POSIX Programmers Guide

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Before the release of Minix 2. There have been a number of revisions and extensions since It's expensive, however. Even paying to have a PDF document e-mailed to you is quite expensive. You might also do well to look in the engineering library of a local university.

Posix Programmers Guide by Donald Lewine

This chapter provides an overview of the thread technologies available in OS X and iOS along with examples of how to use those technologies in your applications. Threading has a real cost to your program and the system in terms of memory use and performance. The core structures needed to manage your thread and coordinate its scheduling are stored in the kernel using wired memory.


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Most of these structures are created and initialized when you first create the thread—a process that can be relatively expensive because of the required interactions with the kernel. Table quantifies the approximate costs associated with creating a new user-level thread in your application.

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Some of these costs are configurable, such as the amount of stack space allocated for secondary threads. The time cost for creating a thread is a rough approximation and should be used only for relative comparisons with each other. Thread creation times can vary greatly depending on processor load, the speed of the computer, and the amount of available system and program memory.

This memory is used to store the thread data structures and attributes, much of which is allocated as wired memory and therefore cannot be paged to disk. The minimum allowed stack size for secondary threads is 16 KB and the stack size must be a multiple of 4 KB. The space for this memory is set aside in your process space at thread creation time, but the actual pages associated with that memory are not created until they are needed.

Another cost to consider when writing threaded code is the production costs. Making those changes might be necessary to avoid the use of synchronization, which can itself impose a tremendous performance penalty on poorly designed applications.

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Designing those data structures, and debugging problems in threaded code, can increase the time it takes to develop a threaded application. Avoiding those costs can create bigger problems at runtime, however, if your threads spend too much time waiting on locks or doing nothing.

Where Can I Get a Copy of the POSIX Standard?

Creating low-level threads is relatively simple. The following sections show the basic creation process for the more commonly used thread technologies. Threads created using these techniques inherit a default set of attributes, determined by the technology you use.

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For information on how to configure your threads, see Configuring Thread Attributes. There are two ways to create a thread using the NSThread class:. Create a new NSThread object and call its start method.

What is Linux? Unix? POSIX?

Both techniques create a detached thread in your application. It also means that your code does not have to join explicitly with the thread later.


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Because the detachNewThreadSelector:toTarget:withObject: method is supported in all versions of OS X, it is often found in existing Cocoa applications that use threads. The following example shows a basic invocation of this method that spawns a thread using a custom method of the current object. Prior to OS X v Although you could get an NSThread object and access some thread attributes, you could only do so from the thread itself after it was running.

In OS X v This support is also available in iOS. This support made it possible to get and set various thread attributes prior to starting the thread. It also made it possible to use that thread object to refer to the running thread later. This method takes the exact same information as the detachNewThreadSelector:toTarget:withObject: method and uses it to initialize a new NSThread instance. It does not start the thread, however. If you have an NSThread object whose thread is currently running, one way you can send messages to that thread is to use the performSelector:onThread:withObject:waitUntilDone: method of almost any object in your application.

Support for performing selectors on threads other than the main thread was introduced in OS X v The messages you send using this technique are executed directly by the other thread as part of its normal run-loop processing.

see Of course, this does mean that the target thread has to be running in its run loop; see Run Loops. You may still need some form of synchronization when you communicate this way, but it is simpler than setting up communications ports between the threads.

Table of Contents

For a list of other thread communication options, see Setting the Detached State of a Thread. This technology can actually be used in any type of application including Cocoa and Cocoa Touch applications and might be more convenient if you are writing your software for multiple platforms. The LaunchThread function creates a new thread whose main routine is implemented in the PosixThreadMainRoutine function. Marking the thread as detached gives the system a chance to reclaim the resources for that thread immediately when it exits.

If you add the code from the preceding listing to one of your source files and call the LaunchThread function, it would create a new detached thread in your application. Of course, new threads created using this code would not do anything useful. The threads would launch and almost immediately exit. To make things more interesting, you would need to add code to the PosixThreadMainRoutine function to do some actual work.

To ensure that a thread knows what work to do, you can pass it a pointer to some data at creation time. For C-based applications, there are several ways to communicate between threads, including the use of ports, conditions, or shared memory. The performSelectorInBackground:withObject: method creates a new detached thread and uses the specified method as the entry point for the new thread. It only defines the contract between them.


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POSIX-compliant application source code should be able to run across many systems because the standard is defined at the source code level. So the binary executable may not run even on similar machines with identical hardware and operating systems. Only source code portability is addressed in the standard. But developers can implement it in any language they like. The standard only deals with aspects of the operating system that interacts with applications. The standard is kept succinct in terms of length and broad in terms of scope to cover a large array of systems.

POSIX was designed to simplify portability. So it will save time and money in the long run. However, if your applications are not POSIX-compliant, it might require significant time and resource investment at the beginning. Zak H Zak H. View all posts.

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