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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 REDHAT

mlm

FBB::mlm(3bobcat)                                             OFoldStream manipulator                                            FBB::mlm(3bobcat)

NAME

       FBB::mlm - Manipulator modifying left margins of OFoldStream objects

SYNOPSIS

       #include <bobcat/ofoldstream> or
       #include <bobcat/ofoldstreambuf>
       Linking option: -lbobcat

DESCRIPTION

       The  mlm  class  implements  a  manipulator that can be inserted into OFoldStream objects to modify the stream's left margin by a requested
       amount. The request cannot result in a negative left margin value. If a negative left margin would be the arithmetic result of the  request
       then left margin 0 will silently be used.

       Depending  on  the  tab-setting  of  the  OFoldStream  the  inserted value represents the number of blank space characters or the number of
       tab-characters that will be added to the left margin.

       The request will be processed at the next newline character or std::flush or std::endl manipulator that is inserted into the stream.  If  a
       line  is  still empty once an mlm object and a flush manipulator are inserted into the stream then the new left margin will be effective at
       the next word inserted into that line (cf., the example section below)

       A bad_cast exception is thrown when the manipulator is inserted into an ostream that is not using a OFoldStreambuf buffer.

NAMESPACE

       FBB
       All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

INHERITS FROM

       -

CONSTRUCTOR

       o      mlm(int addValue):
              The standard copy constructor is available.

MEMBER FUNCTIONS

       There are no public or protected member functions in this class.

EXAMPLE

           #include <iostream>
           #include <bobcat/ofoldstream>

           using namespace std;
           using namespace FBB;

           int main()
           {
               OFoldStream out(cout, 0, 80);
               out << "hello world (left margin is 0)" << mlm(4) << "
"
                      "this uses a 4 character wide left margin
" <<
                       mlm(-10) << flush <<
                       "left margin -6 changed to 0, active on this line
";
               return 0;
           }

FILES

       bobcat/mlm - defines the class interface

SEE ALSO

       bobcat(7), manipulators(3bobcat), lm(3bobcat), ofoldstream(3bobcat)

BUGS

       None Reported.

DISTRIBUTION FILES

       o      bobcat_3.01.00-x.dsc: detached signature;

       o      bobcat_3.01.00-x.tar.gz: source archive;

       o      bobcat_3.01.00-x_i386.changes: change log;

       o      libbobcat1_3.01.00-x_*.deb: debian package holding the libraries;

       o      libbobcat1-dev_3.01.00-x_*.deb: debian package holding the libraries, headers and manual pages;

       o      http://sourceforge.net/projects/bobcat: public archive location;

BOBCAT

       Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

COPYRIGHT

       This is free software, distributed under the terms of the GNU General Public License (GPL).

AUTHOR

       Frank B. Brokken (f.b.brokken@rug.nl).

libbobcat1-dev_3.01.00-x.tar.gz                                      2005-2012                                                   FBB::mlm(3bobcat)
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