This question already has answers here:
Passing compiler options in CMake
(6 answers)
Closed 5 years ago.
I am wondering if it is possible to append to inferred variables when calling cmake. Due either the compiler or the app supplied CMakeLists.txt C++11 support isn't detected by cmake even though it is supported and the application requires it. If I do:
cmake-DCMAKE_CXX_FLAGS='-std=c++11' ../source_dir
The flags get overwritten and I wouldn't like to lose the inferred flags. I could manually run without overwriting and just copy the flags and append c++11 but I think there must be a better solution.
I have found a number of posts on adding c++11 support to CMakeLists.txt but not in cmake call, so I am wondering if this is possible at all. Please let me know.
Yes, this is possible. First, create a variable called MY_FLAGS in the command line with the flags you want.
cmake -DMY_FLAGS='-std=c++11' ../source_dir
Then, in your CMakeLists.txt, do the following:
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${MY_FLAGS}")
This will effectively amend your flags to the flags that CMake generates. However, if you want to set the C++ standard to C++11, you can just do the following without directly dealing with flags:
cmake -DCMAKE_CXX_STANDARD=11 -DCMAKE_CXX_STANDARD_REQUIRED=ON ../source_dir
On a side note, make sure that the ../source_dir is after the CMake options, since otherwise CMAKE_CXX_STANDARD_REQUIRED would not work. (Read)
I recently did this, when building a dependency for one of the applications I work to.
There are 2 CMake variables that you can make use of, namely:
-DCMAKE_BUILD_TYPE=Debug/Release
-DCMAKE_CXX_FLAGS_[DEBUG/RELEASE]
When specifying a build type, cmake will add to the compiler flags the flags that are specified in that second variable. So for me, using:
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_FLAGS_RELEASE=-fPIC
did the thing.
In your case, you can try the following approaches:
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_FLAGS_RELEASE=-std=c++11
or
cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS_DEBUG=-std=c++11
Of course, this will work if the variables are not overwritten in the script. Try both approaches.
You are probably looking for
set (CMAKE_CXX_STANDARD 11)
set (CMAKE_CXX_STANDARD_REQUIRED ON)
Or via command line:
cmake -DCMAKE_CXX_STANDARD=11 -DCMAKE_CXX_STANDARD_REQUIRED=ON
That being said, CMAKE_CXX_FLAGS is not overridden by cmake, so your CMakeLists.txt must be doing it somewhere.
The polite way of setting the flags from within a script is to append to the variable, that is:
set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} stuff")
Related
I am new to CMake and was going through the CMake documentations and tutorials. I was able to understand that the target_include_directories command is just the -I option for the compiler (gcc for me). I tried doing it adding the directories manually by using set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -I <Path>") and it worked perfectly fine!!
My CMakeLists.txt looks something like this:
cmake_minimum_required(VERSION 3.6)
project(Project VERSION 1.0 DESCRIPTION "C Project" LANGUAGES C)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -I <path-to-header>")
add_library(basic file1.c file2.c)
#target_include_directories(basic PUBLIC "path-to-header")
#target_include_directories() is commented out as CMAKE_C_FLAGS have been set
add_executable(main.out main.c)
target_link_libraries(main.out PRIVATE basic)
I wanted to know if there any similar and alternative for the target_link_libraries command using which we can pass -L and -l options to linker in CMake??
To answer your question literally: There is the variable CMAKE_EXE_LINKER_FLAGS and its specializations CMAKE_EXE_LINKER_FLAGS_<CONFIG> for certain configurations like RELEASE or DEBUG or whatever configurations you might have defined. See the CMake documentation for more.
BUT, I highly disrecommend to use these unless you need to pass a very special compiler/linker option CMake does not know about, because these are specific to your compiler.
The point of using CMake is to describe the build process independent of the concrete compiler and platform, such that you can easily switch to another one or at least update your compiler without having to refactor your build code. So, better stick with the generic target_include_directories, target_link_libraries, and similar commands as much as possible.
If you really have to use CMAKE_C_FLAGS or CMAKE_EXE_LINKER_FLAGS I'd recommend to wrap them into an if clause making sure that you are building with the expected compiler on the expected platform and put a fatal error message into the else clause to make future users aware of a possible problem.
This question already has answers here:
Change default value of CMAKE_CXX_FLAGS_DEBUG and friends in CMake
(2 answers)
Closed 5 years ago.
In cmake, the default release compiler flags for gcc are -O2 -DNDEBUG. I want them to be -Ofast -NDEBUG. And I want that to be true for every project I do things with in cmake without imposing that choice on any other consumers of my project.
How do I do that?
I could edit them into that project's CMakeLists.txt file. But then I'm forcing other people to agree with my choice. Also, I have to be really careful about how I do it to make sure that I only affect compilers for which that is a valid set of flags to use.
I could use ccmake on every project every time I check out a new tree or create a new build environment. But that's a huge hassle when I want to set the flags to the same thing every time.
Is there a way to set it up so that for me, personally, the default compiler flags for clang and gcc for the various build types are what I want them to be?
