Updated exit codes as constant values

Re-allocated exit status codes according to three references, which follow:

BSD sysexits.h:http://www.gsp.com/cgi-bin/man.cgi?section=3&topic=sysexits
GNU recomendations:http://www.gnu.org/software/libc/manual/html_node/Exit-Status.html
Bash scripting:http://tldp.org/LDP/abs/html/exitcodes.html

The GNU recommendations stem from and expand upon the standard C/C++ library (stdlibc)
definitions, while also suggesting some best-practice conventions which happen to prevent
exit status code collisions with bash, and probably other shells.

The re-allocated codes are now mapped to constant values, set in *application/config/constants.php*,
and used throughout the CodeIgniter core.  They would additionally be used in *index.php*,
but the constants file hasn't been loaded at that point, so the integer values are used
instead, and a comment follows each such use with amplifying information on why that
particular value was selected.

Finally, the errors documentation has been updated accordingly.

Signed-off-by: Daniel Hunsaker <danhunsaker@gmail.com>

1 files changed, 18 insertions, 3 deletions

diff --git a/user_guide_src/source/general/errors.rst b/user_guide_src/source/general/errors.rst
index 8c941aadb..1d6f8120d 100644
--- a/user_guide_src/source/general/errors.rst
+++ b/user_guide_src/source/general/errors.rst
@@ -18,6 +18,15 @@ procedural interfaces that are available globally throughout the
 application. This approach permits error messages to get triggered
 without having to worry about class/function scoping.
 
+CodeIgniter also returns a status code whenever a portion of the core
+calls ``exit()``. This exit status code is separate from the HTTP status
+code, and serves as a notice to other processes that may be watching of
+whether the script completed successfully, or if not, what kind of 
+problem it encountered that caused it to abort. These values are 
+defined in *application/config/constants.php*. While exit status codes
+are most useful in CLI settings, returning the proper code helps server
+software keep track of your scripts and the health of your application.
+
 The following functions let you generate errors:
 
 show_error()
@@ -36,7 +45,12 @@ following error template::
 	application/errors/error_general.php
 
 The optional parameter ``$status_code`` determines what HTTP status
-code should be sent with the error.
+code should be sent with the error. If ``$status_code`` is less than 100,
+the HTTP status code will be set to 500, and the exit status code will
+be set to ``$status_code + EXIT__AUTO_MIN``. If that value is larger than
+``EXIT__AUTO_MAX``, or if ``$status_code`` is 100 or higher, the exit
+status code will be set to ``EXIT_FAILURE``. You can check in 
+*application/config/constants.php* for more detail.
 
 show_404()
 ==========
@@ -53,8 +67,9 @@ the following error template::
 	application/errors/error_404.php
 
 The function expects the string passed to it to be the file path to the
-page that isn't found. Note that CodeIgniter automatically shows 404
-messages if controllers are not found.
+page that isn't found. The exit status code will be set to ``EXIT_UNK_FILE``.
+Note that CodeIgniter automatically shows 404 messages if controllers are
+not found.
 
 CodeIgniter automatically logs any ``show_404()`` calls. Setting the
 optional second parameter to FALSE will skip logging.