Documentation/make/headers_install.txt

Some documentation for "make headers_install".

Signed-off-by: Rob Landley <rob@landley.net>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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+Exporting kernel headers for use by userspace
+=============================================
+
+The "make headers_install" command exports the kernel's header files in a
+form suitable for use by userspace programs.
+
+The linux kernel's exported header files describe the API for user space
+programs attempting to use kernel services.  These kernel header files are
+used by the system's C library (such as glibc or uClibc) to define available
+system calls, as well as constants and structures to be used with these
+system calls.  The C library's header files include the kernel header files
+from the "linux" subdirectory.  The system's libc headers are usually
+installed at the default location /usr/include and the kernel headers in
+subdirectories under that (most notably /usr/include/linux and
+/usr/include/asm).
+
+Kernel headers are backwards compatible, but not forwards compatible.  This
+means that a program built against a C library using older kernel headers
+should run on a newer kernel (although it may not have access to new
+features), but a program built against newer kernel headers may not work on an
+older kernel.
+
+The "make headers_install" command can be run in the top level directory of the
+kernel source code (or using a standard out-of-tree build).  It takes two
+optional arguments:
+
+  make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr/include
+
+ARCH indicates which architecture to produce headers for, and defaults to the
+current architecture.  The linux/asm directory of the exported kernel headers
+is platform-specific, to see a complete list of supported architectures use
+the command:
+
+  ls -d include/asm-* | sed 's/.*-//'
+
+INSTALL_HDR_PATH indicates where to install the headers.  It defaults to
+"./usr/include".
+
+The command "make headers_install_all" exports headers for all architectures
+simultaneously.  (This is mostly of interest to distribution maintainers,
+who create an architecture-independent tarball from the resulting include
+directory.)  Remember to provide the appropriate linux/asm directory via "mv"
+or "ln -s" before building a C library with headers exported this way.
+
+The kernel header export infrastructure is maintained by David Woodhouse
+<dwmw2@infradead.org>.