I'm trying to port our project from autotools to CMake and got stuck on fortran compiler settings. The project consists of C++, Fortran and Fortran90 pieces. In our autoconf/automake project we have simply defined FC and F77 together with corresponding flags like FCFLAGS and FFLAGS. But in CMake it seems to work a bit differently.
What I did so far:
project (blah-proj CXX Fortran)
... in ibm case ....
set (CMAKE_Fortran_FLAGS "-qfullpath -qinitauto -qnolm -qinit=f90ptr -qfree=f90 " )
and in the source directory CMakeLists.txt:
file (GLOB_RECURSE libblah_f_sources *.f90 *.f)
add_library (blah_f ${libblah_f_sources})
set_target_properties (blah_f PROPERTIES LINKER_LANGUAGE Fortran)
With gfortran on linux it works fine as it's only one binary, but on AIX there are 2 different compilers called xlf and xlf90. Of course it doesn't work as CMake tries to compile both *.f and *.f90 files with same compiler.
It doesn't seem I got what I really wanted, but at least it solved the problem at the first approximation. I kept same structure but then I let compiler itself decide what kind of file it is. Of course I had to remove the version-dependent directives like -qfree=f90 or -qfixed=128 in the hope that compiler is smart enough to guess them from the file extension.
Related
I have never worked with CMake before, so please forgive any rookie mistakes. Most of the following working frame has been given to me by my project group.
The goal is to build GoogleTest into a .dll, to be used in different, indepentent parts of our project. I'm having troubles setting up CMake the right way.
The work-flow so far has been:
Clone gtest from git --> also downloads a CMake List file
Alter variables in CMakeCache.txt to have it produce a Code::Blocks project file
Compile the project file in Code::Blocks
So far, it produces a static library (.a files) that can be used in our project. I'm having troubles genereating .dll files.
Variables I have tried changing:
BUILD_SHARED_LIBS:BOOL=ON --> the files generated by Code::Blocks now have a .dll.a double extension
CMAKE_C_FLAGS and all the corresponding C++ flags where set to -DGTEST_CREATE_SHARED_LIBRARY=1 as given here
CMAKE_EXE_LINKER_FLAGS has been set to -shared to make the linker produce .dll files
I have worked my way through the GoogleTest documentation here and here but in both, building it into a .dll is merely a 2-sentence-topic.
As #Tsyvarev pointed out, the .dll files were created in a (very) different folder.
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 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.
This is similar to Force CMake to use C++ compiler for C files with Visual Studio, but its not quite the same. Its not the same because a its CMake file causing the failure; and I'm working on nearly every modern platform, from BSDs and OS X through Solaris and Unix.
I tried to avoid useless checks being performed by CMake:
project(cryptopp, CXX)
Here's what happens when I attempt to generate the makefile:
$ cmake .
-- Check if the system is big endian
-- Searching 16 bit integer
-- Check size of unsigned short
CMake Error at /usr/share/cmake-2.8/Modules/CheckTypeSize.cmake:82 (try_compile):
Unknown extension ".c" for file
/home/jeffrey/cryptopp/CMakeFiles/CheckTypeSize/CMAKE_SIZEOF_UNSIGNED_SHORT.c
try_compile() works only for enabled languages. Currently these are:
CXX
See project() command to enable other languages.
Call Stack (most recent call first):
/usr/share/cmake-2.8/Modules/CheckTypeSize.cmake:167 (__check_type_size_impl)
/usr/share/cmake-2.8/Modules/TestBigEndian.cmake:27 (CHECK_TYPE_SIZE)
CMakeLists.txt:49 (TEST_BIG_ENDIAN)
We don't have any C files in our project, so we should be safe with project(cryptopp, CXX) (if I am reading cmake --help-command project correctly).
The cited question talks about project files, but not CMake files.
How do I tell CMake to use C++ compiler for all files, including its own CMake files?
