Related
I would like to use the IAR compiler. I noticed CMake has already have a bunch of files about this compiler:
https://github.com/jevinskie/cmake/blob/master/Modules/Compiler/IAR.cmake
From what I read the common solution is to specify manually ALL the toolchain in my CMakeLists.txt:
set(CMAKE_C_COMPILER iccarm)
set(CMAKE_CPP_COMPILER iccarm)
How CMake can link these definitions with `Modules/Compiler/IAR.cmake"?
I thought I would just have to do
include("Modules/Compiler/IAR.cmake")
What is the correct way to specify my IAR compiler?
When I do
cmake .
It still tries to use gcc instead of my IAR compiler. Why?
To select a specific compiler, you have several solutions, as exaplained in CMake wiki:
Method 1: use environment variables
For C and C++, set the CC and CXX environment variables. This method is not guaranteed to work for all generators. (Specifically, if you are trying to set Xcode's GCC_VERSION, this method confuses Xcode.)
For example:
CC=gcc-4.2 CXX=/usr/bin/g++-4.2 cmake -G "Your Generator" path/to/your/source
Method 2: use cmake -D
Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path on the command-line using cmake -D.
For example:
cmake -G "Your Generator" -D CMAKE_C_COMPILER=gcc-4.2 -D CMAKE_CXX_COMPILER=g++-4.2 path/to/your/source
Method 3 (avoid): use set()
Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path in a list file using set(). This must be done before any language is set (ie: before any project() or enable_language() command).
For example:
set(CMAKE_C_COMPILER "gcc-4.2")
set(CMAKE_CXX_COMPILER "/usr/bin/g++-4.2")
project("YourProjectName")
The wiki doesn't provide reason why 3rd method should be avoided...
I see more and more people who set CMAKE_C_COMPILER and other compiler-related variables in the CMakeLists.txt after the project call and wonder why this approach breaks sometimes.
What happens actually
When CMake executes the project() call, it looks for a default compiler executable and determines the way for use it: default compiler flags, default linker flags, compile features, etc.
And CMake stores path to that default compiler executable in the CMAKE_C_COMPILER variable.
When one sets CMAKE_C_COMPILER variable after the project() call, this only changes the compiler executable: default flags, features all remains set for the default compiler.
AS RESULT: When the project is built, a build system calls the project-specified compiler executable but with parameters suitable for the default compiler.
As one could guess, this approach would work only when one replaces a default compiler with a highly compatible one. E.g. replacement of gcc with clang could work sometimes.
This approach will never work for replacement of cl compiler (used in Visual Studio) with gcc one. Nor this will work when replacing a native compiler with a cross-compiler.
What to do
Never set a compiler in CMakeLists.txt.
If you want, e.g., to use clang instead of defaulted gcc, then either:
Pass -DCMAKE_C_COMPILER=<compiler> to cmake when configure the project. That way CMake will use this compiler instead of default one and on the project() call it will adjust all flags for the specified compiler.
Set CC environment variable (CXX for C++ compiler). CMake checks this variable when selects a default compiler.
(Only in rare cases) Set CMAKE_C_COMPILER variable before the project() call. This approach is similar to the first one, but makes the project less flexible.
If the ways above do not work
If on setting CMAKE_C_COMPILER in the command line CMake errors that a compiler cannot "compile a simple project", then something wrong in your environment.. or you specify a compiler incompatible for chosen generator or platform.
Examples:
Visual Studio generators work with cl compiler but cannot work with gcc.
A MinGW compiler usually requires MinGW Makefiles generator.
Incompatible generator cannot be fixed in CMakeLists.txt. One need to pass the proper -G option to the cmake executable (or select the proper generator in CMake GUI).
Cross-compiling
Cross-compiling usually requires setting CMAKE_SYSTEM_NAME variable, and this setting should normally be done in the toolchain file. That toolchain file is also responsible for set a compiler.
Setting CMAKE_SYSTEM_NAME in the CMakeLists.txt is almost always an error.
You need to create a toolchain file, and use the CmakeForceCompiler module.
