# Hdden compiler and link options being added to compile and link line

**URL:** <https://discourse.cmake.org/t/hdden-compiler-and-link-options-being-added-to-compile-and-link-line/10629>\
**Category:** Code\
**Tags:** os:windows\
**Created:** [April 11, 2024, 4:03am UTC](https://discourse.cmake.org/t/hdden-compiler-and-link-options-being-added-to-compile-and-link-line/10629 "2024-04-11T04:03:04Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![loopless](https://discourse.cmake.org/user_avatar/discourse.cmake.org/loopless/32/2163_2.png) [@loopless](https://discourse.cmake.org/u/loopless)\
**Post date:** [April 11, 2024, 4:03am UTC](https://discourse.cmake.org/t/hdden-compiler-and-link-options-being-added-to-compile-and-link-line/10629/1 "2024-04-11T04:03:04Z")

</div>

I am using Conan/CMake with QT 6.5.3  
When running CMake 3.24.4 to generate Ninja files for a Release build I am finding “hidden” options added to the compile and link line.

For example, I note the link flags in ninja.build contains “-LTCG”

Nowhere in my CMakeLists.txt do I add that option. It seems to be added by a QT6 CMake Module.

If I add the following

target\_link\_options(${PROJECT\_NAME} PRIVATE  
$\<$CONFIG:RELEASE:/LTCG:OFF\>)

I get two options, and of course the last one “dominates”

LINK\_FLAGS = /machine:x64 /INCREMENTAL:NO /subsystem:windows /entry:WinMainCRTStartup /LTCG:OFF -LTCG

I have similar issues with compile options and a sticky /GL flag. It seems to be related to QT6.

I cannot find these options when running CMake. I can print all the various variables, and none of them contain these “extra” flags. They just appear in the ninja.build file.  
For example I can get the property “COMPILE\_OPTIONS” and the mystery option is not there.

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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:** [April 11, 2024, 5:40pm UTC](https://discourse.cmake.org/t/hdden-compiler-and-link-options-being-added-to-compile-and-link-line/10629/2 "2024-04-11T17:40:57Z")

</div>

You can see all code that CMake sees with `--trace-expand`. This may be a Qt issue to ask for a way to suppress the flag.

---

<div class="post-metadata">

**Author:** ![loopless](https://discourse.cmake.org/user_avatar/discourse.cmake.org/loopless/32/2163_2.png) [@loopless](https://discourse.cmake.org/u/loopless)\
**Post date:** [April 15, 2024, 3:49pm UTC](https://discourse.cmake.org/t/hdden-compiler-and-link-options-being-added-to-compile-and-link-line/10629/3 "2024-04-15T15:49:29Z")

</div>

> [@ben.boeckel](#):
>
> `--trace-expand`

Thank you. I found the culprit, it was not QT, it was pybind11

```auto
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(265): function(_pybind11_return_if_cxx_and_linker_flags_work result cxxflags linkerflags cxxflags_out linkerflags_out )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(279): function(_pybind11_generate_lto target prefer_thin_lto )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(366): _pybind11_generate_lto(pybind11::lto FALSE )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(280): if(MINGW )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(285): if(CMAKE_CXX_COMPILER_ID MATCHES GNU|Clang )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(314): elseif(CMAKE_CXX_COMPILER_ID MATCHES Intel )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(318): elseif(MSVC )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(322): _pybind11_return_if_cxx_and_linker_flags_work(HAS_MSVC_GL_LTCG /GL -LTCG PYBIND11_LTO_CXX_FLAGS PYBIND11_LTO_LINKER_FLAGS )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(267): set(CMAKE_REQUIRED_LIBRARIES -LTCG )
D:/conan/.conan/data/pybind11/2.10.1/vaone-dev/va1-1.19.x/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/lib/cmake/pybind11/pybind11Common.cmake(268): check_cxx_compiler_flag(/GL HAS_MSVC_GL_LTCG )
C:/Program Files/CMake/share/cmake-3.24/Modules/CheckCXXCompilerFlag.cmake(40): cmake_check_compiler_flag(CXX /GL HAS_MSVC_GL_LTCG )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckCompilerFlag.cmake(11): cmake_parse_arguments(PARSE_ARGV 3 CHECK_COMPILER_FLAG OUTPUT_VARIABLE )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckCompilerFlag.cmake(13): cmake_check_flag_common_init(check_compiler_flag CXX _lang_src _lang_fail_regex )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(15): if(CXX STREQUAL C )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(19): elseif(CXX STREQUAL CXX )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(20): set(_lang_src int main() { return 0; } )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(21): set(_lang_fail_regex FAIL_REGEX command[-]line option .* is valid for .* but not for C\+\+ FAIL_REGEX -Werror=.* argument .* is not valid for C\+\+ )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(56): get_property(_supported_languages GLOBAL PROPERTY ENABLED_LANGUAGES )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(57): if(NOT CXX IN_LIST _supported_languages )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(62): set(_CFCC_locale_vars LC_ALL LC_MESSAGES LANG )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(63): foreach(v IN LISTS _CFCC_locale_vars )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(64): set(_CMAKE_CHECK_FLAG_COMMON_CONFIG_locale_vars_saved_LC_ALL )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(65): set(ENV{LC_ALL} C )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(64): set(_CMAKE_CHECK_FLAG_COMMON_CONFIG_locale_vars_saved_LC_MESSAGES )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(65): set(ENV{LC_MESSAGES} C )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(64): set(_CMAKE_CHECK_FLAG_COMMON_CONFIG_locale_vars_saved_LANG )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckFlagCommonConfig.cmake(65): set(ENV{LANG} C )
C:/Program Files/CMake/share/cmake-3.24/Modules/Internal/CheckCompilerFlag.cmake(15): set(CMAKE_REQUIRED_DEFINITIONS /GL )

```