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waiting for multiple childs - C - waitpid

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# 1  
Old 02-28-2011
waiting for multiple childs - C - waitpid
Hi gurus, I would like to fork more children and then write their return values: so far I tried:

Code:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void)
{
        pid_t pid;
        int rv=0, i;
        pid_t child_pids[5];


        for (i=1; i<=5; i++){

                /* Multiple child forking */
                switch(pid = fork()){
                        case -1:
                                /* something went wrong */
                                /* parent exits */
                                perror("fork");
                                exit(1);

                        case 0:
                                /*Children process*/
                                child_pids[i] = getpid();
                                printf(" CHILD: number (and return value): %d PID: %d PPID: %d \n", i, getpid(), getppid());
                                exit(i);
                                break;

                                /*Missing code for parent process - will be executed out of loop*/
                }
        }
        /*Parent process*/
        if (pid!=0 && pid!=-1) {
                printf("PARENT: my PID is %d\n", getpid());

                for (i=1; i<=5; i++){
                        waitpid(child_pids[i], NULL, 0);
                        printf("PARENT: Child: %d returned value is: %d\n", i, WEXITSTATUS(child_pids[i]));
                }
        }
}

But returns any random variables for child process:

...
PARENT: Child: 3 returned value is: 13
PARENT: Child: 3 returned value is: 127
...


also tried following
Code:
int main(void)
{
        pid_t pid;
        int rv=0, i;
        pid_t child_pids[5];
        int child_state = 9;
        pid_t rc_pid;


        for (i=1; i<=5; i++){

                /* Multiple child forking */
                switch(pid = child_pids[i] = fork()){
                        case -1:
                                /* something went wrong */
                                /* parent exits */
                                perror("fork");
                                exit(1);

                        case 0:
                                /*Children process*/
                                //child_pids[i] = getpid();
                                printf(" CHILD: number (and return value): %d PID: %d PPID: %d \n", i, getpid(), getppid());
                                exit(i);
                                break;

                                /*Missing code for parent process*/
                }
        }
        /*Parent process*/
        if (pid!=0 && pid!=-1) {
                printf("PARENT: my PID is %d\n", getpid());

                for (i=1; i<=5; i++){
                        rc_pid = waitpid(child_pids[i], NULL, 0);
                        printf("PARENT: Child: %d returned value is: %d\n", i, WEXITSTATUS(child_pids[i]));
                }
        }
}

but returns always number 19 as return code of children
...
PARENT: Child: 4 returned value is: 19
...

Probably something something is wrong with waitpid() function
Thanks a lot.
# 2  
Old 02-28-2011
Yes, you're not supposed to use WEXITSTATUS() on a PID, it's a different integer, but you're throwing that value away. Try:

Code:
{
        int status;
        waitpid(child_pids[i], &status, 0);
        printf("exit status %d = %d\n", i, WEXITSTATUS(status));
}

This User Gave Thanks to Corona688 For This Post:
wakatana
# 3  
Old 03-01-2011
Quote:
Originally Posted by Corona688
Yes, you're not supposed to use WEXITSTATUS() on a PID, it's a different integer, but you're throwing that value away. Try:

Code:
{
        int status;
        waitpid(child_pids[i], &status, 0);
        printf("exit status %d = %d\n", i, WEXITSTATUS(status));
}

Thanks a lot that worked as expected Smilie just one question is this right way to create children ?
# 4  
Old 03-01-2011
Instead of 'missing code for parent section' I'd do this:

Code:
default: /* Parent does nothing in the loop */
        break;

Some compilers need it.

I'd also count errors to make sure you're not waiting for more processes than you actually created.

Otherwise I see nothing incorrect in your code.
Last edited by Corona688; 03-01-2011 at 04:55 PM..
# 5  
Old 03-01-2011
if there is
Code:
                        case -1:
                                /* something went wrong */
                                /* parent exits */
                                perror("fork");
                                exit(1);

program should quit if some errors happens and no waiting will be called. or am I missed something ?
# 6  
Old 03-01-2011
You're right, I didn't quite catch that.
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