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Getting ready
To realize our first char driver, we need the module presented in the previous chapter. This is because using kernel modules is the simplest method we have to inject code into kernel space. Of course, we can decide to compile our driver as built in to the kernel but, in this manner, we have to fully recompile the kernel and reboot our system each time we have to modify the code (it's a possibility but definitely not the best!).
Just a note before carrying on: to provide a clearer explanation regarding how a char driver works and to present a really simple example, I decided to use the legacy way to register a char driver into the kernel. There's nothing to be concerned about, since this mode of operation is perfectly legal and still supported and, in any case, in the Using a device tree to describe a character driver recipe, in Chapter 4, Using the Device Tre e , I'm going to present the currently advised way of registering char drivers.