What's the difference between /usr/include/linux and the include folder in linux kernel source?

Question

On a freshly installed Ubuntu , i found kernel headers in both /usr/include/linux , and /usr/src/kernel-version-headers/include/linux

Are they mutually the same ?

Answer 1

They are very different; the /usr/include/linux headers are the headers that were used when compiling the system's standard C library. They are owned by the C library packaging, and updated in lockstep with the standard C library. They exist to provide the userland interface to the kernel, as understood and "brokered"¹ by the C library.

The /usr/src/linux-headers-$(uname -r)/include/linux headers are used via the /lib/modules/$(uname -r)/build symbolic links. They are owned by the kernel headers packages and updated in lockstep with the kernel. These are a subset of the kernel headers and enough of the Kbuild system required to build out-of-tree kernel modules. These files represent the kernel internals -- modules must build against these if they are to properly understand in-memory objects. See the kernel's Documentation/kbuild/modules.txt file for some details.

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1: "Mediated" was my first word choice, but it implies some sort of access controls, which isn't the case. "Brokered" implies a third-party process, but that is also not the case. Consider: when a C program calls _exit(), it is actually calling the Standard C library's _exit() wrapper, which calls the exit(2) system call. The select(2) interface has an upper limit on the number of file descriptors that can be tracked, and that limit is compiled into the standard C library. Even if the kernel's interface were extended, the C library would also need to be recompiled.