How to Use Flex In Cmake?

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To use flex in CMake, you first need to install the flex software on your system. Flex is a tool that generates lexical analyzers for use in compilers and interpreters.

Once you have installed flex, you can create a custom command in your CMakeLists.txt file to run the flex tool on your input file. This can be done using the ADD_CUSTOM_COMMAND() function in CMake.

You also need to add a rule to compile the generated lex.yy.c file using your C compiler. This can be done using the ADD_CUSTOM_TARGET() function in CMake.

Finally, you can link the generated lex.yy.c file with your project executable by adding it to the list of source files in your CMakeLists.txt file.

Overall, using flex in CMake involves generating the lex.yy.c file using flex, compiling it using your C compiler, and linking it with your project executable.

How to configure CMake to work with flex-generated files?

To configure CMake to work with flex-generated files, you need to add the following commands to your CMakeLists.txt file:

  1. Use the FLEX command to generate the lexer source file from the flex input file. This command requires the path to the flex input file as input and the path to the generated source file as output. For example:
FLEX_TARGET(MyLexer lexer.l ${CMAKE_CURRENT_BINARY_DIR}/lexer.cpp)

  1. Add the generated source file to your target executable or library using the add_executable or add_library command. For example:
add_executable(MyApp main.cpp ${FLEX_MyLexer_OUTPUTS})

  1. Include the directory containing the generated source file in the include directories using the include_directories command. For example:

  1. Optionally, you can also specify the output directory for the generated files using the set command. For example:

After adding these commands to your CMakeLists.txt file, you can run CMake to generate the build files for your project. Make sure to run make or cmake --build . to build your project and generate the executable.

What is the impact of using flex on the size of a CMake build?

Using flex in a CMake build can have both advantages and disadvantages in terms of the build size.


  1. Reduced code size: Flex generates efficient lexical analyzers which can result in smaller and more optimized code compared to handwritten lexers.
  2. Faster build times: Flex can improve compile times by generating efficient code for scanning input files.


  1. Increased build size: Including a lexer generated with Flex in the build can increase the size of the final executable or library, as the generated code adds extra overhead.
  2. Dependency management: Adding Flex-generated files to the build may introduce additional dependencies, which can complicate the build system and increase the complexity of the project.

Overall, the impact of using Flex on the size of a CMake build will depend on the specific project requirements and trade-offs between code size, build times, and dependencies. Developers should carefully consider these factors when deciding whether to use Flex in their CMake builds.

What is the significance of using flex in CMake projects?

Using Flex in a CMake project allows for the generation of a lexical analyzer, which can be used to parse and tokenize input data. This is particularly useful in programming languages and compilers, where a lexical analyzer is needed to break down source code into individual tokens for further processing.

By incorporating Flex into a CMake project, developers can easily generate the necessary lexer code, integrate it with their project, and improve the overall efficiency and readability of their codebase. Additionally, Flex offers a more flexible and efficient alternative to manual lexical analysis, saving time and effort in the development process.

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