I'm on Ubuntu 12 LTS, and it provides:
$ cmake --version
cmake version 2.8.7
There are ways to add .c as a valid file extension for the CXX compiler. Even this being very advanced CMake stuff, you may need - if you are bound to support older versions of CMake - a "make rules overwrite script" anyway.
So I've successfully tested the following:
CryptoppMakeRulesOverwrite.cmake
list(APPEND CMAKE_CXX_SOURCE_FILE_EXTENSIONS c)
CMakeLists.txt
cmake_minimum_required(VERSION 2.8.7 FATAL_ERROR)
set(CMAKE_USER_MAKE_RULES_OVERRIDE "CryptoppMakeRulesOverwrite.cmake")
project(cryptopp CXX)
include(CheckTypeSize)
CHECK_TYPE_SIZE("unsigned short" CMAKE_SIZEOF_UNSIGNED_SHORT)
As #Tsyvarev has commented check_type_size() supports a LANGUAGE parameter, but unfortunately not for CMake version 2.8.7. But this older version does already support CMAKE_USER_MAKE_RULES_OVERRIDE.
So I'm still wondering if not the best solution would be to go to a newer version of CMake (forcing some users of older CMake versions to upgrade). Or writing your own try_compile() snippets.
References
How to add in a CMake project a global file extension (*.pde) to GCC which is treated like C++ code
Change default value of CMAKE_CXX_FLAGS_DEBUG and friends in CMake
set_source_files_properties
The CMake setting of (my) choice here would be the set_source_files_properties command. https://cmake.org/cmake/help/latest/command/set_source_files_properties.html
set(qpid_dispatch_SOURCES
alloc.c
alloc_pool.c
aprintf.c
amqp.c
atomic.c
# [...]
)
set_source_files_properties(${qpid_dispatch_SOURCES} PROPERTIES LANGUAGE CXX)
add_library(qpid-dispatch OBJECT ${qpid_dispatch_SOURCES})
As described in the linked docs, CMake 3.18 changed the scoped effect of set_source_files_properties. See the DIRECTORY and TARGET_DIRECTORY options. Therefore, to apply source file property recursively to all files in your project, your CMakeLists.txt should look something like this
cmake_minimum_required(VERSION 3.20)
project(qpid-dispatch LANGUAGES C CXX)
# [...]
add_subdirectory(src)
add_subdirectory(tests)
add_subdirectory(router)
# [...]
file(GLOB_RECURSE CFILES "*.c")
set_source_files_properties(${CFILES}
DIRECTORY src tests router
PROPERTIES LANGUAGE CXX)
Copy the world
There is another approach that I've heard can be made to work: in your CMakeLists.txt, recursively copy all *.c files into *.cpp files and declare separate CMake targets to compile the copies. This has the advantage of allowing you to produce C-compiled and CXX-compiled artifacts at the same time, should you need anything like that.
Another braindead way of doing this (I just tried the set_source_files_properties() route and it just didn't even try to compile the files).
Rather than figure out the rats nest of cmake stuff I'm saddled with, I just added a C++ file that #includes the .c file, and added the C++ file to the CMakeLists.txt sources.
Braindead and stupid, but I'm having to deal with horrific code I'm importing. C header files without extern "C" in them, being used by C++ files.
This is similar to Force CMake to use C++ compiler for C files with Visual Studio, but its not quite the same. Its not the same because a its CMake file causing the failure; and I'm working on nearly every modern platform, from BSDs and OS X through Solaris and Unix.
I tried to avoid useless checks being performed by CMake:
project(cryptopp, CXX)
Here's what happens when I attempt to generate the makefile:
$ cmake .
-- Check if the system is big endian
-- Searching 16 bit integer
-- Check size of unsigned short
CMake Error at /usr/share/cmake-2.8/Modules/CheckTypeSize.cmake:82 (try_compile):
Unknown extension ".c" for file
/home/jeffrey/cryptopp/CMakeFiles/CheckTypeSize/CMAKE_SIZEOF_UNSIGNED_SHORT.c
try_compile() works only for enabled languages. Currently these are:
CXX
See project() command to enable other languages.
