Tool-CMake-OPTION

https://clubjuggler.livejournal.com/138364.html

includes a component

As an example, consider a project that optionally includes a component that communiates via USB and allows the user, at compile time, to specify whether to include that component or not.
The CMakeLists.txt file to do that looks like this:

PROJECT(myproject)
OPTION(WITH_USB "Include our USB component" OFF)
SET(SRCS file1.c file2.c file3.c)
IF(WITH_USB)
  SET(SRCS ${SRCS} usbfile1.c usbfile2.c)
ENDIF(WITH_USB)
ADD_EXECUTABLE(myproject ${SRCS})

The OPTION(...) line creates a parameter WITH_USB and set its default value to OFF. CMake also includes a help string to describe what the option does. Based on the value of WITH_USB, the variable SRCS either includes or excludes the two USB-related files usbfile1.c and usbfile2.c. SRCS is then passed to the ADD_EXECUTABLE call to define the program’s source files.
To include USB support, simply enable WITH_USB like this:
$ cmake –DWITH_USB=ON /path/to/source/files

the optional USB support a library in a subdirectory

PROJECT(myproject)
OPTION(WITH_USB "Include our USB component" OFF)
SET(SRCS file1.c file2.c file3.c)
ADD_EXECUTABLE(myproject ${SRCS})
IF(WITH_USB)
  ADD_DIRECTORY(usblib)
  TARGET_LINK_LIBRARIES(myproject usblib)
ENDIF(WITH_USB)

Given this file, the USB source files would be placed in a subdirectory called usblib along with a new CMakeLists.txt file:

PROJECT(usblib)
SET(SRCS usbfile1.c usbfile2.c)
ADD_LIBRARY(usblib ${SRCS})

Now, if USB support is enabled, CMake builds the USB library and link sit into the executable.

But how does the code code know to use the optional USB code?

First, the USB-specific code should be surrounded with a #ifdef WITH_USB.
Next, CMake processes a special configuration file, substituting placeholders with CMake variables.

Let’s make a new file called config.h.cmake that contains this:

#ifndef TEST_CONFIG_H
#define TEST_CONFIG_H

#cmakedefine WITH_USB

#endif

Then, in the CMakeLists.txt file, modify the first part to look like this:

PROJECT(myproject)
OPTION(WITH_USB "Include our USB component" OFF)
CONFIGURE_FILE(${CMAKE_SOURCE_DIR}/config.h.cmake 
  ${CMAKE_BINARY_DIR}/config.h)
INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR} ${CMAKE_BINARY_DIR})

Given this, CMake translates config.h.cmake to create config.h in the build directory. The INCLUDE_DIRECTORIES command tells the compiler to add both the source directory and the build directory to the include path. If WITH_USB is enabled, CMake replaces #cmakedefine WITH_USB with #define WITH_USB. Otherwise, it replaces #cmakedefine WITH_USB with /*#undef WITH_USB*/. As long as you #include config.h and surround the USB code with #ifdef WITH_USB, everything just works. The only downside at the moment is that config.h.cmake must be created manually.