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Start to document some of the differences in cmake and ninja.

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git-svn-id: svn://svn.linuxfromscratch.org/BLFS/trunk/BOOK@21415 af4574ff-66df-0310-9fd7-8a98e5e911e0
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Ken Moffat 2019-03-30 21:36:23 +00:00
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general.ent
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@ -1,12 +1,12 @@
<!-- $LastChangedBy$ $Date$ -->
<!ENTITY day "29"> <!-- Always 2 digits -->
<!ENTITY day "30"> <!-- Always 2 digits -->
<!ENTITY month "03"> <!-- Always 2 digits -->
<!ENTITY year "2019">
<!ENTITY copyrightdate "2001-&year;">
<!ENTITY copyholder "The BLFS Development Team">
<!ENTITY version "&year;-&month;-&day;">
<!ENTITY releasedate "March 29th, &year;">
<!ENTITY releasedate "March 30th, &year;">
<!ENTITY pubdate "&year;-&month;-&day;"> <!-- metadata req. by TLDP -->
<!ENTITY blfs-version "svn"> <!-- svn|[release #] -->
<!ENTITY lfs-version "development"> <!-- x.y|development -->

186
introduction/important/building-notes.xml
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@ -453,4 +453,190 @@ chmod 755 blfs-yes-test2</userinput></screen>
</sect2>
-->
<sect2 id="buildsystems">
<title>Working with different build systems</title>
<para>
There are now three different build systems in common use for
converting C or C++ source code into compiled programs or
libraries, and their details (particularly, finding out about available
options and their default values) differ. It may be easiest to understand
the issues caused by some choices (typically, slow execution, or
unexpected use of, or omission of, optimizatons) by starting with the item
which drew attention to this, the CFLAGS and CXXFLAGS.
</para>
<para>
Most LFS and BLFS builders are probably aware of the basics of CFLAGS
and CXXFLAGS for altering how a program is compiled. Typically, some
form of optimization is used for a released program (-O2 or -O3),
sometimes with the creation of debug symbols (-g) when using -O2.
</para>
<para>
If there are contradictory flags (e.g. multiple different -O values),
the <emphasis>last</emphasis> value will be used. Sometimes this means
that flags specifiedi in environment variables will be picked up before
values hardcoded in the Makefile, and therefore ignored. For example,
where a user specifies '-O2' and that is followed by '-O3' the build will
use '-O3'.
</para>
<para>
There are various other things which can be passed in CFLAGS or
CXXFLAGS, such as forcing compilation for a specific microarchitecture
(e.g. -march=amdfam10, -march=native) or specifying a specific standard
for C or C++ (-std=c++17 for example). But one thing which has now come
to light is that programmers might include debug assertions in their
code, expecting them to be disabled in releases by using -DNDEBUG.
Specifically, if <xref linkend="mesa"/> is built with these assertions
enabled, some activities such as loading levels of games can take
extremely long times, even on high-class video cards.
</para>
<bridgehead renderas="sect3">Autotools with Make</bridgehead>
<para>
This combination is often described as 'CMMI' (configure, make, make
install) and is used here to also cover the few packages which have a
configure script that is not generated by autotools.
</para>
<para>
Sometimes, running <command>./configure --help</command> will produce
useful options about switches which might be used. At other times,
after looking at the output from a run of configure you may need to look
at the details of the script to find out what it was actually searching
for.
</para>
<para>
Many configure scripts will pick up any CFLAGS or CXXFLAGS from the
environment, but CMMI packages vary about how these will be mixed with
any flags which would otherwise be used (<emphasis>variously</emphasis>:
ignored, used to replace the programmer's suggestion, used before the
programmer's suggestion, used after the programmer's suggestion).
</para>
<para>
In most CMMI packages, running 'make' will list each command and run
