# FindFLEX

Find Fast Lexical Analyzer (Flex) executable and provides a macro to generate custom build rules

The module defines the following variables:

    FLEX_FOUND - True is flex executable is found
    FLEX_EXECUTABLE - the path to the flex executable
    FLEX_VERSION - the version of flex
    FLEX_LIBRARIES - The flex libraries
    FLEX_INCLUDE_DIRS - The path to the flex headers

The minimum required version of flex can be specified using the standard syntax, e.g. `find_package(FLEX 2.5.13)`

If flex is found on the system, the module provides the macro:

    FLEX_TARGET(Name FlexInput FlexOutput
                [COMPILE_FLAGS <string>]
                [DEFINES_FILE <string>]
                )

which creates a custom command to generate the `FlexOutput` file from the `FlexInput` file. Name is an alias used to get details of this custom command. If `COMPILE_FLAGS` option is specified, the next parameter is added to the flex command line.

New in version 3.5: If flex is configured to output a header file, the `DEFINES_FILE` option may be used to specify its name.

Changed in version 3.17: When `CMP0098` is set to `NEW`, `flex` runs in the `CMAKE_CURRENT_BINARY_DIR` directory.

The macro defines the following variables:

    FLEX_${Name}_DEFINED - true is the macro ran successfully
    FLEX_${Name}_OUTPUTS - the source file generated by the custom rule, an
    alias for FlexOutput
    FLEX_${Name}_INPUT - the flex source file, an alias for ${FlexInput}
    FLEX_${Name}_OUTPUT_HEADER - the header flex output, if any.

Flex scanners often use tokens defined by Bison: the code generated by Flex depends of the header generated by Bison. This module also defines a macro:

    ADD_FLEX_BISON_DEPENDENCY(FlexTarget BisonTarget)

which adds the required dependency between a scanner and a parser where `FlexTarget` and `BisonTarget` are the first parameters of respectively `FLEX_TARGET` and `BISON_TARGET` macros.

    ====================================================================
    Example:

    find_package(BISON)
    find_package(FLEX)

    BISON_TARGET(MyParser parser.y ${CMAKE_CURRENT_BINARY_DIR}/parser.cpp)
    FLEX_TARGET(MyScanner lexer.l  ${CMAKE_CURRENT_BINARY_DIR}/lexer.cpp)
    ADD_FLEX_BISON_DEPENDENCY(MyScanner MyParser)

     include_directories(${CMAKE_CURRENT_BINARY_DIR})
     add_executable(Foo
        Foo.cc
        ${BISON_MyParser_OUTPUTS}
        ${FLEX_MyScanner_OUTPUTS}
     )
     target_link_libraries(Foo ${FLEX_LIBRARIES})
    ====================================================================