LinuxQuestions.org - FORK( ) ? why not threads

fork() is used to create processes; the process that calls fork() is called the parent and the new process is called the child; so, nope, a "father" is not created, only the "son."

Your shell program does this: you type a command and hit the return/ether key and the shell forks and executes your command in a child process, a new process. The parent waits until the child completes (and dies or you kill it with, say, Ctrl-C) returning you to your shell prompt.

In C programming you would use fork(), wait() and one of the exec() functions in a child process in lieu of using the system() function (it's more efficient to "fork and exec" than to call system()).

POSIX Threads (pthreads) can be used to implement parallelism; not the same thing. POSIX Threads are most effective on multi-processor or multi-core systems where the process flow can be scheduled to run on another processor thus gaining speed through parallel or distributed processing.

Here's a simple example of "fork and exec;" it is a simple command interpreter (the shell is a command interpreter albeit with a lot more function and capabilities) that prompts for a command the executes it:

Code:

#include <stdio.h>

#include <stdlib.h>

#include <errno.h>

#include <unistd.h>

#include <wait.h>

#include <sys/types.h>



void    main    (void)

{

        char    line [BUFSIZ];

        int     process;



        for ( ; ; ) {

                (void) fprintf (stderr, "cmd: ");

                if (gets (line) == (char *) NULL)

                        exit (EXIT_FAILURE);

                /*      create new process      */

                if ((process = fork ()) > 0)

                        (void) wait ((int *) NULL);

                else if (process == 0) {        /* child        */

                        /*      execute program                 */

                        (void) execlp (line, line, NULL);

                        /*      some problem if exec returns    */

                        (void) fprintf (stderr, "can't execute %s\n", line);

                        exit (errno);

                } else if (process == -1) {     /* can't create */

                        (void) fprintf (stderr, "can't fork\n");

                        exit (errno);

                }

        }

}

Pretty straight-forward.

Here's and example (from http://www.yolinux.com/TUTORIALS/Lin...ixThreads.html) that demonstrates thread creation and termination:

Code:

#include <stdio.h>

#include <stdlib.h>

#include <pthread.h>



void *print_message_function( void *ptr );



main()

{

     pthread_t thread1, thread2;

     char *message1 = "Thread 1";

     char *message2 = "Thread 2";

     int  iret1, iret2;



    /* Create independent threads each of which will execute function */



     iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);

     iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);



     /* Wait till threads are complete before main continues. Unless we  */

     /* wait we run the risk of executing an exit which will terminate   */

     /* the process and all threads before the threads have completed.   */



     pthread_join( thread1, NULL);

     pthread_join( thread2, NULL); 



     printf("Thread 1 returns: %d\n",iret1);

     printf("Thread 2 returns: %d\n",iret2);

     exit(0);

}



void *print_message_function( void *ptr )

{

     char *message;

     message = (char *) ptr;

     printf("%s \n", message);

}

Now, bear in mind that POSIX Threads are useful on multi-core or multi-procesor systems where the operating system schedules and manages the threads (Linux does).

Basically, they're not the same things and each has its uses.

Hope this helps some.