Here is an example toolchain file for bare-metal ARM development with IAR:
include(CMakeForceCompiler)
set(CMAKE_SYSTEM_NAME Generic) # Or name of your OS if you have one
set(CMAKE_SYSTEM_PROCESSOR arm) # Or whatever
set(CMAKE_CROSSCOMPILING 1)
set(CMAKE_C_COMPILER iccarm) # Change the arm suffix if appropriate
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY) # Required to make the previous line work for a target that requires a custom linker file
The last line is necessary because CMake will try to compile a test program with the compiler to make sure it works and to get some version information from preprocessor defines. Without this line, CMake will use add_executable() for the test program, and you will get the error "The C compiler "XXX" is not able to compile a simple test program." This is because the test program fails to link, as it doesn't have your custom linker file (I'm assuming bare-metal development since this is what IAR is usually used for). This line tells CMake to use add_library() instead, which makes the test succeed without the linker file. Source of this workaround: this CMake mailing list post.
Then, assuming that your toolchain file is named iar-toolchain.cmake, invoke CMake like this:
cmake -DCMAKE_TOOLCHAIN_FILE=iar-toolchain.cmake .
You can call cmake like this:
cmake -DCMAKE_C_COMPILER=iccarm ...
or
cmake -DCMAKE_CXX_COMPILER=...
If you don't want to use your PC's standard compiler, you have to give CMake the path to the compiler. You do this via environment variables, a toolchain file or direct definitions in the CMake command line (see e.g. CMake Error at CMakeLists.txt:30 (project): No CMAKE_C_COMPILER could be found).
Putting the compiler's name/path into your CMakeLists.txt would stop your project from being cross-platform.
CMake does check for the compiler ids by compiling special C/C++ files. So no need to manually include from Module/Compiler or Module/Platform.
This will be automatically done by CMake based on its compiler and platform checks.
References
CMake: In which Order are Files parsed (Cache, Toolchain, …)?
CMake GitLab Commit: Add support files for C, C++ and ASM for the IAR toolchain.
IAR Systems recently published a basic CMake tutorial with examples under their GitHub profile.
I like the the idea of a generic toolchain file which works seamlessly for both Windows and Linux compilers using find_program().
The following snippet will be used for when using C and can be used similarly for CXX:
# IAR C Compiler
find_program(CMAKE_C_COMPILER
NAMES icc${CMAKE_SYSTEM_PROCESSOR}
PATHS ${TOOLKIT}
"$ENV{ProgramFiles}/IAR Systems/*"
"$ENV{ProgramFiles\(x86\)}/IAR Systems/*"
/opt/iarsystems/bx${CMAKE_SYSTEM_PROCESSOR}
PATH_SUFFIXES bin ${CMAKE_SYSTEM_PROCESSOR}/bin
REQUIRED )
For ASM, I initially got puzzled with the NAMES but then I realized that the toolchain file was made that way for working with old Assemblers shipped with XLINK:
find_program(CMAKE_ASM_COMPILER
NAMES iasm${CMAKE_SYSTEM_PROCESSOR} a${CMAKE_SYSTEM_PROCESSOR}
PATHS ${TOOLKIT}
"$ENV{PROGRAMFILES}/IAR Systems/*"
"$ENV{ProgramFiles\(x86\)}/IAR Systems/*"
/opt/iarsystems/bx${CMAKE_SYSTEM_PROCESSOR}
PATH_SUFFIXES bin ${CMAKE_SYSTEM_PROCESSOR}/bin
REQUIRED )
Also, take a look at the full toolchain file. It will work automatically for "Arm" when the tools are installed on their default locations, otherwise it is just about updating the TOOLKIT variable and the compilers for all the supported languages should adjust automatically.
If your wanting to specify a compiler in cmake then just do ...
cmake_minimum_required(VERSION 3.22)
set(CMAKE_C_COMPILER "clang")
set(CMAKE_CXX_COMPILER "clang++")
Options 1 is only used if you want to specify what compiler you want to use as default for everything that you might compile on your computer. And I don't even think it would work on windows.
Option 2 would be used if you only want to use a different temporarily.
Option 3 is used if that's the compiler that should be used for that particular project. Also option 3 would be the most cross compatible.
I want to setup a custom toolchain with CMake. I've set the compiler but I don't know how to set the linker. This error is reported because CMake try to use the compiler to link:
The C compiler "xgcc.exe" is not able to compile a simple test program.
Here there is a snippet of my toolchain file
# specify the cross compiler
INCLUDE(CMakeForceCompiler)
SET(CMAKE_C_COMPILER xgcc.exe)
SET(CMAKE_CXX_COMPILER xgcc.exe)
#CMAKE_FORCE_C_COMPILER(xgcc.exe GNU)
#CMAKE_FORCE_CXX_COMPILER(xgcc.exe GNU)
I've tried to force the compiler but the linker problem will not be solved.
