With cmake, how would you disable in-source builds?

CMake has two undocumented options: CMAKE_DISABLE_SOURCE_CHANGES and CMAKE_DISABLE_IN_SOURCE_BUILD

cmake_minimum_required (VERSION 2.8)

# add this options before PROJECT keyword
set(CMAKE_DISABLE_SOURCE_CHANGES ON)
set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)

project (HELLO)

add_executable (hello hello.cxx)

andrew@manchester:~/src% cmake .
CMake Error at /usr/local/share/cmake-2.8/Modules/CMakeDetermineSystem.cmake:160 (FILE):
  file attempted to write a file: /home/andrew/src/CMakeFiles/CMakeOutput.log
  into a source directory.

/home/selivanov/cmake-2.8.8/Source/cmMakefile.cxx

bool cmMakefile::CanIWriteThisFile(const char* fileName)
{
  if ( !this->IsOn("CMAKE_DISABLE_SOURCE_CHANGES") )
    {
    return true;
    }
  // If we are doing an in-source build, than the test will always fail
  if ( cmSystemTools::SameFile(this->GetHomeDirectory(),
                               this->GetHomeOutputDirectory()) )
    {
    if ( this->IsOn("CMAKE_DISABLE_IN_SOURCE_BUILD") )
      {
      return false;
      }
    return true;
    }

  // Check if this is subdirectory of the source tree but not a
  // subdirectory of a build tree
  if ( cmSystemTools::IsSubDirectory(fileName,
      this->GetHomeDirectory()) &&
    !cmSystemTools::IsSubDirectory(fileName,
      this->GetHomeOutputDirectory()) )
    {
    return false;
    }
  return true;
}

Include a function like this one. It is similar to what you do with these differences:

It is encapsulated in a function, which is called when you include the PreventInSourceBuilds.cmake module. Your main CMakeLists.txt must include it:
```
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_CURRENT_SOURCE_DIR}/CMake)
include(PreventInSourceBuilds)
```
It uses get_filename_component() with REALPATH parameter that resolves symlinks before comparing the paths.

In case the github link changes, here's the module source code (which should be placed in a PreventInSouceBuilds.cmake, in a directory called CMake, in the above example):

#
# This function will prevent in-source builds
function(AssureOutOfSourceBuilds)
  # make sure the user doesn't play dirty with symlinks
  get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
  get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)

  # disallow in-source builds
  if("${srcdir}" STREQUAL "${bindir}")
    message("######################################################")
    message("# ITK should not be configured & built in the ITK source directory")
    message("# You must run cmake in a build directory.")
    message("# For example:")
    message("# mkdir ITK-Sandbox ; cd ITK-sandbox")
    message("# git clone http://itk.org/ITK.git # or download & unpack the source tarball")
    message("# mkdir ITK-build")
    message("# this will create the following directory structure")
    message("#")
    message("# ITK-Sandbox")
    message("#  +--ITK")
    message("#  +--ITK-build")
    message("#")
    message("# Then you can proceed to configure and build")
    message("# by using the following commands")
    message("#")
    message("# cd ITK-build")
    message("# cmake ../ITK # or ccmake, or cmake-gui ")
    message("# make")
    message("#")
    message("# NOTE: Given that you already tried to make an in-source build")
    message("#       CMake have already created several files & directories")
    message("#       in your source tree. run 'git status' to find them and")
    message("#       remove them by doing:")
    message("#")
    message("#       cd ITK-Sandbox/ITK")
    message("#       git clean -n -d")
    message("#       git clean -f -d")
    message("#       git checkout --")
    message("#")
    message("######################################################")
    message(FATAL_ERROR "Quitting configuration")
  endif()
endfunction()

AssureOutOfSourceBuilds()

I have a cmake() shell function in my .bashrc/.zshrc similar to this one:

function cmake() {
  # Don't invoke cmake from the top-of-tree
  if [ -e "CMakeLists.txt" ]
  then
    echo "CMakeLists.txt file present, cowardly refusing to invoke cmake..."
  else
    /usr/bin/cmake $*
  fi
}

I prefer this low ceremony solution. It got rid of my colleagues' biggest complaint when we switched to CMake, but it doesn't prevent people who really want to do an in-source/top-of-tree build from doing so—they can just invoke /usr/bin/cmake directly (or not use the wrapper function at all). And it's stupid simple.

I think I like your way. The cmake mailing list does a good job at answering these types of questions.

As a side note: you could create a "cmake" executable file in the directory which fails. Depending on whether or not "." is in their path (on linux). You could even symlink /bin/false.

In windows, I am not sure if a file in your current directory is found first or not.

You can configure your .bashrc file like this one

Look at the functions cmakekde and kdebuild. Set BUILD and SRC env. variables and edit these functions according to your needs. This will build only in buildDir rather than srcDir

With cmake, how would you disable in-source builds?

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