Similarly, it's noticing I have ccache and gcc. And that's fine. But it might be nice to force a different default choice for compiler as well, but just for me personally, not for anybody else who chooses to use my project.
Is this possible?
I normally set compiler flags in cmakelists.txt by adding an environment variable into my build script, then referencing that in cmakelists.txt.
#build.sh
export MY_CXXFLAGS="-std=gnu++11 -Ofast -NDEBUG -Wall -Wno-unused-function -Wno-unknown-pragmas"
cmake `pwd` -Dtest=ON
make -j9
Then in my CMakeLists.txt, I'll have the following line:
SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} $ENV{MY_CXXFLAGS}")
It's simple then to pass arguments to build.sh, which is just a bash script, to change the content of MY_CXXFLAGS, based on the users needs. (E.g. build.sh -b DEVELOP|DEBUG|RELEASE etc).
My current answer to this question is to create a global ~/.config/Kitware/default.cmake file that contains initial cache settings and to use cmake -C <path to default.cmake> ... remainder of cmake options ... to run cmake.
In the process of figuring this out I initally named the file ~/.config/Kitware/CMakeCache.txt and found something quite odd that appears to be a bug in cmake. I posted this question about it: Why does this cmake initial cache file result in such strange errors?
I try to set a preprocessor macro in the command line of CMake. I've tried:
set generator="Visual Studio 8 2005"
set params=-D MY_MACRO=1
cmake.exe -G %generator% %params% ..\some_project
but it's neither defined when I compile nor can I find the name MY_MACRO in the files generated by CMake at all, except for CMakeCache.txt where it's present in the form:
MY_MACRO:UNINITIALIZED=1
How can I do it?
A good alternative would be to define a cmake option:
OPTION(DEFINE_MACRO "Option description" ON) # Enabled by default
Followed by a condition:
IF(DEFINE_MACRO)
ADD_DEFINITIONS(-DMACRO)
ENDIF(DEFINE_MACRO)
Then you can turn that option ON/OFF via command line with cmake using the -D flag. Example:
cmake -DDEFINE_MACRO=OFF ..
To make sure the compiler is receiving the definition right, you can call make in verbose mode and check for the macro being defined or not:
make VERBOSE=1
This is a good solution also because make will recompile your code when any of cmake options changes.
Try this: -D CMAKE_CXX_FLAGS=/DMY_MACRO=1
The motivation behind the question was to batch build 3rd party libraries, which is why I wanted to avoid modifying CMakeLists. So years later, even though I don't need that anymore, I figured out that it's easily achievable by means external to CMake:
Invoke CMake as usual, no special flags.
Then:
With MSVC: The compiler reads the CL environment variable to get extra command line arguments. So
set CL=/DMY_MACRO=1 %CL%
then invoke MSBuild to do its job.
With Makefiles: The generated makefiles use the CFLAGS and CXX_FLAGS variables as makefiles are expected to do. So the build can be started by
make CXX_FLAGS=-DMY_MACRO=1
or by setting the corresponding environment variables.
Unless you have a good reason not to, you should use ADD_DEFINITIONS(<name>=<value>[, ...]).
Just add the following line to your CMakeLists.txt:
ADD_DEFINITIONS("MY_MACRO=1")
CMake will take care of the syntax of the switches (be it -D<name>=<value>, or /D<name>=<value>).
Does exist a variable which contains the compiler flags used in some call to CMake's ADD_LIBRARY function, for example the ones used when we add a module:
ADD_LIBRARY(mylib MODULE mysrc.cpp)
Or, is there a way of getting such flags?
Turning my comments into an answer
There is not a single CMake variable to get the all compiler flags. The problem is that the CMake generator will finally put together the compiler flags (from various CMake variables and properties incl. from depending targets). So you don't have all the flags during configuration step.
I see the following possible problem/solution pairs:
CMake is a cross-platform wrapper around your compiler (that's actually what the C stands for), so no need to extract the compiler flags into an external script
If you just want to add sort of a filter to what is called by CMake you can user set "launcher" variables/properties accordingly e.g. CMAKE_CXX_COMPILER_LAUNCHER or RULE_LAUNCH_LINK
If you want the compiler calls in a machine readable JSON format you could export those by setting CMAKE_EXPORT_COMPILE_COMMANDS
If you just want to see the compiler calls incl. all the flags you could set CMAKE_VERBOSE_MAKEFILE
If you really just need the compiler flags on the output and you don't want CMake to actually compile anything, you could - at least for CMake's Makefile generators - modify CMAKE_CXX_COMPILE_OBJECT and CMAKE_CXX_CREATE_SHARED_MODULE like this:
set(CMAKE_DEPFILE_FLAGS_CXX "")
set(
CMAKE_CXX_COMPILE_OBJECT
"<CMAKE_COMMAND> -E echo <FLAGS>"
)
set(
CMAKE_CXX_CREATE_SHARED_MODULE
"<CMAKE_COMMAND> -E echo <CMAKE_SHARED_MODULE_CXX_FLAGS> <LINK_FLAGS> <CMAKE_SHARED_MODULE_CREATE_CXX_FLAGS>"
)
file(WRITE mysrc.cpp "")
add_library(mylib MODULE mysrc.cpp)
References
Is Cmake set variable recursive?
