Conditional if/else

Inevitably you'll need to use if() condition checks to test if some condition is met. Common pre-defined variables to check include: WIN32, UNIX, APPLE, and CYGWIN. There are also a number of enum-like operators that can be used for comparisons. You can view the complete rundown on the if | CMake Documentation page.

cmake_minimum_required(VERSION 3.28)
project(m-yproject)

if(WIN32)
  add_executable(my-app main_windows.c)
elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
  add_executable(my-app main_linux.c)
else()
  add_executable(my-app main.c)
endif()

cmake -B build
cmake --build build
./build/my-app

Notice how the elseif() block uses the STREQUAL operator instead of a more common idiom like == to check if the CMAKE_SYSTEM_NAME variable is equal to "Linux".

Other common string operators include STRLESS, STRGREATER, STREQUAL, STRLESS_EQUAL, and STRGREATER_EQUAL. There's also a bunch of number operators like LESS, GREATER, EQUAL, LESS_EQUAL, and GREATER_EQUAL. There's even some for comparison semver specifiers! Check out the full list at if | CMake Documentation.