The link command line is set in Modules/CMake{C,CXX,Fortran}Information.cmake and defaults to using the compiler, not CMAKE_LINKER (see source code). This can be changed by replacing the rule that builds the link command line, which lives in variables CMAKE_CXX_LINK_EXECUTABLE (and friends). NB that variable does not indicate the path to the linker executable; it says how to link an executable!
One approach is to set that rule to use the linker, e.g.
cmake -DCMAKE_LINKER=/path/to/linker -DCMAKE_CXX_LINK_EXECUTABLE="<CMAKE_LINKER> <FLAGS> <CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>"
See also this post from CMake mailing list and this one - this also makes a natural place to prepend a linker modifier to another linker.
As Mabraham points out, CMake calls the compiler to do the linking. So, by far the simplest solution to this is to LET IT, and instead tell the compiler to run a different linker when called.
Which, as noted in this other answer — but now it's even a documented option in gcc --help=common — is as easy as:
cmake -DCMAKE_CXX_FLAGS="-fuse-ld=lld"
g++ or clang++ will get passed the -fuse-ld=lld1 flag on every call, and when they do any linking they'll use the specified command instead of the built-in default. Easy-peasy, and CMake need not concern itself with such things at all.
(BTW, the option is parsed (-f) (use-ld) (=) (lld), there's no "fuse" option to gcc.)
Notes
When using Clang, lld can be replaced with whatever other linker command you want to use, like ld.exe, ld.gold, mingw32/bin/ld.exe, etc.
GCC isn't as flexible, its -fuse-ld only accepts one of three possible arguments: lld, bfd, or gold. It will invoke the first matching ld.foo executable it finds on the PATH. (Thanks to bviktor for pointing out GCC's limitations for alternate linker selection.)
CMake only gives you direct control over the compiler for each language. To call the linker, it goes through the configured compiler. This means that there is no universal way to set the linker in CMake, you must configure your compiler to use the linker you intend.
Such flags need to be set before CMake's compiler detection routines run because it will try to compile a test binary. The best way to do this is by creating a toolchain file. The best way to set these flags in the toolchain file is like so:
# e.g. to use lld with Clang
set(CMAKE_EXE_LINKER_FLAGS_INIT "-fuse-ld=lld")
set(CMAKE_MODULE_LINKER_FLAGS_INIT "-fuse-ld=lld")
set(CMAKE_SHARED_LINKER_FLAGS_INIT "-fuse-ld=lld")
These three variables control the (default) set of linker flags for executables, loadable modules, and shared libraries, respectively. There is no need to handle CMAKE_STATIC_LINKER_FLAGS_INIT (for static libraries) here because the archiver is invoked, rather than the linker.
You can then set the toolchain file when you first run CMake by setting -DCMAKE_TOOLCHAIN_FILE=/path/to/toolchain.cmake at the command line. As of CMake 3.21, you will be able to pass --toolchain /path/to/toolchain.cmake instead (which is entirely equivalent, but a little less typing).
Set the variable ${CMAKE_LINKER} either in CMakeCache.txt or after ccmake . under advanced options.
I have to use CMAKE_CXX_LINK_EXECUTABLE, CMAKE_C_LINK_EXECUTABLE variable:
SET(CMAKE_C_LINK_EXECUTABLE "c:\\MoSync\\bin\\pipe-tool.exe")
I had success with doing
add_link_options("-fuse-ld=lld")
It is a variation on the previous answers here. The difference is the CMake command I use to set the flag.
Adding it to CMAKE_CXX_FLAGS has the disadvantage of then also having to add -Wno-unused-command-line-argument as the flags get also added to compilation commands, not only to linking ones.
The disadvantage of CMAKE_SHARED_LINKER_FLAGS is that you have to add it multiple times, to _SHARED_, _EXE_, and maybe I forgot something.
Here is a CMake function which sets linker based on some predefined arbitrary rules (Clang -> lld-version or lld, GCC -> gold).
