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Full Discussion: Function main returning int?
Top Forums Programming Function main returning int? Post 302727693 by alister on Tuesday 6th of November 2012 04:20:01 PM
Old 11-06-2012
void exit(int status)
pid_t wait(int *stat_loc);

As already mentioned, only the low 8 bits of exit()'s int argument, status, are meaningful. The rest are practically ignored. However, it is an error to assume that the low 8 bits written by wait() to stat_loc are the low 8 of exit()'s status. This is often not the case. Since the layout of wait()'s *stat_loc bitmask is implementation dependent, POSIX specifies macros for examination and retrieval.

The 139 "exit status" in Gaurav's post isn't a true exit status. When a process exits abnormally, the bits which encode exit status have no meaningful value (which is why you are required to consult WIFEXITED() to confirm that the process exited normally before examining the exit status with WEXITSTATUS()).

If it's not an exit status, then what is it? If it wasn't an exit() argument, and if it wasn't main()'s return value, and if it's not provided by the kernel, then where does the 139 come from?

Before checking the exit status of it's child, the shell confirms that it exited normally, with WIFEXITED(). When the confirmation fails, the shell consults WIFSIGNALED(). Determining that the process was signaled and terminated abnormally, the signal number is retrieved with WTERMSIG(). By convention, the shell adds 128 to the signal number and stores that result in its ? parameter.

If your C code wait()ed on a process that was killed by that same signal, in the signal bits examined by WTERMSIG(), it would see 11 and not 139.

To avoid ambiguity, if you plan to invoke your binaries with the shell, it's a good idea to keep to exit values in the range 0 to 125 inclusive. The remaining values are spoken for: 126 (command found but not executable), 127 (command not found), and values larger than 128 (signal number + 128).

(Some of the following may be x86 specific.)

Returning to the original question: Where is the exit status stored? Inside the kernel.

When you call exit(n), the least significant 8 bits of the integer n are written to a cpu register. The kernel system call implementation will then copy it to a process-related data structure.

What if your code doesn't call exit()? The c runtime library responsible for invoking main() will call exit() (or some variant thereof) on your behalf. The return value of main(), which is passed to the c runtime in a register, is used as the argument to the exit() call.

When the parent calls wait(stat_loc), the exit status value (along with other status information) is copied from the kernel process structure to the address pointed to by wait()'s stat_loc.

Once a dead process is wait()'d on and its status information delivered, the kernel can destroy that process' data structure. Until then, the lingering data structure is the hallmark of a zombie.

Regards,
Alister
Last edited by alister; 11-06-2012 at 10:10 PM.. Reason: amend stack statement (register is used)
 

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LEARN ABOUT SUNOS

wait

wait(3C)						   Standard C Library Functions 						  wait(3C)

NAME

       wait - wait for child process to stop or terminate

SYNOPSIS

       #include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait(int *stat_loc);

DESCRIPTION

       The  wait()  function  will  suspend  execution of the calling thread until status information for one of its terminated child processes is
       available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If  more
       than  one thread is suspended in wait(), waitpid(3C), or waitid(2) awaiting termination of the same process, exactly one thread will return
       the process status at the time of the target process termination. If status information is available prior to the call  to  wait(),  return
       will be immediate.

       If  wait()  returns  because  the  status  of  a child process is available, it returns the process ID of the child process. If the calling
       process specified a non-zero value for stat_loc, the status of the child process is stored in the location pointed  to  by  stat_loc.  That
       status can be evaluated with the macros described on the wait.h(3HEAD) manual page.

       In the following, status is the object pointed to by stat_loc:

	 o  If	the  child process terminated due to an _exit() call, the low order 8 bits of status will be 0 and the high order 8 bits will con-
	    tain the low order 7 bits of the argument that the child process passed to _exit(); see exit(2).

	 o  If the child process terminated due to a signal, the high order 8 bits of status will be 0 and the low order 7bits	will  contain  the
	    number  of the signal that caused the termination. In addition, if	WCOREFLG is set, a "core image" will have been	produced; see sig-
	    nal.h(3HEAD) and wait.h(3HEAD).

       One instance of a SIGCHLD signal is queued for each child process whose status has changed. If wait() returns because the status of a child
       process	is  available,	any  pending  SIGCHLD  signal associated with the process ID of that child process is discarded. Any other pending
       SIGCHLD signals remain pending.

       If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN, and the process has no unwaited children that  were  transformed
       into zombie processes, it will block until all of its children terminate, and wait() will fail and set errno to ECHILD.

       If  a parent process terminates without waiting for its child processes to terminate, the parent process ID of each child process is set to
       1, with the initialization process inheriting the child processes; see intro(2).

RETURN VALUES

       When wait() returns due to a terminated child process, the process ID of the child is returned to the calling  process.	Otherwise,  -1	is
       returned and errno is set to indicate the error.

ERRORS

       The wait() function will fail if:

       ECHILD	       The calling process has no existing unwaited-for child processes.

       EINTR	       The function was interrupted by a signal.

USAGE

       Since  wait()  blocks  on  a  stopped  child,  a calling process wanting to see the return results of such a call should use waitpid(3C) or
       waitid(2) instead of wait(). The wait() function is implemented as a call to waitpid(-1, stat_loc, 0).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Async-Signal-Safe 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       intro(2),  exec(2),  exit(2),  fork(2),	pause(2),  waitid(2),  ptrace(3C),  signal(3C),   signal.h(3HEAD),   waitpid(3C),   wait.h(3HEAD),
       attributes(5)

SunOS 5.10							    9 Jun 2004								  wait(3C)
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