WEXITSTATUS(childStatus) returns 0 but waitpid returns -1

Question

As far as I know if waitpid returns -1 then it is error condition. How it possible to get success (EXIT_SUCCUSS) from child process in WEXITSTATUS(childStatus)?

What is the difference between childStatus in waitpid & return value from WEXITSTATUS(childStatus)? Does it same?

pid_t returnValue = waitpid(Checksum_pid, &childStatus, WNOHANG);
printf("return value = %d", returnValue);
printf("return value = %d", childStatus);

if (WIFEXITED(childStatus))
        {
            printf("Exit Code: _ WEXITSTATUS(childStatus)") ;    
            //Proceed with other calculation.  
        }

Answer 1

When using the option WNOHANG, I would expect that most of the time waitpid will return -1, with errno set to ECHILD.

In any case, whenever waitpid does return -1, you shouldn't be looking at childStatus, which (for all I know) could be just garbage. Instead, look at errno, and handle it appropriately.

Otherwise, your code seems to be ok, as far as it goes, and you should be able to extract a 0 or EXIT_SUCCESS from childStatus.

The man page for waitpid suggests the following sample code:

   if (WIFEXITED(status)) {
       printf("exited, status=%d\n", WEXITSTATUS(status));
   } else if (WIFSIGNALED(status)) {
       printf("killed by signal %d\n", WTERMSIG(status));
   } else if (WIFSTOPPED(status)) {
       printf("stopped by signal %d\n", WSTOPSIG(status));
   } else if (WIFCONTINUED(status)) {
       printf("continued\n");
   }

although it might be a good idea to add a final else printf("oops?\n") statement to this.