# How to force cmake use -MMD flag instead of -MD

**URL:** https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120
**Category:** Code
**Tags:** gen:makefiles
**Created:** [October 1, 2023, 10:54am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120 "2023-10-01T10:54:06Z")
**Posts on this page:** 11
**Page:** 1

<div class="post-metadata">

### Author: ![Valeryan](https://discourse.cmake.org/user_avatar/discourse.cmake.org/valeryan/32/3886_2.png) [@Valeryan](https://discourse.cmake.org/u/Valeryan)
#### Post date: [October 1, 2023, 10:54am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/1 "2023-10-01T10:54:06Z")

</div>

Tool chain used is arm-none-eabi-gcc.  
I need to compile the project using -MMD flag.  
I’ve try CMAKE\_C\_FLAGS and add\_compile\_options().  
Nothing is work.  
CMake will append -MD automatically.  
I believe when compiler see gcc -MMD -MD, compiler will use -MD.  
How do I make CMake use -MMD flag instead of -MD?

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<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [October 1, 2023, 11:00am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/2 "2023-10-01T11:00:10Z")

</div>

It doesn’t look like CMake provides a way to do that. You might be able to replace it in the `CMAKE_DEPFILE_FLAGS_<LANG>` variable though.

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<div class="post-metadata">

### Author: ![ClausKlein](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clausklein/32/352_2.png) [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 1, 2023, 6:49pm UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/3 "2023-10-01T18:49:25Z")

</div>

### At least for `Ninja` build, this doesn’t longer work on OSX!

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<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [October 1, 2023, 7:17pm UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/4 "2023-10-01T19:17:24Z")

</div>

In what way?

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<div class="post-metadata">

### Author: ![ClausKlein](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clausklein/32/352_2.png) [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 1, 2023, 8:46pm UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/5 "2023-10-01T20:46:41Z")

</div>

`set(CMAKE_DEPFILE_FLAGS_CXX "-MMD" CACHE STRING "dependency flag" FORCE)`

```bash
grep -r -- -MD CMakeFiles/rules.ninja 
CMakeFiles/rules.ninja: command = ${LAUNCHER}${CODE_CHECK}/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ $DEFINES $INCLUDES $FLAGS -MD -MT $out -MF $DEP_FILE -o $out -c $in
CMakeFiles/rules.ninja: command = ${LAUNCHER}${CODE_CHECK}/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ $DEFINES $INCLUDES $FLAGS -MD -MT $out -MF $DEP_FILE -o $out -c $in
bash-5.2$ 

```

```auto
bash-5.2$ cmake --workflow --preset default --fresh 2>&1 | grep CMAKE_DEPFILE_FLAGS_
/Users/clausklein/Workspace/cpp/ModernCmakeStarter/CMakeLists.txt(20): set(CMAKE_DEPFILE_FLAGS_CXX -MMD CACHE STRING dependency flag FORCE )
/Users/clausklein/Workspace/cpp/ModernCmakeStarter/CMakeLists.txt(21): set(CMAKE_DEPFILE_FLAGS_C -MMD CACHE STRING dependency flag FORCE )
/usr/local/Cellar/cmake/3.27.6/share/cmake/Modules/Compiler/GNU.cmake(52): set(CMAKE_DEPFILE_FLAGS_CXX -MD -MT <DEP_TARGET> -MF <DEP_FILE> )
/usr/local/Cellar/cmake/3.27.6/share/cmake/Modules/Compiler/AppleClang-CXX.cmake(7): if(( NOT DEFINED CMAKE_DEPENDS_USE_COMPILER OR CMAKE_DEPENDS_USE_COMPILER ) AND CMAKE_GENERATOR MATCHES Makefiles AND CMAKE_DEPFILE_FLAGS_CXX )
/usr/local/Cellar/cmake/3.27.6/share/cmake/Modules/Compiler/GNU.cmake(52): set(CMAKE_DEPFILE_FLAGS_CXX -MD -MT <DEP_TARGET> -MF <DEP_FILE> )
/usr/local/Cellar/cmake/3.27.6/share/cmake/Modules/Compiler/AppleClang-CXX.cmake(7): if(( NOT DEFINED CMAKE_DEPENDS_USE_COMPILER OR CMAKE_DEPENDS_USE_COMPILER ) AND CMAKE_GENERATOR MATCHES Makefiles AND CMAKE_DEPFILE_FLAGS_CXX )
bash-5.2$ 

