Updated: 11 December 1998

Guide to DECthreads

Order Number: AA--QSBPC--TE

This guide reviews the principles of multithreaded programming, as reflected in the IEEE POSIX 1003.1c-1995 standard, and provides implementation guidelines and reference information for DECthreads, Compaq's Multithreading Run-Time Library.

Revision/Update Information: This manual supersedes the Guide to DECthreads, Version 7.1.

Software Version: OpenVMS Alpha Version 7.2
OpenVMS VAX Version 7.2

Compaq Computer Corporation
Houston, Texas

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Contents

Index

Intended Audience

This guide is for system and application programmers who use DECthreads to create multithreaded applications or to create thread-safe code libraries that can be called from single-threaded or multithreaded applications.

Document Structure

This guide consists of the following:

Part 1

• Chapter 1 provides a brief overview of multithreaded programming.
• Chapter 2 discusses the concepts and techniques related to DECthreads.
• Chapter 3 describes thread disciplines and coding issues you may face when writing a multithreaded program.
• Chapter 4 addresses writing thread-safe libraries.
• Chapter 5 introduces and provides conventions for the modular use of the DECthreads exception package.
• Chapter 6 contains an example demonstrating how to call DECthreads routines from a C language program.

Part 2

• This part provides detailed reference information on each pthread interface routine. Routine descriptions appear in alphabetical order by routine name.

Part 3

• This part provides detailed reference information on each tis interface routine. Routine descriptions appear in alphabetical order by routine name.

Part 4 - Appendixes

• Appendix A discusses DECthreads issues and restrictions specific to Compaq's DIGITAL UNIX systems.
• Appendix B discusses DECthreads issues and restrictions specific to OpenVMS systems.
• Appendix C discusses DECthreads issues and restrictions specific to the Microsoft Win32 interfaces on Windows NT systems.
• Appendix D discusses debugging issues for a multithreaded program that uses DECthreads.
• Appendix E summarizes the differences between the Compaq proprietary DECthreads CMA (or cma) interface and the DECthreads pthread interface. Use this appendix to help you migrate your programs and applications to the pthread interface.
• Appendix F summarizes the differences between the DECthreads POSIX 1003.4a/Draft 4 (or d4) interface and the DECthreads pthread interface. Use this appendix to help you migrate your programs and applications to the pthread interface.

Glossary

• The Glossary contains definitions of terms used in this guide, listed alphabetically.

Related Documents

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Conventions

VMScluster systems are now referred to as OpenVMS Cluster systems. Unless otherwise specified, references to OpenVMS Clusters or clusters in this document are synonymous with VMSclusters.

The following conventions are used in this manual:

...	A horizontal ellipsis in examples indicates one of the following possibilities: Additional optional arguments in a statement have been omitted. The preceding item or items can be repeated one or more times. Additional parameters, values, or other information can be entered.
. . .	A vertical ellipsis indicates the omission of items from a code example or command format; the items are omitted because they are not important to the topic being discussed.
( )	In command format descriptions, parentheses indicate that you must enclose the options in parentheses if you choose more than one.
[ ]	In command format descriptions, brackets indicate optional elements. You can choose one, none, or all of the options. (Brackets are not optional, however, in the syntax of a directory name in an OpenVMS file specification or in the syntax of a substring specification in an assignment statement.)
[|]	In command format descriptions, vertical bars separating items inside brackets indicate that you choose one, none, or more than one of the options.
{ }	In command format descriptions, braces indicate required elements; you must choose one of the options listed.
text style	This text style represents the introduction of a new term or the name of an argument, an attribute, or a reason. In the HTML version of this document, this convention appears as italic text.
italic text	Italic text indicates important information, complete titles of manuals, or variables. Variables include information that varies in system output (Internal error number), in command lines (/PRODUCER= name), and in command parameters in text (where dd represents the predefined code for the device type).
UPPERCASE TEXT	Uppercase text indicates a command, the name of a routine, the name of a file, or the abbreviation for a system privilege.
Monospace type	Monospace type indicates code examples and interactive screen displays. In the C programming language, monospace type in text identifies the following elements: keywords, the names of independently compiled external functions and files, syntax summaries, and references to variables or identifiers introduced in an example.
-	A hyphen at the end of a command format description, command line, or code line indicates that the command or statement continues on the following line.
numbers	All numbers in text are assumed to be decimal unless otherwise noted. Nondecimal radixes---binary, octal, or hexadecimal---are explicitly indicated.

