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CABpp

C++ header-only library implementing a Cyclic Asynchronous Buffer (CAB).

Cyclic Asynchronous Buffer

Cyclic Asynchronous Buffer (CAB) is a communication mechanism used between periodic tasks, first proposed by Prof. D. Clark1 (referred to as Periodic Data Buffer), later redefined by Prof. G. Buttazzo2.

CAB architecture implements a block-free buffer, where reading from it returns the last value written to it and writing to it never blocks.

In order to have non-blocking operations, n_reader + n_writer + 1 slots are allocated. Without the additional slot, the writer could be writing and rewriting to the same slot, without giving the reader a chance to read it before it gets overwritten. When reading, CAB simply uses the last slot used for a successful write operation.

Architecture

In order to achieve non-blocking operations, CAB needs to allocate a number of slots equal to the maximum number of concurrent readers and writers plus one additional slot.

concurrent readers and writers refers to all instances that may be blocking a slot. A slot can be blocked by:

  • not releasing the smart pointer returned by the read function
  • reserving a slot (gets released after the corresponding write function call)
  • writing to the slot

Use

CREATION CABpp allows for three modes of operation:

  • internal object construction
  • external object construction
  • using pre-constructed objects.

READING Reading from CAB returns a smart pointer pointing to the last updated object slot (multiple readers can read from the same slot). The slot is freed only once the smart pointer has been destroyed/released. This allows for repeated access to the same slot without having to recall the read function.

WRITING The writer may:

  • copy a pre-allocated object to a free CAB slot
  • move a pre-allocated object to a free CAB slot
  • reserve a free CAB slot, work directly on the slot's object and update once ready (reserve/write mode).

Reserve/write mode allows the user to work without allocating any additional objects (by using the already allocated objects in the CAB).

Library installation

git clone https://github.com/andrejtonev/cabpp.git
cd cabpp
mkdir build
cd build
cmake ..
make
make install

Run examples

git clone https://github.com/andrejtonev/cabpp.git
cd cabpp
mkdir build
cd build
cmake -DOPENCV_EXAMPLE=ON -DMUTEX_EXAMPLE=ON ..
make
./examples/mutex_example/mutex_example-cabpp
./examples/opencv_example/opencv_example-cabpp

Run tests

git clone https://github.com/andrejtonev/cabpp.git
cd cabpp
mkdir build
cd build
cmake -DGTEST=ON ..
make
make test

Include library in other CMake projects

find_package (CABpp)
target_link_libraries(*** cabpp)

Sources

[1] Clark, D.: “HIC: An Operating System for Hierarchies of Servo Loops”, Proc. of IEEE Int. Conf. on Rob. and Autom., pp. 1004-1009,1989.

[2] Buttazzo, G.C.: "HARTIK: a real-time kernel for robotics applications", Proc. of IEEE Real_Time Systems Symposium, pp. 201–205,1993.

Special thanks to Prof. G. Buttazzo, Gabriele Serra and Gabriele Ara for their helpful remarks.