I'm porting a project to linux and cmake. It is part of a huge project, each subproject in a folder. I want to build only my project and its dependencies.
I can comment the other projects from the main CMakeList.txt file, but that doesn't work because there are a few dependencies to other projects (which may also depend on other projects...)
I don't want to modify all the cmake files from other projects to build only the small pieces that I need because is very error prone and time consuming. I could checkin something by mistake which I was not supposed to, for example.
My main problem is that the other projects break continuisly (almost everybody else is just working in windows, so the linux build is not taken good care of) and is slowing me down a lot.
My question is: Is there a way to tell cmake: run through all the project, but compile only what is needed to compile certain subfolder?
You can do:
add_subdirectory(dir EXCLUDE_FROM_ALL)
and then all targets within that subdirectory (and recursive) are by default marked with EXCLUDE_FROM_ALL, that way they will not build by default.
You may also specify the specific targets to be build with:
cmake --build the_build_dir --target this_target -t another_target
I'm attempting to make a CMake package for Crypto++ inclusion in CMake projects, this will end up in the noloader/cryptopp-cmake repo if it gets done.
The ultimate goal is to come up with a working cross-platform FindCryptoPP.cmake file which can be dropped in the Crypto++ source directory to do things like:
find_package(CryptoPP REQUIRED)
target_link_libraries(libbiocoin cryptopp-static)
Or:
find_package(CryptoPP REQUIRED)
target_link_libraries(libbiocoin cryptopp-shared)
In a finished application and have it "just work."
My current best solution within a CMake application is to build Crypto++ for the platform, stick the resulting archive or library in a lib directory, reference that within the CMakeLists.txt and pull it in that way, but of course that requires packaging a binary distribution of the compiled Crypto++ for every platform targeted by the application, which would be nasty to maintain and generally bad even if it weren't crypto code.
It's better to provide a CMake configuration file. find_package will look for a configuration file if no FindFoo.cmake find script is provided. One advantage over a find script is that you won't end with different, maybe conflicting versions of the find script.
See https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html, especially the section Create Layout.
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It is notoriously difficult to get any useful information on CMake as a beginner. So far, I've seen a few tutorials on how to set up some very basic project or another. However, none of these explain the reasoning behind anything that is shown in them, always leaving many holes to fill.
What does calling CMake on a CMakeLists mean? Is it supposed to be called once per build tree or what? How do I use different settings for each build if they all use the same CMakeLists.txt file from the same source?
Why does each subdirectory need its own CMakeLists file? Would it make sense to use CMake on a CMakeLists.txt other than the one at the root of the project? If so, in what cases?
What's the difference between specifying how to build an executable or library from the CMakeLists file in their own subdirectory versus doing it in the CMakeLists file at the root of all source?
Can I make a project for Eclipse and another for Visual Studio, just changing the -G option when calling CMake? Is that even how it's used?
None of the tutorials, documentation pages or questions/answers I've seen so far give any useful insight towards understanding how to use CMake. The examples are just not thorough. No matter what tutorials I read, I feel like I'm missing something important.
There are many questions asked by CMake newbies like me that don't ask this explicitly, but that make obvious the fact that, as newbs, we have no idea how to deal with CMake or what to make of it.
What is CMake for?
According to Wikipedia:
CMake is [...] software for managing the build process of software
using a compiler-independent method. It is designed to support
directory hierarchies and applications that depend on multiple
libraries. It is used in conjunction with native build environments
such as make, Apple's Xcode, and Microsoft Visual Studio.
With CMake, you no longer need to maintain separate settings specific to your compiler/build environment. You have one configuration, and that works for many environments.
CMake can generate a Microsoft Visual Studio solution, an Eclipse project or a Makefile maze from the same files without changing anything in them.
Given a bunch of directories with code in them, CMake manages all the dependencies, build orders and other tasks that your project needs done before it can be compiled. It does NOT actually compile anything. To use CMake, you must tell it (using configuration files called CMakeLists.txt) what executables you need compiled, what libraries they link to, what directories there are in your project and what is inside of them, as well as any details like flags or anything else you need (CMake is quite powerful).
If this is correctly set up, you then use CMake to create all of the files that your "native build environment" of choice needs to do its job. In Linux, by default, this means Makefiles. So once you run CMake, it will create a bunch of files for its own use plus some Makefiles. All you need to do thereafter is type "make" in the console from the root folder every time you're done editing your code, and bam, a compiled and linked executable is made.
