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Old 09-25-2012, 09:42 AM   #1
elico
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FORK( ) ? why not threads


The FORK statement generates a father and a son, each is doing it's own things .

What is the big deal in this kind of
coding structure ?

Why not threads that do not depend on each other
and can communicate .

What is the big advantage of "Ftaher and Son "

Thanks
Elico
 
Old 09-25-2012, 11:26 AM   #2
tronayne
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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.

Last edited by tronayne; 09-25-2012 at 11:28 AM.
 
1 members found this post helpful.
Old 09-27-2012, 12:28 AM   #3
elico
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Thnaks
I will go into it.

Elico
 
  


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