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CMake build system

    Building lwIP
    =============

    lwIP uses a CMake based build system.

    The CMake files in this project are designed to
    be included into your own CMake files. They are
    mainly variable definitions containing a list of
    source files and predefined static libraries to
    be linked against application code.

    1) lwIP sources:
       src/Filelists.cmake provides file lists containing
       the lwIP core library.
       The file also contains two static libraries, lwipcore
       and lwipallapps, where you can link your app against.
       This is the file that is useful to all lwIP users.

    2) Example applications:
       contrib/Filelists.cmake provides several file lists
       containing the example applications.
       The file also contains several static libraries
       for these example apps.
       This file is only useful for you, if you can use one
       of the examples in your application, which is normally
       not the case.

    3) OS/platform port:
       Usually the OS port needs to be provided by the user.
       If a port to Linux, Windows or MacOS is useful for
       you, you can use
       contrib/ports/{win32, unix}/Filelists.cmake
       that contains file lists and libraries for
       these operating systems.

    VARIABLES
    =========
    In all cases, you need to provide two variables.

    "LWIP_DIR" pointing to the lwIP directory
    Example:
    set(LWIP_DIR ${CMAKE_CURRENT_SOURCE_DIR}/externals/lwip)

    "LWIP_INCLUDE_DIRS" that contains the include base paths
    - for lwIP itself (${LWIP_DIR}/src/include)
    - for lwIP contrib if you use it (${LWIP_DIR}/contrib)
    - to a directory containing an OS port
    - to a directory containing lwipopts.h

    Example:
    set (LWIP_INCLUDE_DIRS
        "${LWIP_DIR}/src/include"
        "${LWIP_DIR}/contrib"
        "${LWIP_DIR}/contrib/ports/unix/port/include"
        "${LWIP_DIR}/contrib/examples/example_app"
    )

    Putting it all together
    =======================
    To get a working application, your CMake system
    needs to provide the variables described above, e.g.
    set (LWIP_DIR <path to lwip sources>)
    set (LWIP_INCLUDE_DIRS
        "${LWIP_DIR}/src/include"
        "${LWIP_DIR}/contrib"
        "<path to my port>/include"
        "<path to lwipopts.h>"
    )

    You may add some defines:
    set (LWIP_DEFINITIONS LWIP_DEBUG=1)

    Then include the filelists you need:
    include(${LWIP_DIR}/src/Filelists.cmake)
    include(${LWIP_DIR}/contrib/Filelists.cmake)

    Then, declare you executable:
    add_executable(my_app <my source files> <my lwip port files>)

    Add lwIP include dirs to your app:
    target_include_directories(my_app PRIVATE ${LWIP_INCLUDE_DIRS})

    Link your app against the lwIP libs from the filelists you need:
    target_link_libraries(my_app lwipcontribapps lwipallapps lwipcore)

    Working example
    ===============
    Working build examples can be found in the
    contrib/ports/{win32, unix}/example_app
    subdirectory.
    To use them, create a build directory and call cmake with
    the lwIP root dir:

    - mkdir build
    - cd build
    - cmake ..
    - cmake --build .

    The CMakeLists.txt will autoselect the correct port
    for your system (supported: Linux, Windows, Darwin).

    To configure the example app to your needs, you need to copy the file
        contrib/examples/example_app/lwipcfg.h.example
    to
        contrib/examples/example_app/lwipcfg.h
    and edit to your needs.

    Makefile based build system
    ===========================
    lwIP also maintains file lists for Makefile-based
    build systems. Look for Filelists.mk files
    in the source tree. We try to maintain them,
    but lwIP's mainly focused build system is CMake.

    MS Visual Studio
    ================
    lwIP also provides basic support for MSVS in the win32 port
    folder under 'msvc'. We try to maintain these files,
    but lwIP's mainly focused build system is CMake.