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>

5 files changed, 14 insertions, 10 deletions

diff --git a/system/core/CodeIgniter.php b/system/core/CodeIgniter.php
index 5a872ef21..8f5271add 100644
--- a/system/core/CodeIgniter.php
+++ b/system/core/CodeIgniter.php
@@ -188,7 +188,7 @@ defined('BASEPATH') OR exit('No direct script access allowed');
 	if ($EXT->call_hook('cache_override') === FALSE
 		&& $OUT->_display_cache($CFG, $URI) === TRUE)
 	{
-		exit(0);
+		exit(EXIT_SUCCESS);
 	}
 
 /*
diff --git a/system/core/Common.php b/system/core/Common.php
index 3cd97dc2e..479f0da7f 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -177,7 +177,7 @@ if ( ! function_exists('load_class'))
 			// self-referencing loop with the Exceptions class
 			set_status_header(503);
 			echo 'Unable to locate the specified class: '.$class.'.php';
-			exit(2);
+			exit(EXIT_UNK_CLASS);
 		}
 
 		// Keep track of what we just loaded
@@ -251,7 +251,7 @@ if ( ! function_exists('get_config'))
 		{
 			set_status_header(503);
 			echo 'The configuration file does not exist.';
-			exit(1);
+			exit(EXIT_CONFIG);
 		}
 
 		// Does the $config array exist in the file?
@@ -259,7 +259,7 @@ if ( ! function_exists('get_config'))
 		{
 			set_status_header(503);
 			echo 'Your config file does not appear to be formatted correctly.';
-			exit(1);
+			exit(EXIT_CONFIG);
 		}
 
 		// Are any values being dynamically replaced?
@@ -374,12 +374,16 @@ if ( ! function_exists('show_error'))
 		$status_code = abs($status_code);
 		if ($status_code < 100)
 		{
-			$exit_status = $status_code + 28;
+			$exit_status = $status_code + EXIT__AUTO_MIN;
+			if ($exit_status > EXIT__AUTO_MAX)
+			{
+				$exit_status = EXIT_FAILURE;
+			}
 			$status_code = 500;
 		}
 		else
 		{
-			$exit_status = 27;
+			$exit_status = EXIT_FAILURE;
 		}
 		
 		$_error =& load_class('Exceptions', 'core');
@@ -407,7 +411,7 @@ if ( ! function_exists('show_404'))
 	{
 		$_error =& load_class('Exceptions', 'core');
 		$_error->show_404($page, $log_error);
-		exit(4);
+		exit(EXIT_UNK_FILE);
 	}
 }
 
diff --git a/system/core/Exceptions.php b/system/core/Exceptions.php
index f799d6027..423387ff9 100644
--- a/system/core/Exceptions.php
+++ b/system/core/Exceptions.php
@@ -117,7 +117,7 @@ class CI_Exceptions {
 		}
 
 		echo $this->show_error($heading, $message, 'error_404', 404);
-		exit(4);
+		exit(EXIT_UNK_FILE);
 	}
 
 	// --------------------------------------------------------------------
diff --git a/system/core/Input.php b/system/core/Input.php
index 904f4d6e9..8d491e055 100644
--- a/system/core/Input.php
+++ b/system/core/Input.php
@@ -746,7 +746,7 @@ class CI_Input {
 		{
 			set_status_header(503);
 			echo 'Disallowed Key Characters.';
-			exit(6);
+			exit(EXIT_USER_INPUT);
 		}
 
 		// Clean UTF-8 if supported
diff --git a/system/core/Output.php b/system/core/Output.php
index d4abe871d..1025703dc 100644
--- a/system/core/Output.php
+++ b/system/core/Output.php
@@ -696,7 +696,7 @@ class CI_Output {
 		if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && $last_modified <= strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']))
 		{
 			$this->set_status_header(304);
-			exit(0);
+			exit(EXIT_SUCCESS);
 		}
 		else
 		{