```

---

<div class="post-metadata">

### Author: ![ClausKlein](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clausklein/32/352_2.png) [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 2, 2023, 9:46am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/6 "2023-10-02T09:46:08Z")

</div>

Here maybe a problem too on OSX **Modules/Compiler/Clang-CXX.cmake**

```cmake
if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
  if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU")
    string(CONCAT CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE
      "\"${CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS}\""
      " -format=p1689"
      " --"
      " <CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS>"
      " -x c++ <SOURCE> -c -o <OBJECT>"
      " -MT <DYNDEP_FILE>"
      " -MD -MF <DEP_FILE>"
      " > <DYNDEP_FILE>")
    set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "clang")
    set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
    set(CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG "--precompile")
  endif()
endif()

```

… and there are more:

```auto
bash-5.2$ grep -r -- -MD Modules 
Modules/CMakeASM_NASMInformation.cmake: set(CMAKE_DEPFILE_FLAGS_ASM_NASM "-MD <DEP_FILE> -MT <DEP_TARGET>")
Modules/Platform/Windows-NVIDIA-CUDA.cmake: set(_MDd "-MDd ")
Modules/Platform/Windows-NVIDIA-CUDA.cmake: set(_MD "-MD ")
Modules/Platform/Windows-NVIDIA-CUDA.cmake:set(CMAKE_CUDA_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDLL -Xcompiler=-MD)
Modules/Platform/Windows-NVIDIA-CUDA.cmake:set(CMAKE_CUDA_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDebugDLL -Xcompiler=-MDd)
Modules/Platform/Windows-Clang.cmake: set(CMAKE_DEPFILE_FLAGS_${lang} "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Platform/Windows-Clang.cmake: set(CMAKE_DEPFILE_FLAGS_RC "${clang_option_prefix}-MD ${clang_option_prefix}-MF ${clang_option_prefix}<DEP_FILE>")
Modules/Platform/Windows-MSVC.cmake: set(CMAKE_${lang}_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDLL -MD)
Modules/Platform/Windows-MSVC.cmake: set(CMAKE_${lang}_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDebugDLL -MDd)
Modules/UseSWIG.cmake: set(swig_depends_flags -MF "${swig_depends_filename}" -MD)
Modules/Compiler/NVIDIA-CUDA.cmake: # The -MD flag was only added to nvcc in 10.2 so
Modules/Compiler/NVIDIA-CUDA.cmake: set(CMAKE_DEPFILE_FLAGS_CUDA "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Binary file Modules/Compiler/.Clang-CXX.cmake.swp matches
Modules/Compiler/IntelLLVM.cmake: set(CMAKE_DEPFILE_FLAGS_${lang} "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/Intel-C.cmake:set(CMAKE_DEPFILE_FLAGS_C "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/GNU-ASM.cmake: set(CMAKE_DEPFILE_FLAGS_ASM${ASM_DIALECT} "--MD <DEP_FILE>")
Modules/Compiler/LCC.cmake: set(CMAKE_DEPFILE_FLAGS_${lang} "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/NVHPC-C.cmake: set(CMAKE_DEPFILE_FLAGS_C "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/NVHPC-C.cmake: # Before NVHPC 21.07 the `-MD` flag implicitly
Modules/Compiler/NVHPC-C.cmake: set(CMAKE_C_DEPENDS_EXTRA_COMMANDS "<CMAKE_C_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -x c -M <SOURCE> -MT <OBJECT> -MD<DEP_FILE>")
Modules/Compiler/QCC.cmake: set(CMAKE_DEPFILE_FLAGS_${lang} "-Wp,-MD,<DEP_FILE> -Wp,-MT,<DEP_TARGET> -Wp,-MF,<DEP_FILE>")
Modules/Compiler/Clang-CXX.cmake: " -MD -MF <DEP_FILE>"
Modules/Compiler/Intel-CXX.cmake:set(CMAKE_DEPFILE_FLAGS_CXX "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/NVHPC-CXX.cmake: set(CMAKE_DEPFILE_FLAGS_CXX "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/NVHPC-CXX.cmake: # Before NVHPC 21.07 the `-MD` flag implicitly
Modules/Compiler/NVHPC-CXX.cmake: set(CMAKE_CXX_DEPENDS_EXTRA_COMMANDS "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -x c++ -M <SOURCE> -MT <OBJECT> -MD<DEP_FILE>")
Modules/Compiler/GNU.cmake: set(CMAKE_DEPFILE_FLAGS_${lang} "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
Modules/Compiler/Clang-CUDA.cmake:set(CMAKE_DEPFILE_FLAGS_CUDA "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")
bash-5.2$ 

