CMake package guidelines
32-bit – CLR – CMake – Cross – DKMS – Eclipse – Electron – Font – Free Pascal – GNOME – Go – Haskell – Java – KDE – Kernel modules – Lisp – Meson – MinGW – Node.js – Nonfree – OCaml – Perl – PHP – Python – R – Ruby – Rust - Security – Shell – VCS – Web – Wine
This document covers standards and guidelines on writing PKGBUILDs for software that uses cmake.
From the CMake web page:
- CMake is an open-source, cross-platform family of tools designed to build, test and package software. CMake is used to control the software compilation process using simple platform and compiler independent configuration files, and generate native makefiles and workspaces that can be used in the compiler environment of your choice.
Typical usage
The typical usage consists of running the cmake
command and after that execute the building command. The cmake
command usually sets some parameters, checks for the needed dependencies and creates the build files, letting the software ready to be built by other tools like make
and ninja
.
CMake undesired behaviors
Due to its own internal characteristics for generating the build files, sometimes CMake can behave in undesired ways. Being such, some steps should be noted when writing PKGBUILDs for CMake-based software.
CMake can automatically override the default compiler optimization flag
It is very common to see people running CMake with the -DCMAKE_BUILD_TYPE=Release
option. Some upstream projects even inadvertently include this option in their building instructions, but this produces an undesired behavior.
Each build type causes CMake to automatically append a set of flags to CFLAGS
and CXXFLAGS
. When using the common Release
build type, it automatically appends the -O3
[1] compiler optimization flag, and this overrides the default Arch Linux flag which currently is -O2
(defined in the makepkg configuration file). This is undesired, as it deviates from the Arch Linux targeted optimization level.
Notes about -O3
Using -O3
does not guarantee that the software will perform better, and sometimes it can even slow down the program. It can also break software in some situations. There is a good reason why the Arch Linux developers choose -O2
as the target optimization level and we should stick with it. Unless you know exactly what you are doing, or if upstream explicitly tells or implies that -O3
is needed, we should avoid using it in our packages.
Fixing the automatic optimization flag override
Fixing this in a 100% guaranteed way is not a simple question due to CMake flexibility. Please note that there is no standard solution that can be applied to all cases. This section will discuss possible solutions and some points that should be observed.
The default CMake build type is None
and it does not append any flags to CFLAGS
and CXXFLAGS
by default, so simply omitting the usage of the CMAKE_BUILD_TYPE
option can work as it will default to None
. But note that omitting this option is not guaranteed to fix the problem, as many software projects automatically set the build type to Release
(or other type) in the CMake files if CMAKE_BUILD_TYPE
is not set at command line. Also be aware of possible references to source files in the resulting package and the corresponding makepkg's WARNING: Package contains reference to $srcdir
caused by a missing NDEBUG
definition in the None
build type.
Since the default None
build type does not append any flags to CFLAGS
and CXXFLAGS
by default, using the -DCMAKE_BUILD_TYPE=None
option can also work. Generally speaking, using the -DCMAKE_BUILD_TYPE=None
option is better than omitting the usage of CMAKE_BUILD_TYPE
. It will cover the case when upstream automatically sets the build type to Release
when CMAKE_BUILD_TYPE
is omitted, it will not append any flags by default and it is uncommon to see software setting undesired flags for the None
build type.
But unfortunately, things are not that simple like only using -DCMAKE_BUILD_TYPE=None
to fix this. When using the None
build type to fix the -O3
issue, one may fall into another problem. It is a common practice for many software projects to define some required compiler flags for the Release
build type in the CMake files (for example, like setting the CMAKE_C_FLAGS_RELEASE
and CMAKE_CXX_FLAGS_RELEASE
CMake variables). Such software may break or misbehave when compiled without these upstream defined flags if you use the None
build type. In order to determine if you are missing some flags you will need to look at the CMake files, or you can compare the output of make VERBOSE=1
for the None
and Release
build types. What to do if the None
build type causes some upstream defined flags to be missed? In this case you may be at the middle of two problematic situations, because if you use the Release
build type you may be using the undesired -O3
flag, and if you use the None
build type you will miss some required upstream defined flags. There is no standard way of solving this situation and it should be analyzed on a case-by-case basis. If upstream defines -O2
for the Release
build type, you can use -DCMAKE_BUILD_TYPE=Release
(see bellow). Otherwise, patching the CMake files may be a solution.
Some few software projects hardcode -O2
for the Release
build type in their CMake files, and thus -DCMAKE_BUILD_TYPE=Release
can be safely set in this case if you are sure that -O2
is the optimization level being used.
