Rendezvous facilities: Concurrent C and the Ada language

The concurrent programming facilities in both Concurrent C and the Ada language are based on the rendezvous concept. Although these facilities are similar, there are substantial differences. Facilities in Concurrent C were designed keeping in perspective the concurrent programming facilities in the Ada language and their limitations. Concurrent C facilities have also been modified as a result of experience with its initial implementations. The authors compare the concurrent programming facilities in Concurrent C and Ada and show that it is easier to write a variety of concurrent programs in Concurrent C than in Ada     

Concurrent programming in the Ada® language: The polling bias

The rendezvous is an important concept in concurrent programming—two processes need to synchronize, i.e. rendezvous, to exchange information. The Ada programming language is the first programming language to use the rendezvous as the basis of its concurrent programming facilities. Our experience with rendezvous facilities in the Ada language shows that these facilities lead to and encourage the design of programs that poll. Polling is generally, but not always, undesirable because it is wasteful of system resources. We illustrate and examine the reasons for polling bias in the Ada language. We give suggestions on how to avoid polling programs, and suggest changes to the rendezvous facilities to eliminate the polling bias. The ramifications of these changes to the implementation of the Ada language are also discussed. Although we have focused on the rendezvous facilities in the Ada language our analysis is also applicable to other languages. A polling bias can occur in any concurrent programming language based on the rendezvous mechanism if it does not provide appropriate facilities.     

Coping with Java threads

A thread is a basic unit of program execution that can share a single address space with other threads - that is, they can read and write the same variables and data structures. Originally, only assembly programmers used threads. A few older programming languages such as PL/I supported thread concurrency, but newer languages such as C and C++ use libraries instead. Only recently have programming languages again begun to build in direct support for threads. Java and Ada are examples of industry-strength languages for multithreading. The Java thread model has its roots in traditional concurrent programming. As the "real-time specification for Java" sidebar describes, RTSJ attempts to remove some of the limitations relative to real-time applications - primarily by circumventing garbage collection. But RTSJ does not make the language safer. It retains standard Java's threading pitfalls and is a risky candidate for critical concurrent applications.     

Parallel software development in the DISC programming environment

This paper describes the architecture of DISC, a system for parallel software development. The system is designed for programming computer systems having several autonomous units, not memory-sharing, and linked by means of a communication network.

The system consists of three parts. The concurrent programming language DISC (DIStributed C), which is an extension of the C language based on the concurrent mechanisms envisaged by the CSP computational model. The programming environment, designed to promote software engineering techniques in the development of distributed-programs. The language run-time support, which provides for the distributed execution of programs.     

IAda: A language for robot programming based on Ada

We present a programming language for robots which we have implemented based on the Ada language. It is an interpreted language which permits dynamic configuration of software. It manipulates Ada tasks and subroutines. One of the Ada tasks is an inference engine of a logic programming language adapted to real-time constraints. We show how the conjunction of Ada tasks, to perform perception and action functions on the robot, to logic programs, for the control of these tasks, both manipulated by the IAda language, gives a powerful environment for robot programming.     

Implementing atomic actions in Ada 95

Atomic actions are an important dynamic structuring technique that aid the construction of fault-tolerant concurrent systems. Although they were developed some years ago, none of the well-known commercially-available programming languages directly support their use. This paper summarizes software fault tolerance techniques for concurrent systems, evaluates the Ada 95 programming language from the perspective of its support for software fault tolerance, and shows how Ada 95 can be used to implement software fault tolerance techniques. In particular, it shows how packages, protected objects, requeue, exceptions, asynchronous transfer of control, tagged types, and controlled types can be used as building blocks from which to construct atomic actions with forward and backward error recovery, which are resilient to deserter tasks and task abortion     

The concurrency models of Ada and occam—a practical comparison

A comparison of the concurrency models of Ada and occam is presented. This comparison is performed in terms of an Ada-to-occam compiler. A subset of the Ada programming language may be compiled allowing a study of how programs and algorithms expressed using the Ada concurrency model can be mapped to the occam concurrency model. The resultant occam code may then be executed on a transputer. This paper describes the Ada and occam concurrency models, highlights the major differences between them, discusses the problems encountered in trying to map concurrent Ada programs to equivalent occam programs as a result of these differences and explains how these problems were overcome in the present compiler.     

Steps to an Advanced Ada1Programming Environment

Conceptual simplicity, tight coupling of tools, and effective support of host-target software development will characterize advanced Ada programming support environments. Several important principles have been demonstrated in the Arcturus system, including template-assisted Ada editing, command completion using Ada as a command language, and combining the advantages of interpretation and compilation. Other principles, relating to analysis, testing, and debugging of concurrent Ada programs, have appeared in other contexts. This paper discusses several of these topics, considers how they can be integrated, and argues for their inclusion in an environment appropriate for software development in the late 1980's.     

