CSC/ECE 517 Summer 2008/wiki1 5 bk

Understanding Hooks

The hooking mechanism in Ruby allows certain functional events inside a running program to be identified while providing an action that will occur upon detection of the specified event. In this way, hooks serve as a detector of actions within the code, when event x is encountered, peform y. Examples of where such a technique would prove useful include:

• Recording when or how many times a method is called.
• Recording when an object is created.
• Debugging problematic code.

Example of Hooking System Calls (citation)

The following is an example of a hook being inserted to perform a certain action when the program makes a system call. In order for this to occur, an alias must be created so that the original functionality can still be accessed. In this case, original_system is aliased to the unaltered system. Once this is done, we can freely modify the definition of system to perform just about any operation. This is similar to the way in which a nefarious hook would work by still performing the intended operation, but also appending something extra... alias_method :original_system, :system The following code will then proceed to print the command executed ("ls -al" in this case), the timestamp, and then finally the output of the command to standard output.

module Kernel

 alias_method :original_system, :system
 def system(*args)
   puts "Your program executed the \"#{args.join(', ')}\" command." 
   puts "It was executed at: " + `date`
   puts "The command output the following: "
   original_system(*args)
 end

end system("ls -al")

Your program executed the "ls -al" command. It was executed at: Mon Jun 2 21:19:27 EDT 2008 The command output the following: total 20 drwxr-xr-x 2 brking brking 4096 2008-06-02 21:19 . drwxr-xr-x 18 brking brking 4096 2008-06-02 21:19 .. -rw-r--r-- 1 brking brking 134 2008-06-01 22:01 fib.rb -rw-r--r-- 1 brking brking 235 2008-06-02 20:29 hooking.rb -rw-r--r-- 1 brking brking 287 2008-06-02 21:19 moreHooking.rb