[SOLVED] waiting for multiple childs - C

wakatana · 02-28-2011, 04:38 AM

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.

dwhitney67 · 02-28-2011, 05:53 AM

You should focus your eyes on the following:

Code:

pid_t child_pids[5];


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

Indices for arrays are zero-based, not one-based.

wakatana · 02-28-2011, 06:56 AM

Thanks for reply - I repair this mistake but still did not working as expected - seems that waitpid did not catch the correct return value

dwhitney67 · 02-28-2011, 07:48 AM

You are not obtaining the exit status that the child process posts when it exits. Call waitpid() like this:

Code:

int status;

waitpid(child_pids[i], &status, 0);

printf("PARENT: Child: %d returned value is: %d\n", child_pids[i], WEXITSTATUS(status));

rupertwh · 02-28-2011, 06:26 PM

Quote:

Originally Posted by wakatana

But returns any random variables for child process:

Maybe that's because you are feeding it random data?

Your parent process never fills it's child_pids[] array -- you do that inside the child processes instead of the parent.

So the parent's child_pids[] is uninitialized and hence filled with random data. (The children's child_pids[] each have one value set)

wakatana · 03-01-2011, 02:45 PM

Quote:

Originally Posted by dwhitney67

You are not obtaining the exit status that the child process posts when it exits. Call waitpid() like this:

Code:

int status;

waitpid(child_pids[i], &status, 0);

printf("PARENT: Child: %d returned value is: %d\n", child_pids[i], WEXITSTATUS(status));

Thanks a lot that worked as expected

just one question is this right way to create children ?

dwhitney67 · 03-01-2011, 03:08 PM

Quote:

Originally Posted by wakatana

... just one question is this right way to create children ?

I'm not sure there's any other way other than using fork(). You can create a separate process using one of the "exec" calls (e.g. execl(), execv(), etc), however these don't share resources in the manner that fork() affords us.

wakatana · 03-01-2011, 03:38 PM

Thank you