This chapter introduces the concepts of threads and multithreaded programming. It describes four functional models that can be a basis for constructing multithreaded applications. The concepts and techniques introduced here are described in more detail in Chapter 2 and in this guide's platform-specific appendixes.

This chapter's last section introduces the components of the DECthreads package, in particular the pthread and tis interfaces, and how those components support building multithreaded applications and thread-safe libraries.

1.1 Advantages of Using Threads

Multithreaded programming means organizing and coding a program so that instances of its routines, called threads, can execute concurrently in the same process. You use threads to improve a program's performance---that is, its throughput, computational speed, responsiveness, or some combination.

Using threads can improve a program's performance on uniprocessor systems by permitting the overlap of input, output, or other slow operations with computational operations. Threads are useful in driving slow devices such as disks, networks, terminals, and printers. A multithreaded program can perform other useful work while waiting for the device to produce its next event, such as the completion of a disk transfer or the receipt of a packet from the network.

Using threads can also be advantageous when constructing an application's user interface. Consider the typical arrangement of a window system. Each time the user invokes an action (for example, by clicking on a mouse button), the program can use a separate thread to implement the action. If the user invokes multiple actions, multiple threads can perform the actions in parallel.

Using threads is especially advantageous when building a distributed system. These systems frequently contain a shared network server, where the server services requests from multiple clients. Using multiple threads allows the server to handle clients' requests in parallel, instead of artificially serializing them or creating (at great expense) one server process per client.

A program with multiple threads can be especially suited to run on a multiprocessor system, where threads run concurrently on separate processors. Threads created using the DECthreads library are capable of utilizing multiprocessors, if the target platform supports parallelism within a process. Compaq's DIGITAL UNIX platforms and OpenVMS Alpha platforms support parallelism; the OpenVMS VAX platform does not support parallelism.

1.2 Overview of Threads

A thread is a single, sequential flow of control within a process. Within each thread there is a single point of execution. Most traditional programs execute as a process with a single thread. Figure 1-1 and Figure 1-2 show the differences between a single-threaded process and a multithreaded process.

Figure 1-1 Single-Threaded Process

In Figure 1-2, notice that multiple threads share heap storage, static storage, and code but that each thread has its own register set and stack.

Using DECthreads, Compaq's multithreading run-time library, a programmer can create several threads within a process. The process's threads execute concurrently. Within a multithreaded program there are at any time multiple points of execution.

Threads execute within (and share) a single address space; therefore, a process's threads can read and write the same memory locations. When the threads access the same memory locations, your program must use synchronization elements, such as mutexes and condition variables, to ensure that the shared memory is accessed correctly. DECthreads provides routines that allow you to use these and other synchronization objects. Section 2.4 describes the synchronization objects that DECthreads offers as well as the operations your program can perform on them.

Figure 1-2 Multithreaded Process

You should design and code a multithreaded program with the assumption that its threads execute simultaneously. That is, your program cannot make assumptions about the relative start or finish times of its threads or the sequence in which they execute. These are governed by the DECthreads thread scheduler, part of the run-time environment that DECthreads establishes before your program begins running. Nevertheless, your program can influence how DECthreads schedules its threads, by setting each thread's scheduling policy and scheduling priority. ( Section 2.3.6 describes how thread scheduling works.)

Each thread has its own thread identifier, which distinguishes it from all other threads in the process. In addition to the thread's scheduling policy and scheduling priority, each thread is associated with any thread-specific instances of thread-common data objects and with thread-specific system resources to support a flow of control.