```

---

<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [October 2, 2023, 10:26am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/7 "2023-10-02T10:26:01Z")

</div>

> [@ClausKlein](#):
>
> `set(CMAKE_DEPFILE_FLAGS_CXX "-MMD" CACHE STRING "dependency flag" FORCE)`

You’ll want to `string(REPLACE)` on the variable (and the others you found) to not drop other important flags, Also, local variables mask cache variables, so using the cache is unlikely to be effective.

---

<div class="post-metadata">

### Author: ![ClausKlein](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clausklein/32/352_2.png) [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 2, 2023, 1:24pm UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/8 "2023-10-02T13:24:48Z")

</div>

> [@ClausKlein](#):
>
> ```auto
> /usr/local/Cellar/cmake/3.27.6/share/cmake/Modules/Compiler/GNU.cmake(52): set(CMAKE_DEPFILE_FLAGS_CXX -MD -MT <DEP_TARGET> -MF <DEP_FILE> )
> 
> ```

That cause my problem!

---

<div class="post-metadata">

### Author: ![ClausKlein](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clausklein/32/352_2.png) [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 4, 2023, 9:34am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/9 "2023-10-04T09:34:35Z")

</div>

@ben.boeckel with this setting after `project()`

```cmake
if(UNIX)
  set(CMAKE_DEPFILE_FLAGS_C "-MMD")
  set(CMAKE_DEPFILE_FLAGS_CXX "-MMD")
endif()

```

### the flags are used, but than there are no dependecies at all?

```bash
klein_cl:~/Workspace/cpp/ModernCmakeStarter/build/user$ ninja -t deps
_deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o: #deps 0, deps mtime 1696411728991046200 (VALID)

_deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o: #deps 0, deps mtime 1696411731741763100 (VALID)

CMakeFiles/greeter.dir/Unity/unity_0_cxx.cxx.o: #deps 0, deps mtime 1696411732034372400 (VALID)

klein_cl:~/Workspace/cpp/ModernCmakeStarter/build/user$
```

---

<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [October 4, 2023, 9:59am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/10 "2023-10-04T09:59:52Z")

</div>

You removed the `-MF` and `-MT` flags that are needed to:

- write them to a file
- with an output that `ninja` accepts

---

<div class="post-metadata">

### Author: ![ClausKlein](https://discourse.cmake.org/user_avatar/discourse.cmake.org/clausklein/32/352_2.png) [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 4, 2023, 10:37am UTC](https://discourse.cmake.org/t/how-to-force-cmake-use-mmd-flag-instead-of-md/9120/11 "2023-10-04T10:37:36Z")

</div>

So the only correct _undocumented_ way would be after `project(...)`:

```cmake
if(UNIX AND CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
  set(CMAKE_DEPFILE_FLAGS_C "-MMD -MT <DEP_TARGET> -MF <DEP_FILE>")
  set(CMAKE_DEPFILE_FLAGS_CXX "-MMD -MT <DEP_TARGET> -MF <DEP_FILE>")
endif()

```