How does CMake work? What does it do?
Here is an example project setup that I will use throughout:
simple/
CMakeLists.txt
src/
tutorial.cxx
CMakeLists.txt
lib/
TestLib.cxx
TestLib.h
CMakeLists.txt
build/
The contents of each file are shown and discussed later on.
CMake sets your project up according to the root CMakeLists.txt of your project, and does so in whatever directory you executed cmake from in the console. Doing this from a folder that isn't the root of your project produces what is called an out-of-source build, which means files created during compilation (obj files, lib files, executables, you know) will be placed in said folder, kept separate from the actual code. It helps reduce clutter and is preferred for other reasons as well, which I will not discuss.
I do not know what happens if you execute cmake on any other than the root CMakeLists.txt.
In this example, since I want it all placed inside the build/ folder, first I have to navigate there, then pass CMake the directory in which the root CMakeLists.txt resides.
cd build
cmake ..
By default, this sets everything up using Makefiles as I've said. Here is what the build folder should look like now:
simple/build/
CMakeCache.txt
cmake_install.cmake
Makefile
CMakeFiles/
(...)
src/
CMakeFiles/
(...)
cmake_install.cmake
Makefile
lib/
CMakeFiles/
(...)
cmake_install.cmake
Makefile
What are all of these files? The only thing you have to worry about is the Makefile and the project folders.
Notice the src/ and lib/ folders. These have been created because simple/CMakeLists.txt points to them using the command add_subdirectory(<folder>). This command tells CMake to look in said folder for another CMakeLists.txt file and execute that script, so every subdirectory added this way must have a CMakeLists.txt file within. In this project, simple/src/CMakeLists.txt describes how to build the actual executable and simple/lib/CMakeLists.txt describes how to build the library. Every target that a CMakeLists.txt describes will be placed by default in its subdirectory within the build tree. So, after a quick
make
in console done from build/, some files are added:
simple/build/
(...)
lib/
libTestLib.a
(...)
src/
Tutorial
(...)
The project is built, and the executable is ready to be executed. What do you do if you want the executables put in a specific folder? Set the appropriate CMake variable, or change the properties of a specific target. More on CMake variables later.
How do I tell CMake how to build my project?
Here are the contents, explained, of each file in the source directory:
simple/CMakeLists.txt:
cmake_minimum_required(VERSION 2.6)
project(Tutorial)
# Add all subdirectories in this project
add_subdirectory(lib)
add_subdirectory(src)
The minimum required version should always be set, according to the warning CMake throws when you don't. Use whatever your version of CMake is.
The name of your project can be used later on, and hints towards the fact you can manage more than one project from the same CMake files. I won't delve into that, though.
As mentioned before, add_subdirectory() adds a folder to the project, which means CMake expects it to have a CMakeLists.txt within, which it will then run before continuing. By the way, if you happen to have a CMake function defined you can use it from other CMakeLists.txts in subdirectories, but you have to define it before you use add_subdirectory() or it won't find it. CMake is smarter about libraries, though, so this is likely the only time you will run into this kind of problem.
simple/lib/CMakeLists.txt:
add_library(TestLib TestLib.cxx)
To make your very own library, you give it a name and then list all the files it's built from. Straightforward. If it needed another file, foo.cxx, to be compiled, you would instead write add_library(TestLib TestLib.cxx foo.cxx). This also works for files in other directories, for instance add_library(TestLib TestLib.cxx ${CMAKE_SOURCE_DIR}/foo.cxx). More on the CMAKE_SOURCE_DIR variable later.
Another thing you can do with this is specify that you want a shared library. The example: add_library(TestLib SHARED TestLib.cxx). Fear not, this is where CMake begins to make your life easier. Whether it's shared or not, now all you need to handle to use a library created in this way is the name you gave it here. The name of this library is now TestLib, and you can reference it from anywhere in the project. CMake will find it.
Is there a better way to list dependencies? Definitely yes. Check down below for more on this.
simple/lib/TestLib.cxx:
#include <stdio.h>
void test() {
printf("testing...\n");
}
simple/lib/TestLib.h:
#ifndef TestLib
#define TestLib
void test();
#endif
simple/src/CMakeLists.txt:
# Name the executable and all resources it depends on directly
add_executable(Tutorial tutorial.cxx)
# Link to needed libraries
target_link_libraries(Tutorial TestLib)
# Tell CMake where to look for the .h files
target_include_directories(Tutorial PUBLIC ${CMAKE_SOURCE_DIR}/lib)
The command add_executable() works exactly the same as add_library(), except, of course, it will generate an executable instead. This executable can now be referenced as a target for things like target_link_libraries(). Since tutorial.cxx uses code found in the TestLib library, you point this out to CMake as shown.
