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FAbian's Realtime Box o' Tricks

This is a collection of design patterns which can be used in realtime code. This library was introduced to the public at meeting C++ on 16th of November 2019.

This library and the containing design patterns were presented at meeting C++ 2019. You can find the slides here: https://hogliux.github.io/farbot/presentations/meetingcpp_2019/index.html

It contains the following realtime design patterns:

RealtimeObject

RealtimeObject<T, RealtimeObjectOptions> is a templated class with which you can share data of type T between non-realtime threads and a single realtime thread. Depending on the non-type template parameter option RealtimeObjectOptions either only the realtime or the non-realtime thread may mutate the data.

You can use it liked this:

struct BiquadCoeffecients  {  float b0, b1, b2, a1, a2; };
RealtimeObject<BiquadCoeffecients, RealtimeObjectOptions::nonRealtimeMutatable> biquadCoeffs;

/* called on realtime thread */
void processAudio (float* buffer)
{
    RealtimeObject<BiquadCoeffecients, RealtimeObjectOptions::nonRealtimeMutatable>::ScopedAccess<ThreadType::realtime> coeffs(biquadCoeffs);
    processBiquad (*coeffs, buffer);
}

/* called on non-realtime thread */
void changeBiquadParameters (BiquadCoeffecients newCoeffs)
{
    RealtimeObject<BiquadCoeffecients, RealtimeObjectOptions::nonRealtimeMutatable>::ScopedAccess<ThreadType::nonRealtime> coeffs(biquadCoeffs);
    *coeffs = newCoeffs;
}

where the ThreadType::Realtime/ThreadType::nonRealtime template parameter of the ScopedAccess class indicates if the thread requiring access to the data is a realtime thread or not.

Here is an example where only the realtime thread can mutate the data:

using FrequencySpectrum = std::array<float, 512>;
RealtimeObject<FrequencySpectrum, RealtimeObjectOptions::realtimeMutatable> mostRecentSpectrum;

/* called on realtime thread */
void processAudio (const float* buffer, size_t n) {
    RealtimeObject<FrequencySpectrum, RealtimeObjectOptions::realtimeMutatable>::ScopedAccess<ThreadType::realtime> freqSpec(mostRecentSpectrum);
    *freqSpec = calculateSpectrum (buffer, n);
}

/* called on non-realtime thread */
void updateSpectrumUIButtonClicked() {
    RealtimeObject<FrequencySpectrum, RealtimeObjectOptions::realtimeMutatable>::ScopedAccess<ThreadType::nonRealtime> recentSpectrum(mostRecentSpectrum);
    displaySpectrum(*recentSpectrum);
}

fifo

fifo is a versatile realtime-safe ringbuffer class which supports various types of fifos: you can choose the consumer/producer to either be accessed from a single or multiple thread. This need not be the same for the consumer and producer, for example, you could have a multi-producer, single-consumer fifo. This is controlled by the farbot::fifo_options::concurrency::single/farbot::fifo_options::concurrency::multiple template parameter.

In addition, you can also choose what happens on an underrun (during a pop) or an overrun (during a push). A fifo with farbot::fifo_options::full_empty_failure_mode::return_false_on_full_or_empty will return false on a push/pop if the fifo is full/empty respectively. A fifo with farbot::fifo_options::full_empty_failure_mode::overwrite_or_return_default will overwrite on full (pop) or return a default constructed element on empty (pop). Again, this option can be chosen independently for the consumer or producer. Note, that with the farbot::fifo_options::full_empty_failure_mode::overwrite_or_return_default option the ordering of the FIFO is lost when overrunning or underruning, i.e. newer elements may be returned before older elements in this case.

The fifo will never lock nor block. Additionally, depending on the above options the push/pop operation may be wait-free: if the consumer/producer is accessed from only a single thread or the consumer/producer uses overwrite_or_return_default then the pop/push will be wait-free respectively. Otherwise the perticular (i.e. push or pop) operation will not be wait-free.

Usage:

fifo<std::function<void()>*,
                  fifo_options::concurrency::single,
                  fifo_options::concurrency::multiple,
                  fifo_options::full_empty_failure_mode::return_false_on_full_or_empty,
                  fifo_options::full_empty_failure_mode::overwrite_or_return_default> my_fifo; // <- this fifo is wait-free on push and pop

fifo.push (mylambda);
std::function<void()>* almabda;
fifo.pop (alambda);

AsyncCaller

AsyncCaller is a class which contains a method called callAsync with which a lambda can be deferred to be processed on a non-realtime thread. This is useful to be able to execute potential non-realtime safe code on a realtime thread (like logging, or deallocations, ...).

Realtime traits

The farbot library also contains very limited type traits to check if a specific type is realtime movable/copyable. Currently this only works for trivially movable/copyable and a few STL containers.

farbot::is_realtime_copy_assignable, farbot::is_realtime_copy_constructable farbot::is_realtime_move_assignable, farbot::is_realtime_move_constructable