Perl Tutorials - Herong's Tutorial Examples - v6.01, by Dr. Herong Yang
BEGIN(), CHECK(), INIT() and END() Functions
This section provides a tutorial example on how to 4 special subroutine used by the Perl compilation process and execution process: BEGIN(), CHECK(), INIT() and END().
In a Perl source code file, you can define 4 special subroutines, which will be executed automatically by the compilation process and the execution process. The key word "sub" is optional when defining those subroutines.
1. BEGIN() - Called during the compilation time immediately after it has been defined. After its execution, the subroutine will be removed and become undefined. Multiple BEGIN()s will be called in "first in, first out" order.
2. CHECK() - Called at the end of the compilation process. Multiple CHECK()s will be called in "last in, first out" order.
3. INIT() - Called at the beginning of the execution process. Multiple INIT()s will be called in "first in, first out" order.
4. END() - Called at the end of the execution process. Multiple END()s will be called in "last in, first out" order.
Here is a demonstration program, BeginEndTest.pl,
#- BeginEndTest.pl #- Copyright (c) HerongYang.com. All Rights Reserved. # $f = "Fortran"; print("Printing before the first BEGIN() code...\n"); sub BEGIN { print("In BEGIN: name space = ",__PACKAGE__,"\n"); print("In BEGIN: \$f = $f...\n"); } sub CHECK { print("In CHECK: name space = ",__PACKAGE__,"\n"); print("In CHECK: \$f = $f...\n"); } sub INIT { print("In INIT: name space = ",__PACKAGE__,"\n"); print("In INIT: \$f = $f...\n"); } sub END { print("In END: name space = ",__PACKAGE__,"\n"); print("In END: \$f = $f...\n"); } print("Printing after the first END() code...\n"); package MyPackage; print("Printing before the second BEGIN() code...\n"); sub BEGIN { print("In BEGIN: name space = ",__PACKAGE__,"\n"); print("In BEGIN: \$f = $f...\n"); } sub CHECK { print("In CHECK: name space = ",__PACKAGE__,"\n"); print("In CHECK: \$f = $f...\n"); } sub INIT { print("In INIT: name space = ",__PACKAGE__,"\n"); print("In INIT: \$f = $f...\n"); } sub END { print("In END: name space = ",__PACKAGE__,"\n"); print("In END: \$f = $f...\n"); } print("Printing after the second END() code...\n"); exit;
No surprises in the output. But the name space (package) name symbol __PACKAGE__ is defined during compilation process:
In BEGIN: name space = main In BEGIN: $f = ... In BEGIN: name space = MyPackage In BEGIN: $f = ... In CHECK: name space = MyPackage In CHECK: $f = ... In CHECK: name space = main In CHECK: $f = ... In INIT: name space = main In INIT: $f = ... In INIT: name space = MyPackage In INIT: $f = ... Printing before the first BEGIN() code... Printing after the first END() code... Printing before the second BEGIN() code... Printing after the second END() code... In END: name space = MyPackage In END: $f = ... In END: name space = main In END: $f = Fortran...
Table of Contents
Data Types: Values and Variables
Expressions, Operations and Simple Statements
►Name Spaces and Perl Module Files
Including Script Codes from Other Files
do() Function - Including Script Files
require() Function - Including Script Files
"package" Statement - Switching Name Space
►BEGIN(), CHECK(), INIT() and END() Functions
CalendarModule.pm - A Sample Perl Module
Hard References - Addresses of Memory Objects
Objects (or References) and Classes (or Packages)
Typeglob and Importing Identifiers from Other Packages
String Built-in Functions and Performance
File Handles and Data Input/Output
Open Directories and Read File Names
File System Functions and Operations
Socket Communication Over the Internet
XML::Simple Module - XML Parser and Generator
SOAP::Lite - SOAP Server-Client Communication Module
Perl Programs as IIS Server CGI Scripts
CGI (Common Gateway Interface)
XML-RPC - Remote Procedure Call with XML and HTTP
RPC::XML - Perl Implementation of XML-RPC
Integrating Perl with Apache Web Server
CGI.pm Module for Building Web Pages
LWP::UserAgent and Web Site Testing
Converting Perl Script to Executable Binary