Call Stack (most recent call first):
/usr/share/cmake-2.8/Modules/CheckTypeSize.cmake:167 (__check_type_size_impl)
/usr/share/cmake-2.8/Modules/TestBigEndian.cmake:27 (CHECK_TYPE_SIZE)
CMakeLists.txt:49 (TEST_BIG_ENDIAN)
We don't have any C files in our project, so we should be safe with project(cryptopp, CXX) (if I am reading cmake --help-command project correctly).
The cited question talks about project files, but not CMake files.
How do I tell CMake to use C++ compiler for all files, including its own CMake files?
I'm on Ubuntu 12 LTS, and it provides:
$ cmake --version
cmake version 2.8.7
There are ways to add .c as a valid file extension for the CXX compiler. Even this being very advanced CMake stuff, you may need - if you are bound to support older versions of CMake - a "make rules overwrite script" anyway.
So I've successfully tested the following:
CryptoppMakeRulesOverwrite.cmake
list(APPEND CMAKE_CXX_SOURCE_FILE_EXTENSIONS c)
CMakeLists.txt
cmake_minimum_required(VERSION 2.8.7 FATAL_ERROR)
set(CMAKE_USER_MAKE_RULES_OVERRIDE "CryptoppMakeRulesOverwrite.cmake")
project(cryptopp CXX)
include(CheckTypeSize)
CHECK_TYPE_SIZE("unsigned short" CMAKE_SIZEOF_UNSIGNED_SHORT)
As #Tsyvarev has commented check_type_size() supports a LANGUAGE parameter, but unfortunately not for CMake version 2.8.7. But this older version does already support CMAKE_USER_MAKE_RULES_OVERRIDE.
So I'm still wondering if not the best solution would be to go to a newer version of CMake (forcing some users of older CMake versions to upgrade). Or writing your own try_compile() snippets.
References
How to add in a CMake project a global file extension (*.pde) to GCC which is treated like C++ code
Change default value of CMAKE_CXX_FLAGS_DEBUG and friends in CMake
set_source_files_properties
The CMake setting of (my) choice here would be the set_source_files_properties command. https://cmake.org/cmake/help/latest/command/set_source_files_properties.html
set(qpid_dispatch_SOURCES
alloc.c
alloc_pool.c
aprintf.c
amqp.c
atomic.c
# [...]
)
set_source_files_properties(${qpid_dispatch_SOURCES} PROPERTIES LANGUAGE CXX)
add_library(qpid-dispatch OBJECT ${qpid_dispatch_SOURCES})
As described in the linked docs, CMake 3.18 changed the scoped effect of set_source_files_properties. See the DIRECTORY and TARGET_DIRECTORY options. Therefore, to apply source file property recursively to all files in your project, your CMakeLists.txt should look something like this
cmake_minimum_required(VERSION 3.20)
project(qpid-dispatch LANGUAGES C CXX)
# [...]
add_subdirectory(src)
add_subdirectory(tests)
add_subdirectory(router)
# [...]
file(GLOB_RECURSE CFILES "*.c")
set_source_files_properties(${CFILES}
DIRECTORY src tests router
PROPERTIES LANGUAGE CXX)
Copy the world
There is another approach that I've heard can be made to work: in your CMakeLists.txt, recursively copy all *.c files into *.cpp files and declare separate CMake targets to compile the copies. This has the advantage of allowing you to produce C-compiled and CXX-compiled artifacts at the same time, should you need anything like that.
Another braindead way of doing this (I just tried the set_source_files_properties() route and it just didn't even try to compile the files).
Rather than figure out the rats nest of cmake stuff I'm saddled with, I just added a C++ file that #includes the .c file, and added the C++ file to the CMakeLists.txt sources.
Braindead and stupid, but I'm having to deal with horrific code I'm importing. C header files without extern "C" in them, being used by C++ files.