POSIX
Processi

Sintassi

• pid_t getpid (void);
• pid_t getppid (void);
• pid_t fork (void);
• pid_t waitpid (pid pid, int * wstatus, opzioni int);
• int execv (const char * percorso, char * const argv []);

Parametri

Crea un processo figlio e attendi fino alla sua uscita

Questo programma mostra come eseguire un altro processo usando fork() e aspetta la sua terminazione usando waitpid() :

• fork() crea una copia identica del processo corrente. Il processo originale è il processo padre, mentre quello appena creato è il processo figlio. Entrambi i processi continuano esattamente dopo il fork() .

• waitpid() blocca fino a quando il processo figlio non termina o termina e restituisce il codice di uscita e il motivo di terminazione.

#include <unistd.h>     /* for fork(), getpid() */
#include <sys/types.h>  /* for waitpid() */
#include <sys/wait.h>   /* for waitpid() */
#include <stdlib.h>     /* for exit() */
#include <stdio.h>      /* for printf(), perror() */

int
main(int argc, char *argv[])
{
    /* Create child process.
     *
     * On success, fork() returns the process ID of the child (> 0) to the
     * parent and 0 to the child. On error, -1 is returned.
     */
    pid_t child_pid = fork();

    if (child_pid < 0) {
        perror("fork() failed");
        exit(EXIT_FAILURE);
    } else if (child_pid == 0) {
        /* Print message from child process.
         *
         * getpid() returns the PID (process identifier) of current process,
         * which is typically int but doesn't have to be, so we cast it.
         *
         * getppid() returns the PID of the parent process.
         */
        printf("from child: pid=%d, parent_pid=%d\n",
                 (int)getpid(), (int)getppid());

        /* We can do something here, e.g. load another program using exec().
         */
        exit(33);
    } else if (child_pid > 0) {
        /* Print message from parent process.
         */
        printf("from parent: pid=%d child_pid=%d\n",
                  (int)getpid(), (int)child_pid); 
        
        /* Wait until child process exits or terminates.
         *
         * The return value of waitpid() is PID of the child process, while
         * its argument is filled with exit code and termination reason.
         */
        int status;
        pid_t waited_pid = waitpid(child_pid, &status, 0);

        if (waited_pid < 0) {
            perror("waitpid() failed");
            exit(EXIT_FAILURE);
        } else if (waited_pid == child_pid) {
            if (WIFEXITED(status)) {
                /* WIFEXITED(status) returns true if the child has terminated 
                 * normally. In this case WEXITSTATUS(status) returns child's
                 * exit code.
                 */
                printf("from parent: child exited with code %d\n",
                          WEXITSTATUS(status));
            }
        }
    }

    exit(EXIT_SUCCESS);
}

Esempio di output:

from parent: pid=2486 child_pid=2487
from child: pid=2487, parent_pid=2486
from parent: child exited with code 33

Copiato da qui , originariamente creato da M.Geiger.