Programming

In one of the Unix Programming FAQ's they have the following headers in the program to catch SIGCHLD

Code:

#include <sys/types.h>  /* include this before any other sys headers */
#include <sys/wait.h>   /* header for waitpid() and various macros */
#include <signal.h>     /* header for signal functions */
#include <stdio.h>      /* header for fprintf() */
#include <unistd.h>     /* header for fork() */

Why does <sys/types.h> have to be placed before any other sys headers in this case?

And the second question. In the following code snippet, when I have status>>8 on a sep. line, I get an exit status of 256 when no one is logged on.

Code:

if (status != 0) {
    fprintf(stderr, "No one logged on.\n");
    WEXITSTATUS(status);
    status>>8;
    printf("%d\n", status);
    exit(EXIT_FAILURE);
    }

However, when I put status>>8 in printf() like in the following

Code:

if (status != 0) {
    fprintf(stderr, "No one logged on.\n");
    WEXITSTATUS(status);
    printf("%d\n", status>>8);
    exit(EXIT_FAILURE);
    }

I get an exit status of 1 when no one is logged on. I don't understand how the placement of status>>8 has an impact on the exit code.

Q2 - POSIX says:
The lower eight bits of something is never 256 -- 8 bits range from 0 - 255. First. Did you check WIFEXITED?

the >> question: I do not know, but I do know function arguments are sequence points. That means they have to be fully evaluated before they are passed as argument to a function.

Are you using an old compiler, like the one that comes as part of HPUX?

Q1- sys/types.h is required to be included first by wait(), followed by sys/wait.h - if you read the man page for wait.

The real reason is so that test macros in the other include files do not "accidentally" turn off a declaration of some datatype, struct or typedef needed by somebody in another include file later on.

The simple answer is "sys/types.h is a fundamental header and contains many types required by other headers".

I'm using the GNU C Compiler. I'm suspecting it might have to do with that fact that the '>> isn't a sequence point.

1. You are not using the macros correctly...

it should be

Code:

x=WEXITSTATUS(status);

saying

Code:

WEXITSTATUS(status);

by itself evaluates the exit status and discards the result.

2. Similarly

Code:

status>>8;

effectively does nothing, it shifts a copy of the number right by 8 places and throws the result away.

A while back, I was was working on a program that expand something like
a-z to abc...xyz. Anyhow, I have having similar issues with stuff being evaluated and discarding the results. I think this is because I'm missing the broader concept. What that broader concept is, I have no idea.

I think I just need to sleep on it and try to figure out what it is I'm not grasping.