The important parts:
Search for lld-version which matches the Clang compiler version (ex. lld-13 if Clang 13.x.x is used), falls back to lld if not found
add_link_options("-fuse-ld=lld-${CLANG_VERSION_MAJOR}")
Use all system threads when linker is set to gold:
add_link_options("-fuse-ld=gold;LINKER:--threads,--thread-count=${HOST_PROC_COUNT}")
The example is a bit too long because of comments, logs and custom logic, but it is self-contained and could be useful staring point for beginners.
function(select_best_linker) #lld for Clang and GNU gold for GCC
if (UNIX AND NOT APPLE)
include(ProcessorCount)
ProcessorCount(HOST_PROC_COUNT)
if(${CMAKE_CXX_COMPILER_ID} MATCHES Clang)
# By default LLD uses all system threads.
# This could be tweaked for versions 11+ (--threads=1), but cannot be disabled for older versions
# add_link_options("-fuse-ld=lld-${CLANG_VERSION_MAJOR};LINKER:--threads=${HOST_PROC_COUNT}") #LLD>=11
# add_link_options("-fuse-ld=lld;LINKER:--threads")#LLD <= 10 this is the default state
string(REPLACE "." ";" VERSION_LIST ${CMAKE_CXX_COMPILER_VERSION})
list(GET VERSION_LIST 0 CLANG_VERSION_MAJOR) #extract major compiler version
find_program(LLD_PROGRAM_MATCH_VER lld-${CLANG_VERSION_MAJOR}) #search for lld-13 when clang 13.x.x is used
find_program(LLD_PROGRAM lld) #else find default lld
if (LLD_PROGRAM_MATCH_VER) #lld matching compiler version
message(STATUS "Set linker to LLD (multi-threaded): ${LLD_PROGRAM_MATCH_VER}")
add_link_options("-fuse-ld=lld-${CLANG_VERSION_MAJOR}")
elseif(LLD_PROGRAM) #default lld
message(STATUS "Set linker to LLD (multi-threaded): ${LLD_PROGRAM}")
add_link_options("-fuse-ld=lld")
endif(LLD_PROGRAM_MATCH_VER)
elseif(${CMAKE_CXX_COMPILER_ID} MATCHES GNU)
find_program(GNU_GOLD_PROGRAM gold)
if (GNU_GOLD_PROGRAM)
message(STATUS "Set linker to GNU gold: ${GNU_GOLD_PROGRAM}, using threads: ${HOST_PROC_COUNT}")
add_link_options("-fuse-ld=gold;LINKER:--threads,--thread-count=${HOST_PROC_COUNT}")
endif(GNU_GOLD_PROGRAM)
endif(${CMAKE_CXX_COMPILER_ID} MATCHES Clang)
endif(UNIX AND NOT APPLE)
endfunction(select_best_linker)
Tested on:
Ubuntu 20.04
CMake 3.16.3
GCC 9.4.0
Clang-12
Clang-13
GNU gold (GNU Binutils 2.37) 1.16
LLD 10.0.0 (compatible with GNU linkers)
Ubuntu LLD 13.0.1 (compatible with GNU linkers)
For completeness, another full-proof option is to just link /usr/bin/ld to ld.gold by running
sudo ln -sf /usr/bin/x86_64-linux-gnu-ld.gold /usr/bin/ld
as suggested here
There is another way to do it, gcc has a "-fuse-ld" option, you can set LINKER_FLAGS in CMakeLists.txt like these:
set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -fuse-ld=lld")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fuse-ld=lld")
then the custom specified linker should be invoked.
I would like to use the IAR compiler. I noticed CMake has already have a bunch of files about this compiler:
https://github.com/jevinskie/cmake/blob/master/Modules/Compiler/IAR.cmake
From what I read the common solution is to specify manually ALL the toolchain in my CMakeLists.txt:
set(CMAKE_C_COMPILER iccarm)
set(CMAKE_CPP_COMPILER iccarm)
How CMake can link these definitions with `Modules/Compiler/IAR.cmake"?
I thought I would just have to do
include("Modules/Compiler/IAR.cmake")
What is the correct way to specify my IAR compiler?
When I do
cmake .
It still tries to use gcc instead of my IAR compiler. Why?
To select a specific compiler, you have several solutions, as exaplained in CMake wiki:
Method 1: use environment variables
For C and C++, set the CC and CXX environment variables. This method is not guaranteed to work for all generators. (Specifically, if you are trying to set Xcode's GCC_VERSION, this method confuses Xcode.)