Similarly, any .h files #included by any sources in add_executable() that are not in the same directory as the source have to be added somehow. If not for the target_include_directories() command, lib/TestLib.h would not be found when compiling Tutorial, so the entire lib/ folder is added to the include directories to be searched for #includes. You might also see the command include_directories() which acts in a similar fashion, except that it does not need you to specify a target since it outright sets it globally, for all executables. Once again, I'll explain CMAKE_SOURCE_DIR later.
simple/src/tutorial.cxx:
#include <stdio.h>
#include "TestLib.h"
int main (int argc, char *argv[])
{
test();
fprintf(stdout, "Main\n");
return 0;
}
Notice how the "TestLib.h" file is included. No need to include the full path: CMake takes care of all that behind the scenes thanks to target_include_directories().
Technically speaking, in a simple source tree like this you can do without the CMakeLists.txts under lib/ and src/ and just adding something like add_executable(Tutorial src/tutorial.cxx) to simple/CMakeLists.txt. It's up to you and your project's needs.
What else should I know to properly use CMake?
(AKA topics relevant to your understanding)
Finding and using packages: The answer to this question explains it better than I ever could.
Declaring variables and functions, using control flow, etc.: check out this tutorial that explains the basics of what CMake has to offer, as well as being a good introduction in general.
CMake variables: there are plenty, so what follows is a crash course to get you on the right track. The CMake wiki is a good place to get more in-depth information on variables and ostensibly other things as well.
You may want to edit some variables without rebuilding the build tree. Use ccmake for this (it edits the CMakeCache.txt file). Remember to configure when done with the changes and then generate makefiles with the updated configuration.
Read the previously referenced tutorial to learn about using variables, but long story short:
set(<variable name> value) to change or create a variable.
${<variable name>} to use it.
CMAKE_SOURCE_DIR: The root directory of source. In the previous example, this is always equal to /simple
CMAKE_BINARY_DIR: The root directory of the build. In the previous example, this is equals to simple/build/, but if you ran cmake simple/ from a folder such as foo/bar/etc/, then all references to CMAKE_BINARY_DIR in that build tree would become /foo/bar/etc.
CMAKE_CURRENT_SOURCE_DIR: The directory in which the current CMakeLists.txt is in. This means it changes throughout: printing this from simple/CMakeLists.txt yields /simple, and printing it from simple/src/CMakeLists.txt yields /simple/src.
CMAKE_CURRENT_BINARY_DIR: You get the idea. This path would depend not only on the folder the build is in, but also on the current CMakeLists.txt script's location.
Why are these important? Source files will obviously not be in the build tree. If you try something like target_include_directories(Tutorial PUBLIC ../lib) in the previous example, that path will be relative to the build tree, that is to say it will be like writing ${CMAKE_BINARY_DIR}/lib, which will look inside simple/build/lib/. There are no .h files in there; at most you will find libTestLib.a. You want ${CMAKE_SOURCE_DIR}/lib instead.
CMAKE_CXX_FLAGS: Flags to pass on to the compiler, in this case the C++ compiler. Also worth noting is CMAKE_CXX_FLAGS_DEBUG which will be used instead if CMAKE_BUILD_TYPE is set to DEBUG. There are more like these; check out the CMake wiki.
CMAKE_RUNTIME_OUTPUT_DIRECTORY: Tell CMake where to put all executables when built. This is a global setting. You can, for instance, set it to bin/ and have everything neatly placed there. EXECUTABLE_OUTPUT_PATH is similar, but deprecated, in case you stumble upon it.
CMAKE_LIBRARY_OUTPUT_DIRECTORY: Likewise, a global setting to tell CMake where to put all library files.
Target properties: you can set properties that affect only one target, be it an executable or a library (or an archive... you get the idea). Here is a good example of how to use it (with set_target_properties().
Is there an easy way to add sources to a target automatically? Use GLOB to list everything in a given directory under the same variable. Example syntax is FILE(GLOB <variable name> <directory>/*.cxx).
Can you specify different build types? Yes, though I'm not sure about how this works or the limitations of this. It probably requires some if/then'ning, but CMake does offer some basic support without configuring anything, like defaults for the CMAKE_CXX_FLAGS_DEBUG, for instance.
