# different compilation flags for C and CXX files

**URL:** https://discourse.cmake.org/t/different-compilation-flags-for-c-and-cxx-files/11033
**Category:** Usage
**Created:** [June 17, 2024, 3:32pm UTC](https://discourse.cmake.org/t/different-compilation-flags-for-c-and-cxx-files/11033 "2024-06-17T15:32:24Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![adaldev](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/a/9dc877/32.png) [@adaldev](https://discourse.cmake.org/u/adaldev)
#### Post date: [June 17, 2024, 3:32pm UTC](https://discourse.cmake.org/t/different-compilation-flags-for-c-and-cxx-files/11033/1 "2024-06-17T15:32:24Z")

</div>

Hi,

I’m passing warning options like this:  
`target_compile_options(${TARGET_NAME} PRIVATE/PUBLIC ${MY_WARNING_FLAGS})`

Yet, among my flags, I’ve got a C++ only warning (`-Wold-style-cast`):  
`set(MY_WARNING_FLAGS -Wall -Wextra -Wold-style-cast -pedantic)`

When configuring and generating, everything’s fine but on building I’ve got:

```auto
[xx%] Building C object CMakeFiles/MyProject.dir/build/CMakeFiles/3.28.3/CompilerIdC/CMakeCCompilerId.c.obj
...
cc1.exe: warning: command-line option '-Wold-style-cast' is valid for C++/ObjC++ but not for C

```

I tried:  
`set(MY_WARNING_FLAGS -Wall -Wextra $<$<COMPILE_LANGUAGE:CXX>:-Wold-style-cast> -pedantic)`

but the generator expression is not parsed and is passed, unchanged, to the compiler command line.

This warning bothers me. How can I get rid of it?

Regards,  
A.

NB Most of my projects are pure C++ but some legacy code still contain C files, yet I think that I’m giving them to a C++ frontend such as g++.

---

<div class="post-metadata">

### Author: ![robert.maynard](https://discourse.cmake.org/user_avatar/discourse.cmake.org/robert.maynard/32/4_2.png) [@robert.maynard](https://discourse.cmake.org/u/robert.maynard)
#### Post date: [June 18, 2024, 2:38pm UTC](https://discourse.cmake.org/t/different-compilation-flags-for-c-and-cxx-files/11033/2 "2024-06-18T14:38:42Z")

</div>

Reproducing your example:

```cmake
cmake_minimum_required(VERSION 3.20 FATAL_ERROR)

project(Example LANGUAGES C CXX)

add_library(target SHARED test.c test.cpp)

set(MY_WARNING_FLAGS -Wall -Wextra $<$<COMPILE_LANGUAGE:CXX>:-Wold-style-cast> -pedantic)
target_compile_options(target PRIVATE ${MY_WARNING_FLAGS})

```

Has everything work as expected locally. I would first verify locally that the above example works for you.

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<div class="post-metadata">

### Author: ![adaldev](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/a/9dc877/32.png) [@adaldev](https://discourse.cmake.org/u/adaldev)
#### Post date: [June 18, 2024, 2:48pm UTC](https://discourse.cmake.org/t/different-compilation-flags-for-c-and-cxx-files/11033/3 "2024-06-18T14:48:08Z")

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Strange indeed.

I cleaned my cache and made a few tries and it behave as expected. Thanks to have report your test to me.

By the way. Any idea why I have `CMakeCCompilerId.c` being compiled with the rest of my project? I don’t see this behavior in my other project using the same framework.

Regards,  
A.  
…

---

<div class="post-metadata">

### Author: ![adaldev](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/a/9dc877/32.png) [@adaldev](https://discourse.cmake.org/u/adaldev)
#### Post date: [June 18, 2024, 2:54pm UTC](https://discourse.cmake.org/t/different-compilation-flags-for-c-and-cxx-files/11033/4 "2024-06-18T14:54:34Z")

</div>

Issue solved!

It was stupid: at some point in time the build directory was erroneously created under my source directory. When recursively globing in it for the list of source files (I know, it’s pure evil), I added `CMakeCCompilerId.c` to my list of files to compile.

Undesired directory removed and now the behavior is consistent and as expected. At least it help me see how to use generator expression to set options 😁

A.