1Exporting kernel headers for use by userspace
2=============================================
3
4The "make headers_install" command exports the kernel's header files in a
5form suitable for use by userspace programs.
6
7The linux kernel's exported header files describe the API for user space
8programs attempting to use kernel services.  These kernel header files are
9used by the system's C library (such as glibc or uClibc) to define available
10system calls, as well as constants and structures to be used with these
11system calls.  The C library's header files include the kernel header files
12from the "linux" subdirectory.  The system's libc headers are usually
13installed at the default location /usr/include and the kernel headers in
14subdirectories under that (most notably /usr/include/linux and
15/usr/include/asm).
16
17Kernel headers are backwards compatible, but not forwards compatible.  This
18means that a program built against a C library using older kernel headers
19should run on a newer kernel (although it may not have access to new
20features), but a program built against newer kernel headers may not work on an
21older kernel.
22
23The "make headers_install" command can be run in the top level directory of the
24kernel source code (or using a standard out-of-tree build).  It takes two
25optional arguments:
26
27  make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr/include
28
29ARCH indicates which architecture to produce headers for, and defaults to the
30current architecture.  The linux/asm directory of the exported kernel headers
31is platform-specific, to see a complete list of supported architectures use
32the command:
33
34  ls -d include/asm-* | sed 's/.*-//'
35
36INSTALL_HDR_PATH indicates where to install the headers.  It defaults to
37"./usr/include".
38
39The command "make headers_install_all" exports headers for all architectures
40simultaneously.  (This is mostly of interest to distribution maintainers,
41who create an architecture-independent tarball from the resulting include
42directory.)  You also can use HDR_ARCH_LIST to specify list of architectures.
43Remember to provide the appropriate linux/asm directory via "mv" or "ln -s"
44before building a C library with headers exported this way.
45
46The kernel header export infrastructure is maintained by David Woodhouse
47<dwmw2@infradead.org>.
48