1. Saving Your Configuration and Exiting menuconfig

After toggling all the options you need inside make menuconfig, navigate to the < Exit > button using the right arrow key and press Enter. A dialog box appears asking whether you want to save your new configuration.

2. The Linux Kernel .config File — What It Is and Where It Lives

When you choose < Yes >, the entire kernel configuration is written to a plain ASCII text file named .config, placed at the root of the kernel source tree. This single file is the authoritative record of every option you enabled, disabled, or marked as a module.

2.1 The Three States of a Kernel Configuration Option

Open any .config file and every option follows a CONFIG_<NAME> pattern. There are exactly three states an option can be in:

2.2 A Real .config Snippet — IKCONFIG Example

If you enabled Kernel .config support and its /proc/config.gz interface during your menuconfig session, your .config will contain the following entries. You can verify this with grep:

$ grep IKCONFIG .config
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y

CONFIG_IKCONFIG=y tells the build system to compile kernel-config support directly into the kernel binary. CONFIG_IKCONFIG_PROC=y means the /proc/config.gz pseudo-file will exist at runtime on any machine running this kernel, letting you inspect the exact configuration that was used to build it.

2.3 How CONFIG Macros Flow Into Kernel C Code

Every option you set becomes a C preprocessor macro. Inside kernel source files you will see patterns like this constantly:

/* From kernel/smp.c — simplified example */
#ifdef CONFIG_SMP
    /* Multi-processor code path */
    smp_call_function_many(mask, func, info, wait);
#else
    /* Single-processor fallback */
    func(info);
#endif

And in kernel Makefiles, the same macros control which source files get compiled:

# drivers/net/ethernet/intel/Makefile — simplified
obj-$(CONFIG_E1000)      += e1000/
obj-$(CONFIG_VIRTIO_NET) += virtio_net.o

When CONFIG_E1000=y, the e1000 folder is compiled into the kernel. When CONFIG_E1000=m, it becomes a loadable module. When it is not set, the folder is completely ignored by the build.

3. What Happens Behind the Scenes — kbuild and syncconfig

Saving .config is not the final step in the kbuild pipeline. As soon as the file is written, kbuild automatically runs an internal target called syncconfig. You never need to call this manually — it triggers on its own. This target was previously named silentoldconfig, a name that was confusing and not descriptive, so it was renamed in modern kernels.

The job of syncconfig is to transform your plain-text .config into multiple generated files that the rest of the build system can actually use.

3.1 autoconf.h — The Bridge Between Config and C Code

The file include/generated/autoconf.h contains one #define per enabled configuration option. Kernel source files include this header indirectly through other headers, which is how #ifdef CONFIG_SMP and similar guards actually work at compile time.

/* Snippet from include/generated/autoconf.h */
#define CONFIG_IKCONFIG 1
#define CONFIG_IKCONFIG_PROC 1
#define CONFIG_SMP 1
#define CONFIG_MODULES 1
/* ... hundreds more options ... */

3.2 auto.conf — The Bridge Between Config and Makefiles

While autoconf.h serves C source files, include/config/auto.conf serves the Makefile infrastructure. It lists CONFIG options in a format GNU Make can evaluate directly in conditional expressions and variable substitutions.

3.3 Empty Sentinel Headers — How kbuild Tracks Changes

Under include/config/ you will find many tiny, empty .h files — one for each configuration option. These files exist for one purpose only: incremental build dependency tracking. When you change a config option, kbuild updates the corresponding sentinel file’s timestamp. Make then knows exactly which object files depend on that option and recompiles only those. This is what makes kernel incremental builds fast even with thousands of source files.

4. Searching for a Config Option Inside menuconfig

The Linux kernel 6.x has over 15,000 configuration options. Nobody navigates to them by hand. The menuconfig UI has a built-in search — triggered exactly like the search in the vi text editor.

4.1 Understanding the Search Result Fields

The result dialog for each matching option gives you far more information than just where to find it. Here is what every field means:

4.2 What Are Kconfig Files?

All the information the search result shows comes from plain text files called Kconfig. Every subsystem in the kernel has its own Kconfig file — for example, drivers/net/Kconfig defines all network driver options, arch/arm64/Kconfig defines ARM64-specific options, and so on. The kbuild system reads every Kconfig file in the entire source tree to build the menuconfig UI.

# What a Kconfig entry looks like (conceptual — from kernel/Kconfig area)
config IKCONFIG
    tristate "Kernel .config support"
    help
      This feature stores the kernel's own configuration inside
      the running kernel. Access it at /proc/config.gz if
      CONFIG_IKCONFIG_PROC is also enabled.

5. Comparing Two Kernel Configurations — scripts/diffconfig

Every time kbuild writes a new .config, it first backs up the old one as .config.old automatically. This gives you a before-and-after pair to compare whenever you want to review what you changed.

Using standard diff on two .config files is painful — the files are hundreds of lines long, full of comments, and the output is hard to interpret. The kernel source tree includes a dedicated script for exactly this task: scripts/diffconfig.

5.1 Basic Usage

# See what you changed compared to your previous configuration
$ scripts/diffconfig .config.old .config

5.2 Reading the Output

# Example diffconfig output
-DEBUG_KERNEL n         <-- was present, now removed/disabled
+IKCONFIG y             <-- newly added, value: y (built-in)
+IKCONFIG_PROC y        <-- newly added, value: y
 SMP y                  <-- unchanged (no prefix)

5.3 The -m Flag — Minimal Config Fragment

# Produce a minimal config fragment (only the differences)
$ scripts/diffconfig -m .config.old .config

The -m flag produces output in a format you can use as a config fragment — a small file containing only the changed options. This is useful when you want to share a patch of config changes or apply specific options on top of a base distribution config.

6. Suggested Images for This Tutorial

7. Quick Reference — Files and Commands

Frequently Asked Questions