf("hello world (pid:%d)\n", (int) getpid());
    int rc = fork();
    if (rc < 0) {
        // fork failed; exit
        fprintf(stderr, "fork failed\n");
        exit(1);
    } else if (rc == 0) {
        // child (new process)
        printf("hello, I am child (pid:%d)\n", (int) getpid());
    sleep(1);
    } else {
        // parent goes down this path (original process)
        int wc = wait(NULL);
        printf("hello, I am parent of %d (wc:%d) (pid:%d)\n",
           rc, wc, (int) getpid());
    }
    return 0;
}

Running the wait() example

prompt> ./p2
hello world (pid:29266)
hello, I am child (pid:29267)
hello, I am parent of 29267 (wc:29267) (pid:29266) 
prompt>

Output is deterministic

The exec() System Call

This system call is useful when you want to run a program that is different from the calling program.
Why?

If fork() was strange, exec() is not so normal either.

What exec does()

Given an executable (e.g., wc), and some arguments (e.g., p3.c), it loads code (and static data) from that executable and over-writes its current code segment (and current static data) with it
the heap and stack and other parts of the memory space of the program are re-initialized.
OS simply runs that program, passing in any arguments as the argv of that process

exec does not create a new process

A program that calls fork(), wait(), exec()

int main(int argc, char *argv[])
{
    printf("hello world (pid:%d)\n", (int) getpid());
    int rc = fork();
    if (rc < 0) {
        // fork failed; exit
        fprintf(stderr, "fork failed\n");
        exit(1);
    } else if (rc == 0) {
        // child (new process)
        printf("hello, I am child (pid:%d)\n", (int) getpid());
        char *myargs[3];
        myargs[0] = strdup("wc");   // program: "wc" (word count)
        myargs[1] = strdup("p3.c"); // argument: file to count
        myargs[2] = NULL;           // marks end of array
        execvp(myargs[0], myargs);  // runs word count
        printf("this shouldn't print out");
    } else {
        // parent goes down this path (original process)
        int wc = wait(NULL);
        printf("hello, I am parent of %d (wc:%d) (pid:%d)\n",
           rc, wc, (int) getpid());
    }
    return 0;
}

Running the exec() example

prompt> ./p3
hello world (pid:29383)
hello, I am child (pid:29384)
29 107 1030 p3.c
hello, I am parent of 29384 (wc:29384) (pid:29383) 
prompt>

Why? Motivating the API | UNIX shell

prompt> wc p3.c > newfile.txt

What: output of the wc is redirected into the output file newfile.txt
How:
- call fork() to create a new child process to run wc
- call exec() to run wc
- wait for wc to complete by calling wait()
- Before calling exec(), the shell closes standard output and opens the file newfile.txt.

Lecture 5 starts here

One more program that does ??

int main(int argc, char *argv[])
{
    int rc = fork();
    if (rc < 0) {
        // fork failed; exit
        fprintf(stderr, "fork failed\n");
        exit(1);
    } else if (rc == 0) {
    // child: redirect standard output to a file
    close(STDOUT_FILENO); 
    open("./p4.output", O_CREAT|O_WRONLY|O_TRUNC, S_IRWXU);

    // now exec "wc"...
        char *myargs[3];
        myargs[0] = strdup("wc");   // program: "wc" (word count)
        myargs[1] = strdup("p4.c"); // argument: file to count
        myargs[2] = NULL;           // marks end of array
        execvp(myargs[0], myargs);  // runs word count
    } else {
        // parent goes down this path (original process)
        int wc = wait(NULL);
    assert(wc >= 0);
    }
    return 0;
}

Running the last program

prompt> ./p4
prompt> cat p4.output
      32     109     846 p4.c
prompt>

Beyond redirection | Pipes

grep foo file | wc -l

Operating systems

Process API

Lecture 4 starts here

fork() System Call

A program that calls fork()

Running the fork() example

The wait() System Call

A program that calls wait()

Running the wait() example

The exec() System Call

What exec does()

A program that calls fork(), wait(), exec()

Running the exec() example

Why? Motivating the API | UNIX shell

Lecture 5 starts here

One more program that does ??

Running the last program

Beyond redirection | Pipes