The customary place to keep source code for the kernel and
standard programs is /usr/src
. In particular,
/usr/src/linux
is usually the directory with the source
code for the running version of the kernel. Kernel source
code, or at least the headers, is needed because the headers of the C library refer to
headers in the kernel source for kernel version dependent
information, such as some types. Because of this,
/usr/include/asm
and /usr/include/linux
are
typically symbolic links to /usr/src/linux/include/asm
and linux , respectively.