You can either set your build type from within the CMakeLists.txt file via set(CMAKE_BUILD_TYPE <type>) or by calling CMake from console with the appropriate flags, for example cmake -DCMAKE_BUILD_TYPE=Debug.
Any good examples of projects that use CMake? Wikipedia has a list of open-source projects that use CMake, if you want to look into that. Online tutorials have been nothing but a letdown to me so far in this regard, however this Stack Overflow question has a pretty cool and easy-to-understand CMake setup. It's worth a look.
Using variables from CMake in your code: Here's a quick and dirty example (adapted from some other tutorial):
simple/CMakeLists.txt:
project (Tutorial)
# Setting variables
set (Tutorial_VERSION_MAJOR 1)
set (Tutorial_VERSION_MINOR 1)
# Configure_file(<input> <output>)
# Copies a file <input> to file <output> and substitutes variable values referenced in the file content.
# So you can pass some CMake variables to the source code (in this case version numbers)
configure_file (
"${PROJECT_SOURCE_DIR}/TutorialConfig.h.in"
"${PROJECT_SOURCE_DIR}/src/TutorialConfig.h"
)
simple/TutorialConfig.h.in:
// Configured options and settings
#define Tutorial_VERSION_MAJOR #Tutorial_VERSION_MAJOR#
#define Tutorial_VERSION_MINOR #Tutorial_VERSION_MINOR#
The resulting file generated by CMake, simple/src/TutorialConfig.h:
// Configured options and settings
#define Tutorial_VERSION_MAJOR 1
#define Tutorial_VERSION_MINOR 1
With clever use of these you can do cool things like turning off a library and such. I do recommend taking a look at that tutorial as there are some slightly more advanced things that are bound to be very useful on larger projects, sooner or later.
For everything else, Stack Overflow is brimming with specific questions and concise answers, which is great for everyone except the uninitiated.
I've just started working with CMake and I noticed that they have both a find_package and a find_library. And this confuses me. Can somebody explain the difference between a package and a library in the world of programming? Or, in the world of CMake?
Appreciate it, guys!
Imagine you want to use zlib in your project, you need to find the header file zlib.h, and the library libz.so (on Linux). You can use the low-level cmake commands find_path and find_library to find them, or you can use find_package(ZLIB). The later command will try to find out all what is necessary to use zlib. It can be extra macro definitions, or dependencies.
Update, more detail about find_package: when the CMake command find_package(SomeThing) is called, as in the documentation, there are two possible modes that cmake can run:
the module mode (that searches for a file FindSomeThing.cmake)
or the config mode (that searches for a file named SomeThingConfig.cmake)
For ZLIB, there is a module named FindZLIB, shipped with CMake itself (on my Linux machine that is the file /usr/share/cmake/Modules/FindZLIB.cmake). That module is a CMake script that uses the CMake API to search for ZLIB files in default locations, or ask the user for the location if it cannot be found automatically.
I'm not asking this for just myself. I hope this question will be a reference for the many newbies who like me, found it utterly perplexing about what exactly what was going on behind the scenes when for such a small CMakeLists.txt file
cmake_minimum_required (VERSION 2.6)
project(Tutorial)
add_executable(Tutorial tutorial.cpp)
and such a small tutorial.cpp
int main() { return 0; }
there are so many files generated
CMakeCache.txt cmake_install.cmake Makefile
CMakeLists.txt tutorial.cpp
and a CMakeFiles folder with so many files and folders
CMakeCCompiler.cmake CMakeOutput.log Makefile.cmake
cmake.check_cache CMakeSystem.cmake progress.marks
CMakeCXXCompiler.cmake CMakeTmp TargetDirectories.txt
CMakeDetermineCompilerABI_C.bin CompilerIdC Tutorial.dir
CMakeDetermineCompilerABI_CXX.bin CompilerIdCXX
CMakeDirectoryInformation.cmake Makefile2
Not understanding what was going on behind the scenes (i.e: why so may files had to be generated and what their purpose was), was the biggest obstacle in being able to learn CMake.
If anyone knows, could you please explain it for the sake of posterity? What is the purpose of these files, and when I type cmake ., what exactly is cmake configuring and generating before it builds the project?
The secret is that you don't have to understand what the generated files do.
CMake introduces a lot of complexity into the build system, most of which only pays off if you use it for building complex software projects.
