Check If C Symbol Exists using CMake

When writing cross-platform C or C++ code, it's common to encounter differences between operating systems or library versions - certain macros, or structure members might exist on one platform but not another. To make the code portable, you often need to test whether a particular symbol (such as a preprocessor macro, variable, or function) is available before using it. This tutorial explains how to check If C symbol exists using CMake.

CMake provides the check_symbol_exists command that can automatically check whether a symbol is defined in specific header files at configuration time. Then it is possible to conditionally enable or disable parts of the code based on the result.

Here's an example CMakeLists.txt demonstrating how to use it:

cmake_minimum_required(VERSION 3.27)
project(myapp)

set(CMAKE_CXX_STANDARD 17)

include(CheckSymbolExists)

check_symbol_exists(O_CLOEXEC "fcntl.h" HAVE_O_CLOEXEC)
if (HAVE_O_CLOEXEC)
    # ...
endif ()

add_executable(${PROJECT_NAME} main.cpp)

When you run CMake, it will try to compile a small test program that includes the given headers and the specified symbol. If compilation succeeds, CMake sets the corresponding variable to TRUE. You can then use that variable within the CMake logic or pass it as a preprocessor definition to the source code to conditionally enable specific functionality.

You can perform similar checks by listing multiple headers using ; separator:

# ...

include(CheckSymbolExists)

check_symbol_exists(stat "sys/types.h;sys/stat.h" HAVE_STAT)
if (HAVE_STAT)
    # ...
endif ()

# ...

By default, the check_symbol_exists command prints diagnostic messages like:

-- Looking for O_CLOEXEC
-- Looking for O_CLOEXEC - found

If you're performing several checks and prefer a quieter configuration phase, you can disable these log messages by setting the variable CMAKE_REQUIRED_QUIET before calling the check function:

# ...

set(CMAKE_REQUIRED_QUIET TRUE)

include(CheckSymbolExists)

check_symbol_exists(O_CLOEXEC "fcntl.h" HAVE_O_CLOEXEC)

# ...