= Compile Flags
:toc:
:toc-placement!:

toc::[]

# Introduction

CMake supports setting compile flags in a number of different ways:

  * using +target_compile_definitions()+ function
  * using the +CMAKE_C_FLAGS+ and +CMAKE_CXX_FLAGS+ variables.

The files in this tutorial are below:

```
$ tree
.
├── CMakeLists.txt
├── main.cpp
```

  * link:CMakeLists.txt[] - Contains the CMake commands you wish to run
  * link:main.cpp[] - The source file with main

# Concepts

# Set Per-Target C++ Flags

The recommended way to set C++ flags in modern CMake is to use per-target flags which can be populated to other targets
through the +target_compile_definitions()+ link:https://cmake.org/cmake/help/v3.0/command/target_compile_definitions.html?highlight=target_compile_definitions[function]. This will populate the link:https://cmake.org/cmake/help/v3.0/prop_tgt/INTERFACE_COMPILE_DEFINITIONS.html#prop_tgt:INTERFACE_COMPILE_DEFINITIONS[INTERFACE_COMPILE_DEFINITIONS] for the library and push the definition to the linked target depending on the scope.

[source,cmake]
----
target_compile_definitions(cmake_examples_compile_flags
    PRIVATE EX3
)
----

This will cause the compiler to add the definition +-DEX3+ when compiling the target.

In the target was a library, and the scope +PUBLIC+ or +INTERFACE+ has been chosen the definition would also be included in any executables that link this target.

For compiler options you can also use the +target_compile_options()+ link:https://cmake.org/cmake/help/v3.0/command/target_compile_options.html[function].

## Set Default C++ Flags

The default `CMAKE_CXX_FLAGS` is either empty or contains the appropriate flags
for the build type.

To set additional default compile flags you can add the following to your
top level CMakeLists.txt

[source,cmake]
----
set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DEX2" CACHE STRING "Set C++ Compiler Flags" FORCE)
----

Similarly to +CMAKE_CXX_FLAGS+ other options include:

  * Setting C compiler flags using +CMAKE_C_FLAGS+
  * Setting linker flags using +CMAKE_LINKER_FLAGS+.

[NOTE]
====
The values `CACHE STRING "Set C++ Compiler Flags" FORCE` from the above command
are used to force this variable to be set in the CMakeCache.txt file.

For more details, see https://cmake.org/cmake/help/v3.0/command/set.html[here]
====


Once set the +CMAKE_C_FLAGS+ and +CMAKE_CXX_FLAGS+ will set a compiler flag / definition globally for all targets in this directory or any included sub-directories. This method is not recommended for general usage now and the +target_compile_definitions+ function is preferred.

### Set CMake Flags

Similar to the build type a global C++ compiler flag can be set using the following methods.

  - Using a gui tool such as ccmake / cmake-gui

image::cmake-gui-set-cxx-flag.png[cmake-gui set cxx flag]

  - Passing into cmake

[source,cmake]
----
cmake .. -DCMAKE_CXX_FLAGS="-DEX3"
----

# Building the Example

[source,bash]
----
$ mkdir build

$ cd build/

$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build

$ make VERBOSE=1
/usr/bin/cmake -H/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags -B/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build'
make -f CMakeFiles/cmake_examples_compile_flags.dir/build.make CMakeFiles/cmake_examples_compile_flags.dir/depend
make[2]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build'
cd /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/DependInfo.cmake --color=
Dependee "/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/DependInfo.cmake" is newer than depender "/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/depend.internal".
Dependee "/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/depend.internal".
Scanning dependencies of target cmake_examples_compile_flags
make[2]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build'
make -f CMakeFiles/cmake_examples_compile_flags.dir/build.make CMakeFiles/cmake_examples_compile_flags.dir/build
make[2]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build'
/usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles 1
[100%] Building CXX object CMakeFiles/cmake_examples_compile_flags.dir/main.cpp.o
/usr/bin/c++    -DEX2   -o CMakeFiles/cmake_examples_compile_flags.dir/main.cpp.o -c /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/main.cpp
Linking CXX executable cmake_examples_compile_flags
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmake_examples_compile_flags.dir/link.txt --verbose=1
/usr/bin/c++    -DEX2    CMakeFiles/cmake_examples_compile_flags.dir/main.cpp.o  -o cmake_examples_compile_flags -rdynamic
make[2]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build'
/usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles  1
[100%] Built target cmake_examples_compile_flags
make[1]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build'
/usr/bin/cmake -E cmake_progress_start /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles 0
----
