The "Oldconfig" Target In The Linux Kernel Makefile

The Linux kernel provides various configuration targets through its Makefile system to help developers and system administrators build customized kernels. The oldconfig target is a crucial configuration option that allows you to update an existing kernel configuration while preserving your previous settings and only prompting for new options introduced in newer kernel versions.

Note Linux commands are case-sensitive.

Understanding the Kernel Build System

The Linux kernel build system uses a sophisticated Makefile structure to manage the compilation process. The configuration system determines which features, drivers, and subsystems get included in the final kernel image. There are five essential components of the kernel Makefile structure

• Makefile The main makefile located in the source root directory

• arch/$(ARCH)/Makefile Architecture-specific makefile that supplements the main Makefile

• .config The kernel configuration file containing all build options

• scripts/Makefile.* Common rules and definitions for all kbuild Makefiles

• Kbuild Makefiles Approximately 500 individual makefiles distributed throughout the source tree

What is the "oldconfig" Target?

The oldconfig target is a configuration method that reads an existing .config file and prompts the user only for new configuration options that weren't present in the previous kernel version. This approach is particularly valuable when upgrading kernel versions, as it preserves your existing configuration choices while ensuring compatibility with new features and options.

How to Use the "oldconfig" Target

Before using oldconfig, ensure you have an existing configuration file. If starting fresh, create a baseline configuration using

make defconfig

To update an existing configuration with the oldconfig target, navigate to the kernel source directory and execute

make oldconfig

The system will read your current .config file and present interactive prompts only for new configuration options. For each new option, you can

• Press Enter Accept the default value (shown in brackets)

• Type 'y' Enable the option

• Type 'n' Disable the option

• Type 'm' Build as a module (if supported)

• Enter a number Select from multiple choice options

Comparison with Other Config Targets

Conclusion

The oldconfig target is an essential tool for maintaining kernel configurations across version upgrades. It preserves your existing settings while ensuring new features are properly configured, making it invaluable for system administrators and developers who need to maintain consistent kernel builds.