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 CENTOS
wait.h
wait.h(3HEAD) Headers wait.h(3HEAD)
NAME
wait.h, wait - wait status
SYNOPSIS
#include <sys/wait.h>
DESCRIPTION
When a process waits for status from its children using either the wait(3C) or waitpid(3C) function, the status returned can be evaluated
with the following macros, defined in <sys/wait.h>. These macros evaluate to integral expressions. The stat argument to these macros is the
integer value returned from wait() or waitpid().
WCOREDUMP(stat) If the value of WIFSIGNALED (stat) is non-zero, this macro evaluates to a non-zero value if a core image of
the terminated child was created.
WEXITSTATUS(stat) If the value of WIFEXITED(stat) is non-zero, this macro evaluates to the exit code that the child process
passed to _exit() (see exit(2)) or exit(3C), or the value that the child process returned from main.
WIFCONTINUED(stat) Evaluates to a non-zero value if status was returned for a child process that has continued.
WIFEXITED(stat) Evaluates to a non-zero value if status was returned for a child process that terminated normally.
WIFSIGNALED(stat) Evaluates to a non-zero value if status was returned for a child process that terminated due to the receipt
of a signal.
WIFSTOPPED(stat) Evaluates to a non-zero value if status was returned for a child process that is currently stopped.
WSTOPSIG(stat) If the value of WIFSTOPPED(stat) is non-zero, this macro evaluates to the number of the signal that caused
the child process to stop.
WTERMSIG(stat) If the value of WIFSIGNALED(stat) is non-zero, this macro evaluates to the number of the signal that
caused the termination of the child process.
The <sys/wait.h> header defines the symbolic constants listed below for use with waitpid(3C).
WNOHANG Do not hang if no status is available; return immediately.
WUNTRACED Report status of stopped child process.
The symbolic constants listed below are defined as possible values for the options argument to waitid(2).
WEXITED Wait for processes that have exited.
WSTOPPED Status is returned for any child that has stopped upon receipt of a signal.
WCONTINUED Status is returned for any child that was stopped and has been continued.
WNOHANG Return immediately if there are no children to wait for.
WNOWAIT Keep the process whose status is returned in infop in a waitable state.
The type idtype_t is defined as an enumeration type whose possible values include the following:
P_ALL
P_PID
P_PGID
The id_t and pid_t types are defined as described in <sys/types.h>.
The siginfo_t type is defined as described in <signal.h>.
The rusage structure is defined as described in <sys/resource.h>.
Inclusion of the <sys/wait.h> header can also make visible all symbols from <signal.h> and <sys/resource.h>.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
SEE ALSO
exit(2), waitid(2), exit(3C), wait(3C), waitpid(3C),attributes(5), standards(5)
SunOS 5.10 10 Sep 2004 wait.h(3HEAD)