  Many Internet of Things and embedded projects are event-driven, and therefore
require asynchronous and concurrent programming. Current proposals for C++20
suggest that coroutines will have native language support. It is timely to
survey the current use of coroutines in embedded systems development. This
paper investigates existing research which uses or describes coroutines on
resource-constrained platforms. The existing research is analysed with regard
to: software platform, hardware platform and capacity; use cases and intended
benefits; and the application programming interface design used for coroutines.
A systematic mapping study was performed, to select studies published between
2007 and 2018 which contained original research into the application of
coroutines on resource-constrained platforms. An initial set of 566 candidate
papers were reduced to only 35 after filters were applied, revealing the
following taxonomy. The C & C++ programming languages were used by 22 studies
out of 35. As regards hardware, 16 studies used 8- or 16-bit processors while
13 used 32-bit processors. The four most common use cases were concurrency (17
papers), network communication (15), sensor readings (9) and data flow (7). The
leading intended benefits were code style and simplicity (12 papers),
scheduling (9) and efficiency (8). A wide variety of techniques have been used
to implement coroutines, including native macros, additional tool chain steps,
new language features and non-portable assembly language. We conclude that
there is widespread demand for coroutines on resource-constrained devices. Our
findings suggest that there is significant demand for a formalised, stable,
well-supported implementation of coroutines in C++, designed with consideration
of the special needs of resource-constrained devices, and further that such an
implementation would bring benefits specific to such devices.
