Short introduction into LTP build system
========================================

******************************************************************************
The following document briefly describes the steps and methodologies used for
the new and improved Makefile system.

Changelog:

 * Initial version: Ngie Cooper <yaneurabeya@gmail.com>
 * Reformated for asciidoc: Cyril Hrubis <chrubis@suse.cz>
******************************************************************************

The Problem
-----------

The problem with the old Makefile system is that it was very difficult to
maintain and it lacked any sense of formal structure, thus developing for LTP
and including new targets was more difficult than it should have been
(maintenance). Furthermore, proper option-based cross-compilation was
impossible due to the fact that the Makefiles didn't support a prefixing
system, and the appropriate implicit / static rules hadn't been configured to
compile into multiple object directories for out-of-tree build support (ease of
use / functionality). Finally, there wasn't a means to setup dependencies
between components, such that if a component required libltp.a in order to
compile, it would go off and compile libltp.a first (ease of use).

These items needed to be fixed to reduce maintenance nightmares for the
development community contributing to LTP, and the project maintainers.

Design
------

The system was designed such that including a single GNU Makefile compatible
set in each new directory component is all that's essentially required to
build the system.

Say you had a directory like the following (with .c files in them which
directly tie into applications, e.g. baz.c -> baz):

-------------------------------------------------------------------------------
.../foo/
     |--> Makefile
     |
      --> bar/
	   |
	    --> Makefile
           |
            --> baz.c
-------------------------------------------------------------------------------

Here's an example of how one would accomplish that:

-------------------------------------------------------------------------------
.../foo/Makefile:
#
# Copyright disclaimer goes here -- please use GPLv2.
#

top_srcdir		?= ..

include $(top_srcdir)/include/mk/env_pre.mk
include $(top_srcdir)/include/mk/generic_trunk_target.mk

.../foo/bar/Makefile:
#
# Copyright disclaimer goes here -- please use GPLv2.
#

top_srcdir		?= ..

include $(top_srcdir)/include/mk/env_pre.mk
include $(top_srcdir)/include/mk/generic_leaf_target.mk
-------------------------------------------------------------------------------

Kernel Modules
--------------

Some of the tests need to build kernel modules, happily LTP has
infrastructure for this.

-------------------------------------------------------------------------------
ifneq ($(KERNELRELEASE),)

obj-m := module01.o

else

top_srcdir	?= ../../../..
include $(top_srcdir)/include/mk/testcases.mk

REQ_VERSION_MAJOR	:= 2
REQ_VERSION_PATCH	:= 6
MAKE_TARGETS		:= test01 test02 module01.ko

include $(top_srcdir)/include/mk/module.mk
include $(top_srcdir)/include/mk/generic_leaf_target.mk

endif
-------------------------------------------------------------------------------

This is example Makefile that allows you build kernel modules inside of LTP.
The prerequisites for the build are detected by the 'configure' script.

The 'REQ_VERSION_MAJOR' and 'REQ_VERSION_PATCH' describe minimal kernel
version for which the build system tries to build the module.

The buildsystem is also forward compatible with changes in Linux kernel
internal API so that if module fails to build the failure is ignored both on
build and installation. If the userspace counterpart of the test fails to load
the module because the file does not exists, the test is skipped.

Note the 'ifneq($(KERNELRELEASE),)', the reason it's there is that the
Makefile is executed twice, once by LTP build system and once by kernel
kbuild, see 'Documentation/kbuild/modules.rst' in the Linux kernel tree for
details on external module build.

Make Rules and Make Variables
-----------------------------

When using make rules, avoid writing ad hoc rules like:

-------------------------------------------------------------------------------
[prog]: [dependencies]
	cc -I../../include $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(LDLIBS) \
	    -o [prog] [dependencies]
-------------------------------------------------------------------------------

etc. This makes cross-compilation and determinism difficult, if not impossible.
Besides, implicit rules are your friends and as long as you use `MAKEOPTS=;' in
the top-level caller (or do $(subst r,$(MAKEOPTS)) to remove -r), the compile
will complete successfully, assuming all other prerequisites have been
fulfilled (libraries, headers, etc).

-------------------------------------------------------------------------------
$(AR)			: The library archiver.

$(CC)			: The system C compiler.

$(CPP)			: The system C preprocessor.

$(CFLAGS)		: C compiler flags.

$(CPPFLAGS)		: Preprocessor flags, e.g. -I arguments.

$(DEBUG_CFLAGS)		: Debug flags to pass to $(CC), -g, etc.

$(KVM_LD)		: Special linker for wrapping KVM payload binaries
			  into linkable object files. Defaults to $(LD).
			  Change this variable if the KVM Makefile fails
			  to build files named *-payload.o.

$(LD)			: The system linker (typically $(CC), but not
			  necessarily).

$(LDFLAGS)		: What to pass in to the linker, including -L arguments
			  and other ld arguments, apart from -l library
			  includes (see $(LDLIBS)).

			  This should be done in the $(CC) args passing style
			  when LD := $(CC), e.g. `-Wl,-foo', as opposed to
			  `-foo'.

$(LDLIBS)		: Libraries to pass to the linker (e.g. -lltp, etc).

$(LTPLDLIBS)            : LTP internal libraries i.e. these in libs/ directory.

$(OPT_CFLAGS)		: Optimization flags to pass into the C compiler, -O2,
			  etc. If you specify -O2 or higher, you should also
			  specify -fno-strict-aliasing, because of gcc
			  fstrict-aliasing optimization bugs in the tree
			  optimizer. Search for `fstrict-aliasing optimization
			  bug' with your favorite search engine.

			  Examples of more recent bugs:
			  1. tree-optimization/17510
			  2. tree-optimization/39100

			  Various bugs have occurred in the past due to buggy
			  logic in the tree-optimization portion of the gcc
			  compiler, from 3.3.x to 4.4.

$(RANLIB)		: What to run after archiving a library.

$(WCFLAGS)		: Warning flags to pass to $(CC), e.g. -Werror,
			  -Wall, etc.
-------------------------------------------------------------------------------

Make System Variables
---------------------

A series of variables are used within the make system that direct what actions
need to be taken. Rather than me listing the variables here, please with their
intended uses, please refer to the comments contained in
+.../include/mk/env_pre.mk+.

Guidelines and Recommendations
------------------------------

Of course, the GNU Make manual is key to understanding the Make system, but
here are the following sections and chapters I suggest reviewing:

link:http://www.gnu.org/software/make/manual/make.html#Implicit-Rules[Implicit Rules]
link:http://www.gnu.org/software/make/manual/make.html#Using-Variables[Variables and Expansion]
link:http://www.gnu.org/software/make/manual/make.html#Origin-Function[Origin Use]
link:http://www.gnu.org/software/make/manual/make.html#Directory-Search[VPath Use]

Before Committing
-----------------

One should rebuild from scratch before committing. Please see INSTALL for more
details.

Other Errata
------------

Please see TODO for any issues related to the Makefile infrastructure, and
build structure / source tree in general.
