CODEWITHSUNDEEP
c++cmake
solti
solti

Reputation: 4519

How do I add a linker or compile flag in a CMake file?

I am using the arm-linux-androideabi-g++ compiler. When I try to compile a simple "Hello, World!" program it compiles fine. When I test it by adding a simple exception handling in that code it works too (after adding -fexceptions .. I guess it is disabled by default).

This is for an Android device, and I only want to use CMake, not ndk-build.

For example - first.cpp

#include <iostream>

using namespace std;

int main()
{
   try
   {
   }
   catch (...)
   {
   }
   return 0;
}

./arm-linux-androideadi-g++ -o first-test first.cpp -fexceptions

It works with no problem...

The problem ... I am trying to compile the file with a CMake file.

I want to add the -fexceptions as a flag. I tried with

set (CMAKE_EXE_LINKER_FLAGS -fexceptions ) or set (CMAKE_EXE_LINKER_FLAGS "fexceptions" )

and

set ( CMAKE_C_FLAGS "fexceptions")

It still displays an error.

Upvotes: 308

Views: 836867

Answers (8)

rustyx
rustyx

Reputation: 85481

Since CMake 3.13 there is the add_link_options command.

It applies to all subsequent add_library and add_executable commands in the same scope and sub-scopes.

This can be useful for project-wide settings. For example:

add_link_options("-fexceptions")

add_executable(first-test first.cpp)

For specific targets there is target_link_options:

add_executable(first-test first.cpp)

target_link_options(first-test PUBLIC "-fexceptions")

PUBLIC|PRIVATE|INTERFACE as usual determines how the option is propagated to downstream dependencies.

There's also a BEFORE flag for cases when the option needs to be prepended instead of appended.

Upvotes: 4

John McFarlane
John McFarlane

Reputation: 6107

Edit: My general advice is to avoid adding compiler flags explicitly via CMake altogether. Toolchain files are for configuring novel toolchains, not customising an existing GCC toolchain. A project's CMake is more clean, portable, and future-proof when it is toolchain-agnostic. Separate the concern of describing how your artefacts are constituted (using CMake) from the concern of customising compiler flags. I find environment variables, such as CXXFLAGS are much better at maintaining this separation of concerns. However, in answer to the OP...

The preferred way to specify toolchain-specific options is using CMake's toolchain facility. This ensures that there is a clean division between:

  • instructions on how to organise source files into targets -- expressed in CMakeLists.txt files, entirely toolchain-agnostic; and
  • details of how certain toolchains should be configured -- separated into CMake script files, extensible by future users of your project, scalable.

Ideally, there should be no compiler/linker flags in your CMakeLists.txt files -- even within if/endif blocks. And your program should build for the native platform with the default toolchain (e.g. GCC on GNU/Linux or MSVC on Windows) without any additional flags.

Steps to add a toolchain:

  1. Create a file, e.g. arm-linux-androideadi-gcc.cmake with global toolchain settings:

    set(CMAKE_CXX_COMPILER arm-linux-gnueabihf-g++)
set(CMAKE_CXX_FLAGS_INIT "-fexceptions")

    (You can find an example Linux cross-compiling toolchain file here.)

  2. When you want to generate a build system with this toolchain, specify the CMAKE_TOOLCHAIN_FILE parameter on the command line:

    mkdir android-arm-build && cd android-arm-build
cmake -DCMAKE_TOOLCHAIN_FILE=$(pwd)/../arm-linux-androideadi-gcc.cmake ..

    (Note: you cannot use a relative path.)

  3. Build as normal:

    cmake --build .

Toolchain files make cross-compilation easier, but they have other uses:

  • Hardened diagnostics for your unit tests.

    set(CMAKE_CXX_FLAGS_INIT "-Werror -Wall -Wextra -Wpedantic")

  • Tricky-to-configure development tools.

    # toolchain file for use with gcov
set(CMAKE_CXX_FLAGS_INIT "--coverage -fno-exceptions -g")

  • Enhanced safety checks.

    # toolchain file for use with gdb
set(CMAKE_CXX_FLAGS_DEBUG_INIT "-fsanitize=address,undefined -fsanitize-undefined-trap-on-error")
set(CMAKE_EXE_LINKER_FLAGS_INIT "-fsanitize=address,undefined -static-libasan")

Upvotes: 9

Offirmo
Offirmo

Reputation: 19860

Please be aware that due to the evolution of CMake since the writing of this answer in 2012, the majority of the recommendations provided here are now obsolete or no longer recommended, with improved alternatives available.

Suppose you want to add those flags (better to declare them in a constant):

SET(GCC_COVERAGE_COMPILE_FLAGS "-fprofile-arcs -ftest-coverage")
SET(GCC_COVERAGE_LINK_FLAGS    "-lgcov")

There are several ways to add them:

  1. The easiest one (not clean, but easy and convenient, and works only for compiler flags, C & C++ at once):

    add_definitions(${GCC_COVERAGE_COMPILE_FLAGS})

  2. Appending to corresponding CMake variables:

    SET(CMAKE_CXX_FLAGS  "${CMAKE_CXX_FLAGS} ${GCC_COVERAGE_COMPILE_FLAGS}")
SET(CMAKE_EXE_LINKER_FLAGS  "${CMAKE_EXE_LINKER_FLAGS} ${GCC_COVERAGE_LINK_FLAGS}")

  3. Using target properties, cf. doc CMake compile flag target property and need to know the target name.

    get_target_property(TEMP ${THE_TARGET} COMPILE_FLAGS)
if(TEMP STREQUAL "TEMP-NOTFOUND")
    SET(TEMP "") # Set to empty string
else()
    SET(TEMP "${TEMP} ") # A space to cleanly separate from existing content
endif()
# Append our values
SET(TEMP "${TEMP}${GCC_COVERAGE_COMPILE_FLAGS}" )
set_target_properties(${THE_TARGET} PROPERTIES COMPILE_FLAGS ${TEMP} )

Right now I use method 2.

Upvotes: 305

vitaut
vitaut

Reputation: 55665

In newer versions of CMake you can set compiler and linker flags for a single target with target_compile_options and target_link_libraries respectively (yes, the latter sets linker options too):

target_compile_options(first-test PRIVATE -fexceptions)

The advantage of this method is that you can control propagation of options to other targets that depend on this one via PUBLIC and PRIVATE.

As of CMake 3.13 you can also use target_link_options to add linker options which makes the intent clearer.

Upvotes: 259

sakra
sakra

Reputation: 65951

Try setting the variable CMAKE_CXX_FLAGS instead of CMAKE_C_FLAGS:

set (CMAKE_CXX_FLAGS "-fexceptions")

The variable CMAKE_C_FLAGS only affects the C compiler, but you are compiling C++ code.

Adding the flag to CMAKE_EXE_LINKER_FLAGS is redundant.

Upvotes: 60

Unmitigated
Unmitigated

Reputation: 89422

With CMake 3.4+, APPEND can be used with the string command to add flags.

string(APPEND CMAKE_EXE_LINKER_FLAGS " -fexceptions")

Upvotes: -1

myuce
myuce

Reputation: 1391

This worked for me when I needed a precompile definition named "NO_DEBUG":

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14 -DNO_DEBUG")

Then from code

#ifdef NO_DEBUG
.....

Upvotes: 1

kaveish
kaveish

Reputation: 1434

You can also add linker flags to a specific target using the LINK_FLAGS property:

set_property(TARGET ${target} APPEND_STRING PROPERTY LINK_FLAGS " ${flag}")

If you want to propagate this change to other targets, you can create a dummy target to link to.

Upvotes: 4

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