/usr/src/linux

The /usr/src/linux directory in a Linux system typically contains the source code for the Linux kernel. This directory is crucial for tasks such as compiling custom kernels, developing kernel modules, or understanding the kernel's internal workings.

Key Components of /usr/src/linux

1. Kernel Source Code:

   • /usr/src/linux: The main directory where the kernel source code is located. It usually contains various subdirectories and files that make up the Linux kernel source tree.

2. Subdirectories:

   • arch/: Architecture-specific code (e.g., x86, ARM).

   • block/: Block device drivers.

   • drivers/: Device drivers for hardware components.

   • fs/: File system code.

   • include/: Header files used throughout the kernel.

   • init/: Initialization code for the kernel.

   • kernel/: Core kernel code.

   • lib/: Library routines used by the kernel.

   • mm/: Memory management code.

   • net/: Networking stack.

3. Files:

   • Makefile: The main Makefile used for compiling the kernel.

   • Kconfig: Configuration files used to set up kernel build options.

   • README: Basic information and instructions on building the kernel.

Common Tasks Involving /usr/src/linux

1. Kernel Compilation:

   • Prerequisites: Ensure necessary tools like gcc, make, and libncurses-dev are installed.

   • Configuration: Configure the kernel with make menuconfig, make xconfig, or make oldconfig.

   • Compilation: Compile the kernel and modules using make and make modules_install.

   • Installation: Install the new kernel with make install.

2. Kernel Module Development:

   • Creating Modules: Write kernel modules (usually .c files) and place them in a subdirectory.

   • Building Modules: Use a Makefile to compile the module against the kernel source.

3. Kernel Hacking and Debugging:

   • Modifying Code: Make changes to the kernel source for custom features or bug fixes.

   • Testing Changes: Compile and test the modified kernel to ensure it works as expected.

Example: Compiling a Kernel

Here’s a step-by-step guide to compile a Linux kernel from source:

1. Navigate to the Source Directory:

   cd /usr/src/linux

2. Clean the Build Environment:

   make mrproper

3. Configure the Kernel:

   make menuconfig

4. Compile the Kernel:

   make

5. Compile and Install Modules:

   make modules
   make modules_install

6. Install the Kernel:

   make install

7. Update Bootloader (e.g., GRUB):

   • Ensure the bootloader is updated to recognize the new kernel. This is usually done automatically by make install, but you might need to manually update grub.cfg or similar configuration files.

Conclusion

The /usr/src/linux directory is fundamental for anyone involved in kernel development, module creation, or customization of the Linux kernel.