waitpid() in loop, not ending correctly

[Gatsu]

Hi, I have a small program that will fork 5 children and after the last child is created dispose of them one atthe time in a loop like this:

for(x = 0 ; x < 5 ; x++)
    {
      int waitValue = waitpid(childPid[x], &status, 0);
      printf("waitpid() status: %d\n", status);
      printf("waitpid() return value: %d\n", waitValue);
    }

the childrens pid is stored in an array childPid

the result when calling wait like this is:

waitpid() status: 0
waitpid() return value: 496
waitpid() status: 0
waitpid() return value: 497
waitpid() status: 0
waitpid() return value: 498
waitpid() status: 0
waitpid() return value: -1
waitpid() status: 0
waitpid() return value: -1

It always works for the 3 first but not for the two last, this is the full code and if anybody can identify why this happens, please share.

[C] #include <stdio.h> #include <unistd.h> #include <sys/types.h> #include <sys/w - Pastebin.com

thank you

[Salem]

waitpid(2): wait for process to change state - Linux man page
You need to use the likes of WEXITSTATUS(status) to actually get the return status out of all the other bits of information encoded into the status integer.

[AndiPersti]

Run the following version of your code (line 58 is added)

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

int randomSleep()
{
 srand(time(NULL));
 return (rand() % 100);
}

/*
 * Main method
 * creates children and prints their info
 */
int main(void)
{
 int pid, i, x, maxChildren = 5, status, childPid[5];

 for(i = 0 ; i < maxChildren ; i++)
 {
  pid = fork();

  if(pid == -1)
  {
   printf("fork() failed! Child number %d was not created. Retrying...\n", i);
   i -= 1;
   sleep(1);
   exit(0); //child creation failed, exit
  }
  else if(pid == 0)
  {
   int currentPid = getpid();
   printf("\nParent PID %d, child %d PID %d, group PID %d\n", getppid(), i, currentPid, getpgrp());
   childPid[i] = currentPid;

   exit(0);
  }
  else
  {
   //create a random number from 1-10
   srand(time(NULL));
   int sleepSecond = (rand() % 10) + 1;

   //sleep after each child did some work
   printf("sleep for %d sec\n", sleepSecond);
   sleep(sleepSecond);

   if(i >= (maxChildren - 1))
   {
    printf("\n5 children created, parent sleeps for 20 secnds...\n");
    sleep(2);

    for(x = 0 ; x < 5 ; x++)
    {
      printf("parent thinks pid of child #%d is %d\n", x, childPid[x]);  // This is the new line!
      int waitValue = waitpid(childPid[x], &status, 0);
      printf("waitpid() status: %d\n", status);
      printf("waitpid() return value: %d\n", waitValue);
    }

    exit(0); //dont create too many children
   }
  }
 }
 return 0;
}

Compare the output with the output of your child processes (focus on the pid's) and then think about how the children are connected with the parent.

Bye, Andreas

[Gatsu]

So I have to use a pointer to the array so that all children add to the same array?

creating array like this:
int *childPid[5];

when child is created, add to this array like this:
*childPid[i] = currentPid;

Then I check if it worked by writing out the added value just after:
printf("pid from array %d\n", *childPid[i]);

But for some reason this only prints out the last childs pid from this array, the 4 first just skips printing anything.

Then my parent loop gives me "segment fault":

for(x = 0 ; x < 5 ; x++)    {
      printf("parent thinks pid of child #%d is %d\n", x, *childPid[x]);  // This is the new line
      int waitValue = waitpid(*childPid[x], &status, 0);
      printf("waitpid() status: %d\n", WEXITSTATUS(status));
      printf("waitpid() return value: %d\n", waitValue);
    }

This means that the parent can not access the childPid array?

[Gatsu]

nevermind, I discovered that if I used another variable as a pointer it works

thanks very much all!

[AndiPersti]

This means that the parent can not access the childPid array?

The parent can access the childPid array.

The problem is this array is not the same as the childPid arrays in the child processes. After forking 5 times you end up with 6 different arrays all having the same name but they are completely independent from each other. Forking means all the variables from the parent process are copied to the child. But there is no connection because the copies are new objects (in different memory locations).

You have added the children's pids to the children's arrays. Thus the parent process didn't notice any changes and the array in the parent just contained garbage.

Bye, Andreas