For example:
CC=gcc-4.2 CXX=/usr/bin/g++-4.2 cmake -G "Your Generator" path/to/your/source
Method 2: use cmake -D
Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path on the command-line using cmake -D.
For example:
cmake -G "Your Generator" -D CMAKE_C_COMPILER=gcc-4.2 -D CMAKE_CXX_COMPILER=g++-4.2 path/to/your/source
Method 3 (avoid): use set()
Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path in a list file using set(). This must be done before any language is set (ie: before any project() or enable_language() command).
For example:
set(CMAKE_C_COMPILER "gcc-4.2")
set(CMAKE_CXX_COMPILER "/usr/bin/g++-4.2")
project("YourProjectName")
The wiki doesn't provide reason why 3rd method should be avoided...
I see more and more people who set CMAKE_C_COMPILER and other compiler-related variables in the CMakeLists.txt after the project call and wonder why this approach breaks sometimes.
What happens actually
When CMake executes the project() call, it looks for a default compiler executable and determines the way for use it: default compiler flags, default linker flags, compile features, etc.
And CMake stores path to that default compiler executable in the CMAKE_C_COMPILER variable.
When one sets CMAKE_C_COMPILER variable after the project() call, this only changes the compiler executable: default flags, features all remains set for the default compiler.
AS RESULT: When the project is built, a build system calls the project-specified compiler executable but with parameters suitable for the default compiler.
As one could guess, this approach would work only when one replaces a default compiler with a highly compatible one. E.g. replacement of gcc with clang could work sometimes.
This approach will never work for replacement of cl compiler (used in Visual Studio) with gcc one. Nor this will work when replacing a native compiler with a cross-compiler.
What to do
Never set a compiler in CMakeLists.txt.
If you want, e.g., to use clang instead of defaulted gcc, then either:
Pass -DCMAKE_C_COMPILER=<compiler> to cmake when configure the project. That way CMake will use this compiler instead of default one and on the project() call it will adjust all flags for the specified compiler.
Set CC environment variable (CXX for C++ compiler). CMake checks this variable when selects a default compiler.
(Only in rare cases) Set CMAKE_C_COMPILER variable before the project() call. This approach is similar to the first one, but makes the project less flexible.
If the ways above do not work
If on setting CMAKE_C_COMPILER in the command line CMake errors that a compiler cannot "compile a simple project", then something wrong in your environment.. or you specify a compiler incompatible for chosen generator or platform.
Examples:
Visual Studio generators work with cl compiler but cannot work with gcc.
A MinGW compiler usually requires MinGW Makefiles generator.
Incompatible generator cannot be fixed in CMakeLists.txt. One need to pass the proper -G option to the cmake executable (or select the proper generator in CMake GUI).
Cross-compiling
Cross-compiling usually requires setting CMAKE_SYSTEM_NAME variable, and this setting should normally be done in the toolchain file. That toolchain file is also responsible for set a compiler.
Setting CMAKE_SYSTEM_NAME in the CMakeLists.txt is almost always an error.
You need to create a toolchain file, and use the CmakeForceCompiler module.
Here is an example toolchain file for bare-metal ARM development with IAR:
include(CMakeForceCompiler)
set(CMAKE_SYSTEM_NAME Generic) # Or name of your OS if you have one
set(CMAKE_SYSTEM_PROCESSOR arm) # Or whatever
set(CMAKE_CROSSCOMPILING 1)
set(CMAKE_C_COMPILER iccarm) # Change the arm suffix if appropriate
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY) # Required to make the previous line work for a target that requires a custom linker file
The last line is necessary because CMake will try to compile a test program with the compiler to make sure it works and to get some version information from preprocessor defines. Without this line, CMake will use add_executable() for the test program, and you will get the error "The C compiler "XXX" is not able to compile a simple test program." This is because the test program fails to link, as it doesn't have your custom linker file (I'm assuming bare-metal development since this is what IAR is usually used for). This line tells CMake to use add_library() instead, which makes the test succeed without the linker file. Source of this workaround: this CMake mailing list post.
Then, assuming that your toolchain file is named iar-toolchain.cmake, invoke CMake like this:
cmake -DCMAKE_TOOLCHAIN_FILE=iar-toolchain.cmake .
You can call cmake like this:
cmake -DCMAKE_C_COMPILER=iccarm ...
or
cmake -DCMAKE_CXX_COMPILER=...