The good news is that CMake does a good job of keeping a lot of this messiness away from you: Use out-of-source builds and you don't even have to look at the generated files. If you didn't do this so far (which I guess is the case, since you wrote cmake .), please check them out before proceeding. Mixing the build and source directory is really painful with CMake and is not how the system is supposed to be used.
In a nutshell: Instead of
cd <source_dir>
cmake .
always use
cd <build_dir_different_from_source_dir>
cmake <source_dir>
I usually use an empty subfolder build inside my source directory as build directory.
To ease your pain, let me give a quick overview of the relevant files which CMake generates:
Project files/Makefiles - What you are actually interested in: The files required to build your project under the selected generator. This can be anything from a Unix Makefile to a Visual Studio solution.
CMakeCache.txt - This is a persistent key/value string storage which is used to cache value between runs. Values stored in here can be paths to library dependencies or whether an optional component is to be built at all. The list of variables is mostly identical to the one you see when running ccmake or cmake-gui. This can be useful to look at from time to time, but I would recommend to use the aforementioned tools for changing any of the values if possible.
Generated files - This can be anything from autogenerated source files to export macros that help you re-integrate your built project with other CMake projects. Most of these are only generated on demand and will not appear in a simple project such as the one from your question.
Anything else is pretty much noise to keep the build system happy. In particular, I never needed to care about anything that is going on inside the CMakeFiles subdirectory.
In general you should not mess with any of the files that CMake generates for you. All problems can be solved from within CMakeLists.txt in one way or the other. As long as the result builds your project as expected, you are probably fine. Do not worry too much about the gory details - as this is what CMake was trying to spare you of in the first place.
As stated on its website:
Cmake is cross-platform, open-source build system for managing the build process of software using a compiler-independent method
In most cases it is used to generate project/make files - in your example it has produced Makefile which are used to build your software (mostly on Linux/Unix platform).
Cmake allows to provide cross platform build files that would generate platform specific project/make files for particular compilation/platform.
For instance you may to try to compile your software on Windows with Visual Studio then with proper syntax in your CMakeLists.txt file you can launch
cmake .
inside your project's directory on Windows platform,Cmake will generate all the necessary project/solution files (.sln etc.).
If you would like to build your software on Linux/Unix platform you would simply go to source directory where you have your CMakeLists.txt file and trigger the same cmake . and it will generate all files necessary for you to build software via simple make or make all.
Here you have some very good presentation about key Cmake functionalities http://www.elpauer.org/stuff/learning_cmake.pdf
EDIT
If you'd like to make platform dependent library includes / variable definitions etc. you can use this syntax in CMakeLists.txt file
IF(WIN32)
...do something...
ELSE(WIN32)
...do something else...
ENDIF(WIN32)
There is also a lot of commands with use of which you are able to prevent the build from failing and in place Cmake will notify you that for instance you do not have boost libraries filesystem and regex installed on your system. To do that you can use the following syntax:
find_package(Boost 1.45.0 COMPONENTS filesystem regex)
Having checked that it will generate the makefiles for appropriate system/IDE/compiler.
Exactly how CMake works is a question for the developers, so this question can't be answered here.
However we can give a touch of useful guidance as far as when you should use CMake and when you therefore need to worry about how it works. I'm not a fan of "oh it just works" answers either - because, especially in software, NOTHING ever "just works" and you ALWAYS have to get into the nitty-gritty details at some point.
CMake is an industrial-strength tool. It automates several VERY complex process and takes into account many variables of which you may not be aware, especially as a fairly new developer, probably working with limited knowledge of all the operating systems and build tools CMake can handle. The reason so many files are generated and why things seem so complex is because all of those other systems are complex and must be accounted for and automated. Additionally there are the issues of "caching" and other time-saving features of the tool To understand everything in CMake would mean understanding everything in these build tools and OS's and all the possible combinations of these variables, which as you can imagine is impossible.
It's important to note that if you're not in charge of managing a large cross-platform build system, and your code base is a few KLOC, maybe up to 100KLOG, using CMake seems a little bit like using a 100,000 dollar forestry tree removal machine to remove weeds from your 2 foot by 2 foot flower garden. (By the way, if you've never seen such a machine, you should look for one on youtube, they're amazing)
If your build system is small and simple it's likely to be better to just write your own makefiles by hand or script them yourself. When your makefiles become unwieldy or you need to build a version of your system on another platform, then you can switch over to CMake. At that point, you'll have lots of problems to solve and you can ask more focused questions about it. In the meantime, check out some of the great books that have been written about CMake, or even better, write one yourself! 8)