Windows Hooks of Interaction With Hardware in C programming language

As the name suggests, the hook mechanism permits us to intercept and alter the flow of messages in the OS before they reach the application. Since hooks are used to alter the messaging mechanism on a system-wide basis the code for hooking has to be written in a DLL. The hooking mechanism involves writing a hook procedure in a DLL file and registering this procedure with the OS. Since the DLL cannot execute on its own we need a separate program that would load and execute the DLL.

For different messages there are different types of hooks. For example, for keyboard messages there is a keyboard hook, for mouse messages there is mouse hook, etc. You can refer MSDN for nearly a dozen more types of hooks. Here we would restrict our discussion only to the keyboard hook. Before we proceed to write our own hook procedure let us understand the normal working of the keyboard messages. This is illustrated

With reference to Figure 19.7 here is a list of steps that are carried out when we press a key from the keyboard

(a) On pressing a key an interrupt occurs and the corresponding kernel routine gets called.
(b) The kernel routine calls the ISR of the keyboard device driver.
(c) The ISR communicates with the keyboard controller and obtains the code of the key pressed.
(d) The ISR calls a OS function keybd_event( ) to post the key code to the System Message Queue.
(e) The OS retrieves the message from the System Message Queue and posts it into the message queue of the application with regard to which the key has been pressed.

Let us now see what needs to be done if we are to alter this procedure. We simply need to register our hook procedure with the OS. As a result, our hook procedure would receive the message before it is dispatched to the appropriate Application Message Queue. Since our hook procedure gets a first shot at the message it can now alter the working in the following three ways:

(a) It can suppress the message altogether
(b) It can change the message
(c) It can post more messages into the System Message Queue using the keybd_event( ) function.
Let us now put all this theory into practice by writing a few programs.