![]() The _init parameters tells the kernel to free the space of this function after the initialization. Usually you alloc the memory you need and do the initialization of you module here. This is called the "init" function of the module, it needs to return 0 on success. This function will execute when we load the module into the kernel. Then, we have 2 functions: static int _init hellokernelmod_init(void) In our case we need kernel.h for the kernel functions (printk) and module.h for the module functions (init, exit.), every module needs this header. MODULE_DESCRIPTION("Simple Kernel Module to display messages on init and exit.") įirst, we need to include the necessary headers for our module to work. Printk(KERN_INFO "Hasta la vista, Kernel!\n") Static void _exit hellokernelmod_exit(void) Static int _init hellokernelmod_init(void) Here is the code of our first kernel module. For an example, instead of using printf() you will use kprintf(). But don't worry about it, the kernel has most functions that you normally use in C translated to Kernel Space, usually they have the same name but beginning with a "k". That means that you should not use some "userland" headers and functions in you code. The linux kernel modules runs in a space called Kernel Space, where the kernel runs and provides its services. You can also built-in the module into the linux kernel, in this way it's not possible to dynamically load and unload. Kernel Module and Kernel SpaceĪ Kernel Module is a program or piece of code, usually written in C, that can be loaded or unloaded dynamically in the linux kernel. Note that in this article I will not focus on a deep explanation about this topic for the moment, since this is only the introduction.īut before we dive into code we need to have the minimum understand what is a Kernel Module and where it runs. For this Linux Kernel Modules Development introduction we will follow the same concept, but instead of the usual "Hello World" we will make a "Hello Kernel!" and you will understand the reason in a few moments. Note: lsmod displays only the dynamically loaded modules not the built-in ones.Our very first program in every language or framework usually is the notorious "Hello World" program. config file in the root folder of your kernel source: DRIVER_1=y // y indicate a builtin moduleĭRIVER_1=m //m inicates a loadable module You can configure a module to be either of the two by editing the. But for the built-in modules if you need to make any changes in the module then you need to compile the whole kernel and then reboot the system with the new image of the kernel. ![]() ![]() Every time you test it and you need to make changes to it, you can easily unload it ( rmmod driver.ko or modprobe -r driver.ko) and then after making changes, you can insert it back. This is good if you are working on a module and you need to test it. The advantage the loadable modules have over the built-in modules is that you can load unload them on run-time. Loadable kernel module (LKM) - A driver that is not automatically loaded by the kernel, the user can insert this module at run-time by insmod driver.ko or modprobe driver.ko.Built-in kernel modules - When the kernel is booted up, the kernel automatically inserts this driver in to the kernel (it's more like it is already part of the kernel code). ![]() Linux kernel supports inserting of modules (aka device drivers) in two ways:
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