fork and wait with two child processes

Question

I need to use the fork   and wait   functions to complete an assignment  We are modelling non-deterministic behaviour and need the program to fork   if there is more than one possible transition   In order to try and work out how fork and wait work  I have just made a simple program  I think I understand now how the calls work and would be fine if the program only branched once because the parent process could use the exit status from the single child process to determine whether the child process reached the accept state or not   As you can see from the code that follows though  I want to be able to handle situations where there must be more than one child processes  My problem is that you seem to only be able to set the status using an  exit function once  So  as in my example the exit status that the parent process tests for shows that the first child process issued 0 as it s exit status  but has no information on the second child process   I tried simply not  exit  -ing on a reject  but then that child process would carry on  and in effect there would seem to be two parent processes   Sorry for the waffle  but I would be grateful if someone could tell me how my parent process could obtain the status information on more than one child process  or I would be happy for the parent process to only notice accept status s from the child processes  but in that case I would successfully need to exit from the child processes which have a reject status   My test code is as follows    include  lt stdio h gt   include  lt unistd h gt   include  lt stdlib h gt   include  lt errno h gt   include  lt sys wait h gt   int main void          pid t child pid  wpid  pid      int status   0      int i       int a 3     1  2  1       for i   1  i  lt  3  i               printf  i    d n   i           pid   getpid            printf  pid after i    d n   pid           if  child pid   fork       0                 printf  In child process n                pid   getpid                printf  pid in child process is  d n   pid                  Is a child process                if a i   lt  2                     printf  Should be accept n                     exit 1                 else                    printf  Should be reject n                     exit 0                                      if child pid  gt  0                Is the parent process            pid   getpid            printf  parent pid    d n   pid           wpid   wait  amp status           if wpid    -1                 printf  Child s exit status was  d n   status               if status  gt  0                     printf  Accept n                  else                    printf  Complete parent process n                    if a 0   lt  2                         printf  Accept n                      else                        printf  Reject n                                                        return 0

User · Accepted Answer

It looks to me as though the basic problem is that you have one wait() call rather than a loop that waits until there are no more children. You also only wait if the last fork() is successful rather than if at least one fork() is successful.

You should only use _exit() if you don't want normal cleanup operations - such as flushing open file streams including stdout. There are occasions to use _exit(); this is not one of them. (In this example, you could also, of course, simply have the children return instead of calling exit() directly because returning from main() is equivalent to exiting with the returned status. However, most often you would be doing the forking and so on in a function other than main(), and then exit() is often appropriate.)

Hacked, simplified version of your code that gives the diagnostics I'd want. Note that your for loop skipped the first element of the array (mine doesn't).

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

int main(void)
{
    pid_t child_pid, wpid;
    int status = 0;
    int i;
    int a[3] = {1, 2, 1};

    printf("parent_pid = %d\n", getpid());
    for (i = 0; i < 3; i++)
    {
        printf("i = %d\n", i);
        if ((child_pid = fork()) == 0)
        {
            printf("In child process (pid = %d)\n", getpid());
            if (a[i] < 2)
            {
                printf("Should be accept\n");
                exit(1);
            }
            else
            {
                printf("Should be reject\n");
                exit(0);
            }
            /*NOTREACHED*/
        }
    }

    while ((wpid = wait(&status)) > 0)
    {
        printf("Exit status of %d was %d (%s)\n", (int)wpid, status,
               (status > 0) ? "accept" : "reject");
    }
    return 0;
}

Example output (MacOS X 10.6.3):

parent_pid = 15820
i = 0
i = 1
In child process (pid = 15821)
Should be accept
i = 2
In child process (pid = 15822)
Should be reject
In child process (pid = 15823)
Should be accept
Exit status of 15823 was 256 (accept)
Exit status of 15822 was 0 (reject)
Exit status of 15821 was 256 (accept)

User · Answer

brilliant example Jonathan Leffler  to make your code work on SLES  I needed to add an additional header to allow the pid t object      include  lt sys types h gt

User · Answer

Put your wait   function in a loop and wait for all the child processes  The wait function will return -1 and errno will be equal to ECHILD if no more child processes are available

[c] fork() and wait() with two child processes

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