Screenplay: Generated Code (add_custom_command())

What We Want

Imagine …

  • In the Linux variant, sensor configurations are read from .ini style config file

  • Deeply embedded cannot do this because there is no filesystem

Solution …

  • Can we generate code from these config files?

  • ⟶ link-time polymorphism

Handwritten Prototype: What A Code Generator Could Do For Us

  • Lets say, sensor config files have the format

    2024-06-20.conf (Download)
    [bottom-right]
Type = Const
Value = -273.15

# ... more ...

  • In a dedicated .cpp file, we mirror that same information (this is what we will generate)

    2024-06-20-manual.cpp (Download)
    #include "conf-sensors.h"       // <--- THE config's extern declaration

#include <sensor-const.h>

SensorConfig conf_sensors;      // <--- THE config's definition

namespace {
    struct conf_sensors_init
    {
        conf_sensors_init()
        {
            conf_sensors.add_sensor("bottom-right", std::make_unique<ConstantSensor>(-273.15));
           // ... more ...
        };
    };
    conf_sensors_init init;
};

  • Main program uses conf_sensors from the extern declaration

    data-logger-deeply-embedded.cpp (Download)
    #include "conf-sensors.h"         // <--- conf_sensors'  extern declaration

DataLogger logger(conf_sensors,   // <--- conf_sensors' usage
                  ...);

One Deeply Embedded Application (2024-06-20-manual.cpp)

  • data-logger-deeply-embedded.cpp uses conf_sensors

  • ⟶ defined in 2024-06-20-manual.cpp

  • … or any other file …

  • ⟶ link-time decision

  • Build instructions for the 2024-06-20-manual.cpp constellation

    add_executable(
  app-data-logger-deeply-embedded-2024-06-20-manual

  data-logger-deeply-embedded.cpp
  2024-06-20-manual.cpp                      # <--- polymorphic
)
target_link_libraries(app-data-logger-deeply-embedded-2024-06-20-manual base data-logger)
install(TARGETS app-data-logger-deeply-embedded-2024-06-20-manual DESTINATION bin)

Another Deeply Embedded Application (Generated From 2024-06-20.conf)

  • Generator demo

    $ pwd
.../firmware
$ ./config-generator.py 2024-06-20.conf /tmp/demo.cpp
$ cat /tmp/demo.cpp

  • Build instructions: link with a not-yet-existig file, 2024-06-20.cpp (from the build tree)

    add_executable(
  app-data-logger-deeply-embedded-2024-06-20

  data-logger-deeply-embedded.cpp
  ${CMAKE_CURRENT_BINARY_DIR}/2024-06-20.cpp    # <--- to be generated

)
target_link_libraries(app-data-logger-deeply-embedded-2024-06-20 base data-logger)
install(TARGETS app-data-logger-deeply-embedded-2024-06-20 DESTINATION bin)

  • Code generation: add_custom_command()

    add_custom_command(
  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/2024-06-20.cpp    # <--- same as in add_executable()

  COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/config-generator.py ${CMAKE_CURRENT_SOURCE_DIR}/2024-06-20.conf ${CMAKE_CURRENT_BINARY_DIR}/2024-06-20.cpp
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/config-generator.py ${CMAKE_CURRENT_SOURCE_DIR}/2024-06-20.conf
)

    Discuss DEPENDS …

  • Generated config includes conf-sensors.h from source directory ⟶ PRIVATE “include” property

    target_include_directories(app-data-logger-deeply-embedded-2024-06-20 PRIVATE .)

Et Voila!