If you don't want to use your PC's standard compiler, you have to give CMake the path to the compiler. You do this via environment variables, a toolchain file or direct definitions in the CMake command line (see e.g. CMake Error at CMakeLists.txt:30 (project): No CMAKE_C_COMPILER could be found).
Putting the compiler's name/path into your CMakeLists.txt would stop your project from being cross-platform.
CMake does check for the compiler ids by compiling special C/C++ files. So no need to manually include from Module/Compiler or Module/Platform.
This will be automatically done by CMake based on its compiler and platform checks.
References
CMake: In which Order are Files parsed (Cache, Toolchain, …)?
CMake GitLab Commit: Add support files for C, C++ and ASM for the IAR toolchain.
IAR Systems recently published a basic CMake tutorial with examples under their GitHub profile.
I like the the idea of a generic toolchain file which works seamlessly for both Windows and Linux compilers using find_program().
The following snippet will be used for when using C and can be used similarly for CXX:
# IAR C Compiler
find_program(CMAKE_C_COMPILER
NAMES icc${CMAKE_SYSTEM_PROCESSOR}
PATHS ${TOOLKIT}
"$ENV{ProgramFiles}/IAR Systems/*"
"$ENV{ProgramFiles\(x86\)}/IAR Systems/*"
/opt/iarsystems/bx${CMAKE_SYSTEM_PROCESSOR}
PATH_SUFFIXES bin ${CMAKE_SYSTEM_PROCESSOR}/bin
REQUIRED )
For ASM, I initially got puzzled with the NAMES but then I realized that the toolchain file was made that way for working with old Assemblers shipped with XLINK:
find_program(CMAKE_ASM_COMPILER
NAMES iasm${CMAKE_SYSTEM_PROCESSOR} a${CMAKE_SYSTEM_PROCESSOR}
PATHS ${TOOLKIT}
"$ENV{PROGRAMFILES}/IAR Systems/*"
"$ENV{ProgramFiles\(x86\)}/IAR Systems/*"
/opt/iarsystems/bx${CMAKE_SYSTEM_PROCESSOR}
PATH_SUFFIXES bin ${CMAKE_SYSTEM_PROCESSOR}/bin
REQUIRED )
Also, take a look at the full toolchain file. It will work automatically for "Arm" when the tools are installed on their default locations, otherwise it is just about updating the TOOLKIT variable and the compilers for all the supported languages should adjust automatically.
If your wanting to specify a compiler in cmake then just do ...
cmake_minimum_required(VERSION 3.22)
set(CMAKE_C_COMPILER "clang")
set(CMAKE_CXX_COMPILER "clang++")
Options 1 is only used if you want to specify what compiler you want to use as default for everything that you might compile on your computer. And I don't even think it would work on windows.
Option 2 would be used if you only want to use a different temporarily.
Option 3 is used if that's the compiler that should be used for that particular project. Also option 3 would be the most cross compatible.
I'm currently working on using cmake to build some projects, with the main platforms being Visual C++, MinGW GCC and Linux GCC. When building with GCC, I need to specify the -Wno-invalid-offsetof compiler option.
My current fix is as follows...
if ( "${CMAKE_GENERATOR}" MATCHES "^Visual Studio"
OR "${CMAKE_GENERATOR}" MATCHES "^NMake"
)
set (CPPLIB_COMPILER_OPTS "")
else ()
set (CPPLIB_COMPILER_OPTS "-Wno-invalid-offsetof")
endif ()
...
set_target_properties(sh_core PROPERTIES COMPILE_FLAGS "${CPPLIB_COMPILER_OPTS}")
# repeated for all targets
This works, but assuming that all generators other than the visual studio ones will build with gcc is obviously unsafe. For a start, there are IIRC generators for Borland compilers. More importantly, using make doesn't always mean using gcc.
Other compilers I'm likely to use are llvm-gcc and clang. Fortunately, I think even clang supports gcc-compatible options. But this logic is only good for as long as the relevant code is never released.
Cmake appears to check for available compilers and generate a makefile specifically for that compiler (raising the question - why not at least have the option of building the project directly, without the need for a middle-man like make?).
That being the case, I was hoping to be able to test directly for gcc in my CMakeLists.txt files. So far, though, I can't find an appropriate variable to test or any other obvious solution.
Is this possible?
To create a portable build system, it is best to not test for platforms, but to test for features.
