Question

Help with wait3() function

Asked by: kinko899

Hi.....I was wondering if anyone can help me with the following problem that I am having.  I just wrote a problem that forks a new child process and tracks the user and system time statistics for it.  I used the getrusage() function to track the statistics of the child process.  However, I used the wait() function to have the parent wait for the child to terminate.  I would like to rewrite my code to use the wait3() function instead of using the wait() function and print out the same results.  I have looked at the Unix man pages and some online resources but I can't seem to get it work correctly.  Below is a copy of my source code.  You will also find a commented out line that I tried to implement wait3() with but it did not seem to work.  Any help will be greatly appreciated.  Thanks :-)

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/resource.h>
#include <sys/time.h>

int main(int argc, char *argv[])
{

        struct rusage ru_start;
        double sys;
        double user;
        pid_t pid;
        pid_t waitpid;
        time_t ltime;
        int status;


      time(&ltime);
      printf("Start Time: %s\n", ctime(&ltime));

      if ((pid = fork()) < 0)
      {
            perror("fork() failed");
            exit(1);
      }

      if (pid == 0)
      {
            printf("Child pid = %d, ppid = %d\n", getpid(), getppid());

            execl("child_program","child_program", NULL);
            perror("execl");
            exit(1);
      }

        /* waitpid = wait3(&status, WNOHANG, &ru_start); */

         waitpid = wait(NULL);  

        getrusage(RUSAGE_CHILDREN, &ru_start);    

        printf("user time is %ld.%06ld\n", ru_start.ru_utime.tv_sec, ru_start.ru_utime.tv_usec);
        printf("system time is %ld.%06ld\n", ru_start.ru_stime.tv_sec, ru_start.ru_stime.tv_usec);


        printf("got to the end\n");

}

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Asked On
2004-05-28 at 16:44:47ID21006377
Tags

rusage_children

Topic

C Programming Language

Participating Experts
2
Points
500
Comments
3

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Answers

 

by: brettmjohnsonPosted on 2004-05-28 at 20:15:54ID: 11185457

wait3() with WNOHANG does not actually wait.   If a child process has exited (or some other
signal is pending),  wait3(..., WNOHANG, ...) returns with the status, otherwise it returns
immediately with a status of 0.  At the time wait3() is called in the parent program, the child
probably hasn't even started executing yet.  Since there is no status, wait3(WNOHANG)
returns immediately, and rusage structure is either empty or only contains information about
the parent.

 

by: sunnycoderPosted on 2004-05-28 at 22:04:20ID: 11185802

brettmjohnson has hit the nail on the head ...

waitpid = wait3(&status, WNOHANG, &ru_start); should actually have been

waitpid = wait3(&status, 0, &ru_start);

WNOHANG is causing you the woes

 

by: kinko899Posted on 2004-05-29 at 15:20:18ID: 11188917

Thanks for the help....Sunny & Brettmjohnson.  Both of you helped solve my problem.  I now got my program working correctly.  Since you both helped me solve this problem I decided to split the points 50/50.  Thanks again :-)

20120131-EE-VQP-002

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