it, interspersed with any warnings. But some packages try to be 'silent'
and only show which file they are compiling or linking instead of showing
the command line. If you need to inspect the command, either because of
an error, or just to see what options and flags are being used, adding
'V=1' to the make invocation may help.
</para>
<bridgehead renderas="sect3">CMake</bridgehead>
<para>
CMake works in a very different way, and it has two backends which can
be used on BLFS: 'make' and 'ninja'. The default backend is make, but
ninja can be faster on large packages with multiple processors. To
use ninja, specify '-G Ninja' in the cmake command.
</para>
<para>
The hardest part of using CMake is knowing what options you might wish
to specify. The only way to get a list of what the package knows about
is to run <command>cmake -LAH</command> and look at the output for that
default configuration.
</para>
<para>
Perhaps the most-important thing about CMake is that it has a variety
of CMAKE_BUILD_TYPE values, and these affect the flags. The default
is that this is not set and no flags are generated. Any CFLAGS or
CXXFLAGS in the environment will be used. If the programmer has coded
any debug assertions, those will be enabled unless -DNDEBUG is used.
The following CMAKE_BUILD_TYPE values will generate the flags shown,
and these will come <emphasis>after</emphasis> any flags in the
environment and therefore take precedence.
</para>
<itemizedlist>
<listitem>
<para>Debug : '-g'</para>
</listitem>
<listitem>
<para>Release : '-O3 -DNDEBUG'</para>
</listitem>
<listitem>
<para>RelWithDebInfo : '-O2 -g -DNDEBUG'</para>
</listitem>
<listitem>
<para>MinSizeRel : '-Os -DNDEBUG'</para>
</listitem>
</itemizedlist>
<para>
CMake tries to produce quiet builds. To see the details of the commands
which are being run, use 'make VERBOSE=1' or 'ninja -v'.
</para>
<bridgehead renderas="sect3">Meson</bridgehead>
<para>
Meson has some similarities to CMake, but many differences. To get
details of the definesthat you may wish to change is slightly involved
using meson-0.49.2:
</para>
<screen><userinput>meson ..
meson configure | less</userinput></screen>
<para>
After identifying what to set, meson then needs to be reconfigured:
</para>
<screen><userinput>meson --prefix=/usr -Dsome_option=true -Dother_option=false --reconfigure</userinput></screen>
<para>
Alternatively, you could remove the build directory where you did this,
recreate it, and then run meson with the desired switches.
</para>
<para>
Meson provides the following buildtype values, and the flags they enable
come <emphasis>after</emphasis> any flags supplied in the environment and
therefore take precedence.
</para>
<itemizedlist>
<listitem>
<para>plain : no added flags. This is for distributors to supply their
own CLFAGS, CXXFLAGS and LDFLAGS. There is no obvious reason to use
this in BLFS.</para>
</listitem>
<listitem>
<para>debug : '-g'</para>
</listitem>
<listitem>
<para>debugoptimized : '-O2 -g' - this is the default if nothing is
specified, it leaves assertions enabled.</para>
</listitem>
<listitem>
<para>release : '-O3 -DNDEBUG'</para>
</listitem>
</itemizedlist>
<para>
Although the release buildtype is described as enabling -DNDEBUG, and all
CMake Release builds pass that, it has so far only been observed (in
verbose builds) for <xref linkend="mesa"/>. That suggests that it might
only be used when there are debug assertions present.
</para>
<para>
The -DNDEBUG flag can also be provided by passing
<command>-Db_ndebug=true</command>.
</para>
<para>
To see the details of the commands which are being run in a package using
meson, again use 'ninja -v'.
</para>
</sect2>
</sect1>

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introduction/welcome/changelog.xml
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@ -41,6 +41,17 @@
</itemizedlist>
</listitem>
-->
<listitem>
<para>March 29th, 2019</para>
<itemizedlist>
<listitem>
<para>[ken] - Add "Working with different build systems" to the end of
"Notes on Building Software", to start to document some of the
differences in CMake and Meson.</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>March 29th, 2019</para>
<itemizedlist>