Instead of testing "if Windows then do this", test for "if the -Wno-invalid-offsetof flag works then use it". You can do that with the CheckCCompilerFlag module, for example:
include(CheckCCompilerFlag)
check_c_compiler_flag(-Wno-invalid-offsetof HAS_NO_INVALID_OFFSETOF)
if (HAS_NO_INVALID_OFFSETOF)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-invalid-offsetof")
endif()
For C++ there is a similar CheckCXXCompilerFlag with a check_cxx_compiler_flag(flag var) command.
I want to setup a custom toolchain with CMake. I've set the compiler but I don't know how to set the linker. This error is reported because CMake try to use the compiler to link:
The C compiler "xgcc.exe" is not able to compile a simple test program.
Here there is a snippet of my toolchain file
# specify the cross compiler
INCLUDE(CMakeForceCompiler)
SET(CMAKE_C_COMPILER xgcc.exe)
SET(CMAKE_CXX_COMPILER xgcc.exe)
#CMAKE_FORCE_C_COMPILER(xgcc.exe GNU)
#CMAKE_FORCE_CXX_COMPILER(xgcc.exe GNU)
I've tried to force the compiler but the linker problem will not be solved.
The link command line is set in Modules/CMake{C,CXX,Fortran}Information.cmake and defaults to using the compiler, not CMAKE_LINKER (see source code). This can be changed by replacing the rule that builds the link command line, which lives in variables CMAKE_CXX_LINK_EXECUTABLE (and friends). NB that variable does not indicate the path to the linker executable; it says how to link an executable!
One approach is to set that rule to use the linker, e.g.
cmake -DCMAKE_LINKER=/path/to/linker -DCMAKE_CXX_LINK_EXECUTABLE="<CMAKE_LINKER> <FLAGS> <CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>"
See also this post from CMake mailing list and this one - this also makes a natural place to prepend a linker modifier to another linker.
As Mabraham points out, CMake calls the compiler to do the linking. So, by far the simplest solution to this is to LET IT, and instead tell the compiler to run a different linker when called.
Which, as noted in this other answer — but now it's even a documented option in gcc --help=common — is as easy as:
cmake -DCMAKE_CXX_FLAGS="-fuse-ld=lld"
g++ or clang++ will get passed the -fuse-ld=lld1 flag on every call, and when they do any linking they'll use the specified command instead of the built-in default. Easy-peasy, and CMake need not concern itself with such things at all.
(BTW, the option is parsed (-f) (use-ld) (=) (lld), there's no "fuse" option to gcc.)
Notes
When using Clang, lld can be replaced with whatever other linker command you want to use, like ld.exe, ld.gold, mingw32/bin/ld.exe, etc.
GCC isn't as flexible, its -fuse-ld only accepts one of three possible arguments: lld, bfd, or gold. It will invoke the first matching ld.foo executable it finds on the PATH. (Thanks to bviktor for pointing out GCC's limitations for alternate linker selection.)
CMake only gives you direct control over the compiler for each language. To call the linker, it goes through the configured compiler. This means that there is no universal way to set the linker in CMake, you must configure your compiler to use the linker you intend.
Such flags need to be set before CMake's compiler detection routines run because it will try to compile a test binary. The best way to do this is by creating a toolchain file. The best way to set these flags in the toolchain file is like so:
# e.g. to use lld with Clang
set(CMAKE_EXE_LINKER_FLAGS_INIT "-fuse-ld=lld")
set(CMAKE_MODULE_LINKER_FLAGS_INIT "-fuse-ld=lld")
set(CMAKE_SHARED_LINKER_FLAGS_INIT "-fuse-ld=lld")
These three variables control the (default) set of linker flags for executables, loadable modules, and shared libraries, respectively. There is no need to handle CMAKE_STATIC_LINKER_FLAGS_INIT (for static libraries) here because the archiver is invoked, rather than the linker.
You can then set the toolchain file when you first run CMake by setting -DCMAKE_TOOLCHAIN_FILE=/path/to/toolchain.cmake at the command line. As of CMake 3.21, you will be able to pass --toolchain /path/to/toolchain.cmake instead (which is entirely equivalent, but a little less typing).
Set the variable ${CMAKE_LINKER} either in CMakeCache.txt or after ccmake . under advanced options.
