How to Access and Use CMake Variables in C Source Code?

Reading CMake Variables in C Source Code

Many developers prefer to store configuration information, such as version numbers, in a central location for ease of management. CMake provides a convenient mechanism for defining such variables within its configuration files. This article explores how to access and utilize these variables within C source code during compilation.

Defining CMake Variables

To define a CMake variable, use the SET command. Here's an example that defines a variable named LIBINTERFACE_VERSION with a value of 1:

SET(LIBINTERFACE_VERSION 1 CACHE INTEGER "Version of libInterface")

Passing Variables to C Code

Once a CMake variable is defined, you can make it available to C code using the add_definitions or configure_file commands.

Using add_definitions

The add_definitions command is a simple and straightforward way to pass a CMake variable to C code. It creates a preprocessor definition that can be used in source code.

add_definitions( -DVERSION_LIBINTERFACE=${LIBINTERFACE_VERSION} )

This will create the preprocessor definition VERSION_LIBINTERFACE with the value of LIBINTERFACE_VERSION, which can then be used within C code.

Using configure_file

configure_file is a more versatile approach that allows you to create a header file template and use CMake to substitute variables into it. This method provides more flexibility and can be used to generate multiple header files with different configurations.

Creating a Template Header File

First, create a template header file, such as version_config.h.in, with the following contents:

#ifndef VERSION_CONFIG_H
#define VERSION_CONFIG_H

// define your version_libinterface
#define VERSION_LIBINTERFACE @LIBINTERFACE_VERSION@

#endif // VERSION_CONFIG_H

This template defines a VERSION_LIBINTERFACE preprocessor macro with the value of LIBINTERFACE_VERSION.

Using configure_file

In your CMakeLists.txt file, use configure_file to generate the version_config.h header file:

configure_file( version_config.h.in ${CMAKE_BINARY_DIR}/generated/version_config.h )

This command will substitute the @LIBINTERFACE_VERSION@ placeholder with the actual value of the variable and generate the header file.

Including the Generated Header File

Finally, make sure to include the generated version_config.h header file in your C source code:

#include <generated></generated>

With either of these methods, you can access and use CMake variables in your C source code. Both add_definitions and configure_file offer different advantages depending on your specific needs and project structure.

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