Our new paper, “Structured Reactive Programming with Céu”, will appear in the Workshop on Reactive and Event-based Languages & Systems (REBLS’14) at SPLASH.
REBLS succeeds REM as the new “reactive workshop” inside SPLASH.
The paper discusses our proposal for “organisms” as an abstraction mechanism for “Structured (Synchronous) Reactive Programming (SSRP)”. It is the textual version of the talk at the Strange Loop.
The preprint version of the paper can be downloaded here.
I’ll present the last advances in Céu at the Future Programming Workshop in two events:
I’m specially excited about the StrangeLoop event, as it will be the first non-academic demonstration of the language in a prestigious conference.
Here’s the abstract:
Francisco Sant’Anna “Dynamic Organisms in Céu”
We propose a new abstraction mechanism for a synchronous reactive programming language: Organisms reconcile objects and threads into a single concept. We enforce lexical scope for dynamic instances and also restrict the use of explicit references to organisms. This way, we can eliminate typical issues in dynamic allocation, such as memory leaks, danging pointers, and garbage collection.
An Interrupt Service Routine (ISR) executes in reaction to an asynchronous hardware request, interrupting the ongoing computation in the CPU.
As an example, in an Arduino, whenever the USART subsystem receives a byte from the serial line, the CPU execution is redirected to the “USART_RX interrupt vector”, which is a predefined memory address containing the ISR to handle the byte received.
Only after the ISR returns that the interrupted computation resumes.
ISRs are often associated with a high-priority functionality that cannot wait long.
Complementing the USART example, if the execution of the ISR is too much delayed, some received bytes can be lost.
Likewise, the execution of an ISR should never take long, because other interrupts will not trigger in the meantime (although it is possible to nest ISRs).
For this reason, a typical USART ISR simply stores received bytes in a buffer so that the program can handle them afterwards.
ISRs in Céu:
Céu has primitive support for ISRs, which are declared similarly to functions.
However, instead of a name identifier, an ISR declaration requires a number that refers to the index in the interrupt vector for the specific platform.
When an interrupt occurs, not only the ISR executes, but Céu also enqueues the predefined event OS_INTERRUPT passing the ISR index.
This mechanism allows the time-critical operation associated with the interruption to be handled in the ISR, but encourage non-critical operations to be postponed and respect the event queue, which might already be holding events that occurred before the interruption.
The code snippets that follow is part of an USART driver for the Arduino.
The driver emits a READ output event to signal a received byte to other applications (i.e. they are awaiting READ).
The ISR just hold incoming bytes in a queue, while the main body is responsible for signaling each byte to all applications (in a lower priority).
/* variables to manage the buffer */ var byte[SZ] rxs; // buffer to hold received bytes var u8 rx_get; // position to get the oldest byte var u8 rx_put; // position to put the newest byte atomic do rx_get = 0; // initialize get/put rx_put = 0; // the `atomic´ block disables interrupts end /* ISR for receiving byte (index "20" in the manual) */ function isr do var u8 put = (rx_put + 1) % SZ; // next position var byte c = _UDR0; // receive the byte if put != rx_get then // check buffer space rxs[rx_put] = c; // save the received byte rx_put = put; // update to the next position end end /* DRIVER body: receive bytes in a loop */ output byte READ; // the driver outputs received bytes to applications loop do var int idx = await OS_INTERRUPT until idx==20; // USART0_RX_vect var byte c; // hold the received byte ... atomic do // protect the buffer manipulation new interrupts c = rxs[rx_get]; // get the next byte rx_get = (rx_get + 1) % SZ; // update to the next position end emit READ => c; // signal other applications ... end
Note how the real-time/high-priority code to store received bytes in the buffer runs in the ISR, while the code that processes the buffer and signal other applications runs in the body of the driver after every occurrence of OS_INTERRUPT.
Given that ISRs share data with and abruptly interrupt the normal execution body, some sort of synchronization between them is necessary.
As a matter of fact, Céu tracks all variables that ISRs access and enforces all other accesses (outside them, in the normal execution body) to be protected with
Céu provides primitive support for handling interrupt requests:
- An ISR is declared similarly to a function, but specifies the interrupt vector index associated with it.
- An ISR should only execute hard real-time operations, leaving lower priority operations to be handled in reaction to the associated OS_INTERRUPT event.
- The static analysis enforces the use of
atomicblocks for memory shared between ISRs and the normal execution body.
The new version of Céu is released!
What’s new in v0.7:
- Added support for functions and class methods (we relied on C for defining new functions).
- Experimental support for interrupt service routines.
- Experimental microkernel for Arduino.
- Experimental `input/output´ requests.
Céu in a Box (CiB) is a Linux-based distribution that comes pre-installed with the Céu Programming Language environment:
CiB is distributed as a single .ova file to be used with VirtualBox.