Using atomic await commands to develop concurrent programs in Java

Systematic techniques for dealing with concurrency can be very helpful in concurrent programming. In his concurrent programming text, Andrews [2] describes a technique for developing concurrent programs based on first developing solutions using course-grained atomic commands and atomic await primitives (which are usually not available in real programming environments), then systematically transforming the solution to an implementation that uses available synchronization primitives. Transformations are known for semaphores, conditional critical regions, and monitors. In this paper, transformations are given for the synchronization constructs provided by the Java programming language [1]. Familiarity with the basic facilities to support concurrency provided by Java is assumed.     

A tactic language for refinement of state-rich concurrent specifications

Circus is a refinement language in which specifications define both data and behavioural aspects of concurrent systems using a combination of Z and CSP. Its refinement theory and calculus are distinctive, but since refinements may be long and repetitive, the practical application of this technique can be hard. Useful strategies have been identified, described, and used, and by documenting them as tactics, they can be expressed and repeatedly applied as single transformation rules. Here, we present ArcAngelC, a language for defining such tactics; we present the language, its semantics, and its application in the formalisation of an existing strategy for verification of Ada implementations of control systems specified by Simulink diagrams. We also discuss its mechanisation in a theorem prover, ProofPower-Z.     

A multiprocessor implementation of joyce

Joyce is a programming language for parallel computers based on CSP and Pascal. A Joyce program defines concurrent agents which communicate through unbuffered channels. This paper describes a multiprocessor implementation of Joyce.     

Implementation of blocking coordinated atomic actions based on forward error recovery

The coordinated atomic action concept was proposed as a means for providing fault tolerance in complex objectoriented systems that incorporate both cooperative and competitive concurrency. This paper has two purposes: to discuss a particular implementation of this concept and to address a number of the implementation issues that are common to any experiments with this concept. Our implementation relies on a detailed set of programming conventions for the standard Ada 95 language and uses a scheme of forward error recovery incorporating concurrent exception handling and resolution. Ada 95 has a number of unique features which make it a particularly good choice for our experiments. We believe that our approach is practical and useful for many critical applications with high dependability requirements.     

Ada95语言评述

Ａｄａ９５语言是在Ａｄａ８３基础上修订而成的，它几乎提供了现代程序设计范型及程序设计实践所需要的一切设施，它可以支持面向对象的程序设计、大型程序设计、实时与并行程序设计等等。     

The Joyce language report

This paper defines a programming language for distributed processing called Joyce. Joyce is based on CSP and Pascal. A Joyce program defines concurrent agents which communicate through synchronous channels. The agents may be recursive.     

A joyce implementation

Joyce is a programming language for distributed systems based on CSP and Pascal. A Joyce program defines concurrent agents which communicate through unbuffered channels. This paper describes a surprisingly simple single-processor implementation of Joyce which runs on an IBM-PC.     

Critical races in Ada programs

It is noted that critical races in concurrent Ada programs are an insidious source of system errors, and that the Ada language does not have concurrency features to ease the development of critical race-free programs. The authors illustrate this using the CRF language, which extends the Ada language by the addition of a powerful caller selection mechanism. It is this facility that implements a discriminatory mutual exclusion (dimutex) construct, an important feature for building less critical race-prone programs. The MLog language supports the CRF caller selection mechanisms; thus it is convenient as a rapid-prototyping platform. A design method for systematically converting CRF designs into Ada designs is presented. The method uses a mixture of mechanical and creative transformations that are guaranteed not to introduce critical races. D. Helmbold and D. Luckham's (1985) gas-station example is used to illustrate the various approaches to applying the design method     

The cyclic executive model and Ada

Periodic processes are major parts of many real-time embedded computer applications. The programming language Ada permits programming simple periodic processes, but it has some serious limitations; producing Ada programs with real-time performance comparable to those produced to date using traditional cyclic executives requires resorting to techniques that are specific to one machine or compiler. We present and evaluate the cyclic executive model for controlling periodic processes. The features and limitations of Ada for programming cyclic executive software are discussed and demonstrated, and some practical techniques for circumventing Ada limitations are described.     

Extensions to the C programming language for enhanced fault detection

The acceptance of the C programming language by academia and industry is partially responsible for the ‘software crisis’. The simple, trusting semantics of C mask many common faults, such as range violations, which would be detected and reported at run-time by programs coded in a robust language such as Ada.

1 Ada is a registered trademark of the U.S. Government (Ada Joint Program Office)

This needlessly complicates the debugging of C programs. Although the assert macro lets programmers add run-time consistency checks to their programs, the number of instantiations of this macro needed to make a C program robust makes it highly unlikely that any programmer could correctly perform the task. We make some unobtrusive extensions to the C language which support the efficient detection of faults at run-time without reducing the readability of the source code. Examples of the extensions are automatic checking of error codes returned by library routines, constrained subtypes and detection of references to uninitialized and/or non-existent array elements.