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FreeRTOS
FreeRTOS is real-time operating system kernel for embedded devices. Think of it as a thread library rather than a general purpose operating system like Linux or Microsoft Windows. Out-of-the-box, FreeRTOS provides support for tasks (threads), mutexes, semaphores and software times. Third-party modules are available to add features. e.g. lwIP can be used to add networking.
FreeRTOS+lwIP is currently supported by Eclipse Cyclone DDS. Support for other network stacks, e.g. FreeRTOS+TCP, may be added in the future.
Eclipse Cyclone DDS does not make use of FreeRTOS+POSIX because it was not available at the time. Future versions of Eclipse Cyclone DDS may or may not require FreeRTOS+POSIX for compatibility when it becomes stable.
Target
FreeRTOS provides an operating system kernel. Batteries are not included. i.e. no C library or device drivers. Third-party distributions, known as board support packages (BSP), for various (hardware) platforms are available though.
Board support packages, apart from FreeRTOS, contain:
- C library. Often ships with the compiler toolchain, e.g. IAR Embedded Workbench includes the DLIB runtime, but open source libraries can also be used. e.g. The Xilinx Software Development Kit includes newlib.
- Device drivers. Generally available from the hardware vendor, e.g. NXP or Xilinx. Device drivers for extra components, like a real-time clock, must also be included in the board support package.
A board support package is linked with the application by the toolchain to generate a binary that can be flashed to the target.
Requirements
Eclipse Cyclone DDS requires certain compile-time options to be enabled in
FreeRTOS (FreeRTOSConfig.h) and lwIP (lwipopts.h) for correct operation.
The compiler will croak when a required compile-time option is not enabled.
Apart from the aforementioned compile-time options, the target and toolchain must provide the following.
- Support for thread-local storage (TLS) from the compiler and linker.
- Berkeley socket API compatible socket interface.
- Real-time clock (RTC). A high-precision real-time clock is preferred, but
the monotonic clock can be combined with an offset obtained from e.g. the
network if the target lacks an actual real-time clock. A proper
clock_gettimeimplementation is required to retrieve the wall clock time.
Thread-local storage
FreeRTOS tasks are not threads and compiler supported thread-local storage (tls) might not work as desired/expected under FreeRTOS on embedded targets. i.e. the address of a given variable defined with __thread may be the same for different tasks.
The compiler generates code to retrieve a unique address per thread when it
encounters a variable defined with __thread. What code it generates depends
on the compiler and the target it generates the code for. e.g. iccarm.exe
that comes with IAR Embedded Workbench requires __aeabi_read_tp to be
implemented and mb-gcc that comes with the Xilinx SDK requires
__tls_get_addr to be implemented.
The implementation for each of these functions is more-or-less the same.
Generally speaking they require the number of bytes to allocate, call
pvTaskGetThreadLocalStoragePointer and return the address of the memory
block to the caller.
Simulator
FreeRTOS ports for Windows and POSIX exist to test compatibility. How to cross-compile Eclipse Cyclone DDS for the FreeRTOS Windows Port or the unofficial FreeRTOS POSIX Port can be found in the msvc and posix port directories.
Known Limitations
Triggering the socket waitset is not (yet) implemented for FreeRTOS+lwIP. This introduces issues in scenarios where it is required.
- Receive threads require a trigger to shutdown or a thread may block indefinitely if no packet arrives from the network.
- Sockets are created dynamically if ManySocketsMode is used and a participant is created, or TCP is used. A trigger is issued after the sockets are added to the set if I/O multiplexing logic does not automatically wait for data on the newly created sockets as well.