- Some software may break when using the
None
build type. Test the software when using theNone
build type to check if it will break or lose functionality. - Some software may work only with the
Release
build type. You will need to experiment and test the software.
Verifying the fixes
You can verify if the fixes are being correctly used by CMake by enabling the verbose mode of the build tool. For example, when using make
(which is the CMake default), this can be done by adding VERBOSE=1
to it (like make VERBOSE=1
). This will enable make
to output the compiler commands that are being executed. You can then run makepkg and examine the output to see if the compiler is using the -D_FORTIFY_SOURCE=2
and -O2
flags. If multiple optimization flags are being displayed in each command line, the last flag in the line will be the one used by the compiler (it means that -O2
needs to be the last optimization flag in order to be effective).
Prefix and library install directories
The standard Arch Linux /usr
prefix can be specified by the -DCMAKE_INSTALL_PREFIX=/usr
CMake option. This is usually needed because a lot of software defaults to install files into the /usr/local
prefix.
Some upstream projects set their CMake files to install libraries into the /usr/lib64
directory. If this is the case, you can correctly set the library installation directory to /usr/lib
by using the -DCMAKE_INSTALL_LIBDIR=lib
CMake option.
Tips and tricks
Specifying directories
Since CMake version 3.13, there is a -B
option that automatically creates the build directory. This avoids the creation of the build directory by a separated mkdir
(or install
) command. The -S
option specifies the source directory (where to search for a CMakeLists.txt
file) and avoids the need of cd
'ing into the source tree before executing cmake
. Combined together, these two options are a convenient way to specify the build and the source directories.
PKGBUILD
build() { cmake -B build -S "$pkgname-$pkgver" [other_cmake_options] cmake --build build }
Reducing possible unneeded output
The -Wno-dev
CMake option will suppress the output of some warnings that are meant only for the upstream project developers who write the CMakeLists.txt
files. Removing these warnings makes the CMake output smoother and reduces the burden on examining it. As a general rule, these warnings usually can be safely ignored by packagers.
Removing insecure RPATH references from binaries
Sometimes the resulting binaries can contain insecure references in RPATH
. This can be verified by running Namcap on the built package and consists in a security issue that should be fixed. There is a good chance to fix this by using the CMAKE_SKIP_INSTALL_RPATH=YES
or CMAKE_SKIP_RPATH=YES
CMake options. You need to experiment with both and see what will work in the software in question (using both options is not needed).
Getting all available CMake options
For getting all "visible" CMake options that are available for a software project, execute cmake -LAH
in the source tree (where the main CMakeLists.txt
file is located).
If you want to save the output for later reference, you can redirect it to a file:
$ cmake -LAH >options.txt 2>&1
Avoiding FetchContent downloads during the build
CMake provides the FetchContent module that allows fetching additional resources or subprojects at build-time. However, ideally all sources should be fetched by makepkg prior to the build, as they are specified in the sources
array. This can be accomplished via the option FETCHCONTENT_SOURCE_DIR_<uppercaseName>, which allows specifying the path to the files that would otherwise be fetched. Additionally, FETCHCONTENT_FULLY_DISCONNECTED=ON can be used to skip all downloads during the build, even if you missed any FetchContent
declarations.
Example
Assume a project fetches the resource foo
:
CMakeLists.txt
FetchContent_Declare( foo URL https://example.com/foo.tar.gz URL_HASH SHA256=cf051bf611a94884ba5e4c2d03932d14e83875c5b77f0fdf55c404cad0e4a6e6 ) FetchContent_MakeAvailable(foo)
Then, instead of downloading it during the build, this resource can be added to the sources
array and declared when generating the build files:
PKGBUILD
sources=( ... "https://example.com/foo.tar.gz" ) sha256sums=( ... "cf051bf611a94884ba5e4c2d03932d14e83875c5b77f0fdf55c404cad0e4a6e6" )
$ cmake -B build -S "$pkgname-$pkgver" -DFETCHCONTENT_FULLY_DISCONNECTED=ON -DFETCHCONTENT_SOURCE_DIR_FOO="$srcdir/foo"
Template
Here is a general template for the build()
function that serves as a starting point for CMake-based packages. Supposing the package is C and C++ based and that it does not define any required compiler flags for the Release
build type in the CMake files.
enable_testing()
and/or add_test()
functionality in CMakeLists.txt
.PKGBUILD
build() { cmake -B build -S "$pkgname-$pkgver" \ -DCMAKE_BUILD_TYPE='None' \ -DCMAKE_INSTALL_PREFIX='/usr' \ -Wno-dev cmake --build build } check() { ctest --test-dir build --output-on-failure } package() { DESTDIR="$pkgdir" cmake --install build }
Do not forget to place cmake in makedepends.