Here are some resources. Before we begin, there are a few required steps you need to take before starting any pthreads coding:. To create a thread, the following function is available:. Before we dive into an example, let's first look at two other important thread functions:. Let's look at an example program exercising the above pthread functions:.
c - Implementing pthread barrier for Mac OS/X - Code Review Stack Exchange
The syntax of the third and fourth argument are particularly important. Of course, you can pass simple data types as pointers if that is all that is needed, or NULL if no arguments are required. However, it is good practice to be able to pass arguments of arbitrary type and size, and is thus illustrated for this purpose. A few things to mention:.
Threads can be assigned various thread attributes at the time of thread creation. Attributes can be retrieved via complimentary get functions. Consult the man pages for the effect of each of these attributes.
Attributes for the mutex can be given through the second parameter. To specify default attributes, pass NULL as the second parameter. Alternatively, mutexes can be initialized to default values through a convenient macro rather than a function call:. Here a mutex object named lock is initialized to the default pthread mutex values.
To perform mutex locking and unlocking, the pthreads provides the following functions:. Each of these calls requires a reference to the mutex object. It is absolutely essential to check the return value of the trylock call to determine if the mutex has been successfully acquired or not.
Let's expand the previous example with code that uses mutexes:. Note that we continue to maintain the lock even through the printf function call as releasing the lock before this and printing can lead to inconsistent results in the output. Recall that the code in-between the lock and unlock calls is called a critical section. Critical sections should be minimized for increased concurrency.
Similar to the mutex initialization call, condition variables can be given non-default attributes through the second parameter. Threads can act on condition variables in three ways: wait, signal or broadcast:. It requires a mutex of the associated shared resource value it is waiting on. Here is an example on using pthread condition variables:. This is because of spurious wakeups problem mentioned previously. Locks that other threads could be waiting on should be released before you signal or broadcast.
Before a barrier can be called, a pthread barrier object must be initialized first:. Barrier objects are initialized like mutexes or condition variables, except there is one additional parameter, count. The count variable defines the number threads that must join the barrier for the barrier to reach completion and unblock all threads waiting at the barrier. If default barrier attributes are used i.
NULL for the second parameter , one can use the initializer macro with the specified count. The actual barrier call follows:. This function would be inside thread code where the barrier is to take place. The performance gains from using threads can be substantial when done properly and in the right problem context, but can it be even better? You should consider the following when analyzing your program for potential bottlenecks:. There are various template libraries available that ease implementation of multithreading in a semi- portable fashion. This tutorial has explored the very basics of multithreaded programming.
What about multiprocess programming?
These topics are beyond the scope of this document, but to perform cross-process synchronization, one would use some form of IPC: pipes, semaphores, message queues, or shared memory. Of all of the forms of IPC, shared memory is usually the fastest excluding doors. Interprocess first. OpenMP is a portable interface for implementing fork-join parallelism on shared memory multi-processor machines.
For a quick introduction, please see the slides here. It is not uncommon to see programs that are both multithreaded and contain MPI calls to take advantage of shared memory within a node and MPI to perform processing across nodes. It is difficult to cover more than an introduction to threads with this short tutorial and overview. There are many excellent online resources regarding pthreads on the web. Use your favorite search engine to find these. Notice: Please do not replicate or copy these pages and host them elsewhere. This is to ensure that the latest version can always be found here.
Disclaimer: The document author has published these pages with the hope that it may be useful to others. However, the document author does not guarantee that all information contained on these webpages are correct or accurate. There is no warranty, expressed or implied, of merchantability or fitness for any purpose. The author does not assume any liability or responsibility for the use of the information contained on these webpages.
If you see an error, please send an email to the address below indicating the error. Your feedback is greatly appreciated and will help to continually improve these pages. Table of Contents: 1. Introduction 2. What is a Thread? Thread Design Patterns 4.
No, they're not. Hell, on my debian box they don't even render correctly. Here's the OpenBSD pthread docs. While I think the documenation is pretty complete, it may not cover everything and may have some OBSD-specific implementation details in there. This still leaves the condition where if the last thread hangs, the parent will too or if all threads hang.
I really appreciate the replies. Part of this is that MacOs is built on an underlying Mach scheduler and pthreads are described as "a light wrapper" for the underlying scheduler.
I know that scheduler somewhat better not really well, all my real experience is with real-time schedulers in embedded stuff. But this project which is the MP stuff for the Ars Testbench incidentally needs to be clean, cross-platform posix threads One of my worries about this is that some function that I pick here won't be supported on other target systems On what platforms might it not be supported?
Can I assume that it is supported on "all reasonable" Linux distros? If it is a guaranteed attribute, then "If a priority N RR task is runnable, no task at lower priority will ever get a timeslice. This implies that the way to do it is to create a "severed" thread that is at high priority, but which is set to sleep Unfortunately, you're diving into a badly underspecified domain when you start talking about priorities and quasi-real-time scheduling.
When POSIX was meeting to specify the stuff, all the vendors had their own proprietary thread layers, and nobody wanted to change their stuff enough to make pthreads really portable. Also, Darwin does seem to be missing things that I'm accustomed to on Linux not that I've done that much thread programming , at least in the manpages. Yep, you're right Mach of course, but what is that doing and why didn't they call it RR, for threads a this level? What am I doing wrong?