Which one to choose waitpid/wait/waitid?

Question

I want to use execl in child process after doing fork. execl will execute the script which will take around 120 seconds of time. I tried almost all the combination with waitpid, wait and waitid with different argument (0, WNOHANG, etc.,) but in all case I am getting -1 return value. So I want to know in which wait function I need to use when? So I can concentrate in one wait function to make it work.

One more interesting thing which I observed from my logs is that when I am doing nothing in child process it is showing my parent thread as orphaned. I don't know how it is possible? How can my parent thread become orphaned?

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

int main(void)
{
    pid_t Checksum_pid = fork();

    if (Checksum_pid < 0)
        printf("Fork Failed\n");
    else if (Checksum_pid == 0)
    {
        execl("/bin/ls","ls",(char *)NULL) ;
        exit(EXIT_FAILURE);
    }
    else
    {
        int childStatus;
        pid_t returnValue = waitpid(Checksum_pid, &childStatus, 0);

        if (returnValue > 0)
        {
            if (WIFEXITED(childStatus))
                printf("Exit Code: %d\n", WEXITSTATUS(childStatus));

        }
        else if (returnValue == 0)
            printf("Child process still running\n");
        else
        {
            if (errno == ECHILD)
                printf(" Error ECHILD!!\n");
            else if (errno == EINTR)
                printf(" Error EINTR!!\n");
            else
                printf("Error EINVAL!!\n");
        }
    }

    return 0;
}

Any of them will do. You're doing something wrong somewhere. For further help, post your code and explanation of what you observe- — nos, Apr 08 '14 at 09:56
Maybe your test for whether you are the child or the parent is the wrong way round. Show us your code. — Mark Setchell, Apr 08 '14 at 09:57
Look at `errno` e.g. using `perror` when a `wait*` function gives -1. — Basile Starynkevitch, Apr 08 '14 at 10:08
If you get a -1 return value, inspect `errno`, or use `perror` to print the error message. — Thomas, Apr 08 '14 at 10:08

Basile Starynkevitch · Accepted Answer · 2014-04-08T10:37:50.987

As I commented: your last else should just be

 else perror("waitpid");

But you are getting ECHILD. So please read waitpid(2) man page:

   ECHILD (for wait()) The calling process does not have any unwaited-
          for children.

  ECHILD (for waitpid() or waitid()) The process specified by pid
          (waitpid()) or idtype and id (waitid()) does not exist or is
          not a child of the calling process.  (This can happen for
          one's own child if the action for SIGCHLD is set to SIG_IGN.
          See also the Linux Notes section about threads.)

BTW, I cannot reproduce your error. Check also your limits as given by ulimit -a on bash.

Maybe your execl fails (especially if you execute some script instead of /bin/ls). Add a call to perror after it.

Also, compile with gcc -Wall -g and use strace and gdb

I used strerror(errno) in printf. I am getting String "No Child Process" from strerror with (-1 return value from waitpid). When Child is running, I am getting String "Succuss" from strerror with 0 as return value from waitpid. — kapilddit, Apr 09 '14 at 17:52

Which one to choose waitpid/wait/waitid?

1 Answers1

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