What does the "c" in cmake stand for?
How to use CMAKE_EXPORT_COMPILE_COMMANDS?
Using CMake with GNU Make: How can I see the exact commands?
Retrieve all link flags in CMake
I've been searching a portable way to force CMake to enable the compiler's C99 features in order to avoid the following gcc error for instance:
error: ‘for’ loop initial declarations are only allowed in C99 mode
for (int s = 1; s <= in_para->StepNumber; s++){
^
I also wouldn't like to check for which compiler and append something like:
set(CMAKE_C_FLAGS "-std=c99") # that would be bad
So I found this post: Enabling C99 in CMake and the associated feature request: 0012300: CMake has no cross-platform way to ask for C99. In this Mantis bug I learned about target_compiler_features and after that I found these SOF answers on it: How to activate C++11 in CMake? and How to detect C++11 support of a compiler with CMake.
So my questions are: this target_compiler_features will provide a way to require a C feature as well as a C++ one? What is the most portable way to achive this by now - I'm currently using CMake 2.8.12.2. The target_compiler_features isn't in CMake's most recent release version (3.0.0). Do you know when it is being released?
After creating a target such as a library or executable, put a line like this in your CMakeLists.txt file:
set_property(TARGET tgt PROPERTY C_STANDARD 99)
where tgt is the name of your target.
I think this was added in CMake 3.1, and the documentation is here:
http://www.cmake.org/cmake/help/latest/prop_tgt/C_STANDARD.html
If you need to support versions of CMake older than 3.1, you can use this macro:
macro(use_c99)
if (CMAKE_VERSION VERSION_LESS "3.1")
if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
set (CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
endif ()
else ()
set (CMAKE_C_STANDARD 99)
endif ()
endmacro(use_c99)
After putting that macro in your top-level file so it is visible everywhere, you can just write use_c99() at the top of any CMakeLists file that defines a target with C99 code in it.
CMake issue #15943 for clang users targeting macOS
If you are using CMake and clang to target MacOS there is a bug that can cause the CMAKE_C_STANDARD feature to simply not work (not add any compiler flags). Make sure that you do one of the following things:
Use cmake_minimum_required to require CMake 3.0 or later, or
Set policy CMP0025 to NEW with the following code at the top of your CMakeLists.txt file before the project command:
# Fix behavior of CMAKE_C_STANDARD when targeting macOS.
if (POLICY CMP0025)
cmake_policy(SET CMP0025 NEW)
endif ()
As this question keeps getting attention I'm summarizing here what I think are the best options today.
The following command sets C99 as a minimum requirement for target:
target_compile_features(target PUBLIC c_std_99)
I consider this the preferred way, as it is per target and exposes a way to control the visibility through the PUBLIC, INTERFACE and PRIVATE keywords - see the reference. Although the target_compile_features command was introduced on the 3.1 version, c_std_99 requires at least CMake 3.8.
Similar to the above, another way to set C99 as the standard for target is the following:
set_property(TARGET target PROPERTY C_STANDARD 99)
This is available since CMake 3.1. A possible drawback is that it doesn't enforce the standard (see the reference). For this reason setting the C_STANDARD_REQUIRED property may be useful:
set_property(TARGET target PROPERTY C_STANDARD_REQUIRED ON)
The above two properties are defaulted to the values of CMAKE_C_STANDARD and CMAKE_C_STANDARD_REQUIRED respectively.
So a possible way to make C99 default for all targets is:
set(CMAKE_C_STANDARD 99)
set(CMAKE_C_STANDARD_REQUIRED TRUE)
As a side note, expanding on the target_compile_features approach, there may be no need to require some specific language standard if all you care about is some specific feature. For instance by setting:
target_compile_features(target PUBLIC c_variadic_macros)
CMake will take care to pick the proper flags that enforce the availability of variadic macros. However currently there are only a few such features available for the C language - see CMAKE_C_KNOWN_FEATURES for the complete list - and loop initial declarations is not among them.
In libevent, add the following in CMakeLists.txt
set (CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
Edit CMakeLists
add on line > 2
set (CMAKE_C_STANDARD 99)
then
cmake 3 ..
The C_STANDARD property will allow use to apply the C standard to a specific target, rather than globally. The following will apply C99 to mytarget.
set_property(TARGET mytarget PROPERTY C_STANDARD 99)
From CMake 3.0.2, it is possible to use add_compile_options (https://cmake.org/cmake/help/latest/command/add_compile_options.html#command:add_compile_options), it is one of the most portable way I found to set C standart.
Take care to use this command before to declare target (with add_library or add_executable).
Below is a CMake script example setting C standart to C99:
add_compile_options(-std=c99)
add_executable(my_exe ${SOURCES})
Add the following to your CMakeLists.txt file and run cmake again
set(CMAKE_C_FLAGS "std=c99")