I have to use CMAKE_CXX_LINK_EXECUTABLE, CMAKE_C_LINK_EXECUTABLE variable:
SET(CMAKE_C_LINK_EXECUTABLE "c:\\MoSync\\bin\\pipe-tool.exe")
I had success with doing
add_link_options("-fuse-ld=lld")
It is a variation on the previous answers here. The difference is the CMake command I use to set the flag.
Adding it to CMAKE_CXX_FLAGS has the disadvantage of then also having to add -Wno-unused-command-line-argument as the flags get also added to compilation commands, not only to linking ones.
The disadvantage of CMAKE_SHARED_LINKER_FLAGS is that you have to add it multiple times, to _SHARED_, _EXE_, and maybe I forgot something.
Here is a CMake function which sets linker based on some predefined arbitrary rules (Clang -> lld-version or lld, GCC -> gold).
The important parts:
Search for lld-version which matches the Clang compiler version (ex. lld-13 if Clang 13.x.x is used), falls back to lld if not found
add_link_options("-fuse-ld=lld-${CLANG_VERSION_MAJOR}")
Use all system threads when linker is set to gold:
add_link_options("-fuse-ld=gold;LINKER:--threads,--thread-count=${HOST_PROC_COUNT}")
The example is a bit too long because of comments, logs and custom logic, but it is self-contained and could be useful staring point for beginners.
function(select_best_linker) #lld for Clang and GNU gold for GCC
if (UNIX AND NOT APPLE)
include(ProcessorCount)
ProcessorCount(HOST_PROC_COUNT)
if(${CMAKE_CXX_COMPILER_ID} MATCHES Clang)
# By default LLD uses all system threads.
# This could be tweaked for versions 11+ (--threads=1), but cannot be disabled for older versions
# add_link_options("-fuse-ld=lld-${CLANG_VERSION_MAJOR};LINKER:--threads=${HOST_PROC_COUNT}") #LLD>=11
# add_link_options("-fuse-ld=lld;LINKER:--threads")#LLD <= 10 this is the default state
string(REPLACE "." ";" VERSION_LIST ${CMAKE_CXX_COMPILER_VERSION})
list(GET VERSION_LIST 0 CLANG_VERSION_MAJOR) #extract major compiler version
find_program(LLD_PROGRAM_MATCH_VER lld-${CLANG_VERSION_MAJOR}) #search for lld-13 when clang 13.x.x is used
find_program(LLD_PROGRAM lld) #else find default lld
if (LLD_PROGRAM_MATCH_VER) #lld matching compiler version
message(STATUS "Set linker to LLD (multi-threaded): ${LLD_PROGRAM_MATCH_VER}")
add_link_options("-fuse-ld=lld-${CLANG_VERSION_MAJOR}")
elseif(LLD_PROGRAM) #default lld
message(STATUS "Set linker to LLD (multi-threaded): ${LLD_PROGRAM}")
add_link_options("-fuse-ld=lld")
endif(LLD_PROGRAM_MATCH_VER)
elseif(${CMAKE_CXX_COMPILER_ID} MATCHES GNU)
find_program(GNU_GOLD_PROGRAM gold)
if (GNU_GOLD_PROGRAM)
message(STATUS "Set linker to GNU gold: ${GNU_GOLD_PROGRAM}, using threads: ${HOST_PROC_COUNT}")
add_link_options("-fuse-ld=gold;LINKER:--threads,--thread-count=${HOST_PROC_COUNT}")
endif(GNU_GOLD_PROGRAM)
endif(${CMAKE_CXX_COMPILER_ID} MATCHES Clang)
endif(UNIX AND NOT APPLE)
endfunction(select_best_linker)
Tested on:
Ubuntu 20.04
CMake 3.16.3
GCC 9.4.0
Clang-12
Clang-13
GNU gold (GNU Binutils 2.37) 1.16
LLD 10.0.0 (compatible with GNU linkers)
Ubuntu LLD 13.0.1 (compatible with GNU linkers)
For completeness, another full-proof option is to just link /usr/bin/ld to ld.gold by running
sudo ln -sf /usr/bin/x86_64-linux-gnu-ld.gold /usr/bin/ld
as suggested here
There is another way to do it, gcc has a "-fuse-ld" option, you can set LINKER_FLAGS in CMakeLists.txt like these:
set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -fuse-ld=lld")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fuse-ld=lld")
then the custom specified linker should be invoked.