A thread changes its state over the course of its execution. A thread is in one of the following states:

• Waiting---The thread is not eligible to execute, because it is synchronizing with another thread or with an external event, such as I/O.
• Ready---The thread is eligible to be executed by a processor.
• Running---The thread is currently being executed by a processor.
• Terminated---The thread has completed all of its work or has been canceled.

Figure 1-3 shows the transitions between states for a typical thread implementation.

Figure 1-3 Thread State Transitions

The following sections describe four functional models of processing information that are especially well suited for implementation in multithreaded programs:

• Boss/worker model
• Work crew model
• Pipelining model
• Combination of models

In a boss/worker model, one thread functions as the "boss" because it assigns tasks for "worker" threads to perform. Each worker performs a distinct task until it has finished, at which point it notifies the boss that it is ready to receive another task. Alternatively, the boss polls workers periodically to see whether any is ready to receive another task.

A variation of the boss/worker model is the work queue model. The boss places tasks in a queue, and workers check the queue and take tasks to perform.

An example of the work queue model in an office environment is a secretarial typing pool. The office manager boss puts documents to be typed in a basket, and worker typists take documents from the basket to work on.

1.4.2 Work Crew Model

In the work crew model, multiple threads work together on a single task. The task is divided into pieces that are performed in parallel, and each thread performs one piece.

An example of a work crew is a group of people cleaning a building. Each person cleans certain rooms or performs certain types of work (washing floors, polishing furniture, and so forth), and each works independently.

In a multithreaded program that reflects the work crew model, each thread executes a task that can be performed in parallel. Figure 1-4 shows a task performed by three threads in a work crew model.

Figure 1-4 Work Crew Model of Thread Operation

In the pipelining model, a task is divided into steps. The steps must be performed in sequence to produce a single instance of the desired result, and the work done in each step (except for the first and last) is based on the previous step and is a prerequisite for the work in the next step. However, the goal is to produce multiple instances of the desired result, and the steps are designed to operate in parallel: while one step is performed on one instance of the result, the preceding step can be performed on the next instance of the result.

An example of the pipelining model is an automobile assembly line. Each step or stage in the assembly line is continually busy receiving the product of the previous stage's work, performing its assigned work, and passing the product along to the next stage.

In a multithreaded program that reflects the pipelining model, each thread executes a step in the task. Figure 1-5 shows a task performed by three threads in a pipelining model.

Figure 1-5 Pipelining Model of Thread Operation

If the task that your program performs is complex, you might find it appropriate to organize it as a combination of the functional models previously described. For example, a program could follow the pipelining model, but with one or more steps performed by a set of threads that follow a work crew model. In addition, threads could be assigned to a work crew by taking a task from a work queue and deciding (based on the task characteristics) which threads are needed for the work crew.

1.5 Potential Issues for Multithreaded Programs

When you design and code a multithreaded program, you must accommodate or eliminate, as appropriate, each of the following issues:

• Program complexity is the most significant issue to consider in any multithreaded programming effort. Although using threads can simplify the coding and designing of a program, a certain level of expertise is required to be sure that the design of the synchronization and interplay among threads is appropriate and correctly specified. This level of expertise is higher than that required to design most single-threaded programs.
• Dependence upon other nonreentrant software means that your multithreaded program calls a routine or library that is not equipped to deal with threads. Given this dependence, your program must use the DECthreads global lock to prevent conflicts with other threads that use the same nonreentrant routine or library. Section 3.9 presents multithreaded programming techniques for managing dependencies upon other nonreentrant software.
• Due to programming errors, race conditions in the program's behavior can cause unpredictable and erroneous program behavior. Similarly, deadlocks can cause two or more threads to be blocked from executing indefinitely. Section 3.6.2 discusses race conditions in more detail, and Section 3.6.3 discusses deadlocks.
• Priority inversion prevents high-priority threads from executing when interdependencies exist among three or more threads of different priorities. Section 3.5.2 discusses techniques for avoiding priority inversion.

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