POSIX Threads

POSIX Threads, usually referred to as Pthreads, is a POSIX standard for threads. The standard, POSIX.1c, Threads extensions (IEEE Std 1003.1c-1995), defines an API for creating and manipulating threads.

Implementations of the API are available on many Unix-like POSIX-conformant operating systems such as FreeBSD, NetBSD, OpenBSD, GNU/Linux, Mac OS X and Solaris. DR-DOS and Microsoft Windows implementations also exist: within the SFU/SUA subsystem which provides a native implementation of a number of POSIX APIs, and also within third-party packages such as pthreads-w32, which implements pthreads on top of existing Windows API.

Contents

Pthreads defines a set of C programming language types, functions and constants. It is implemented with a pthread.h header and a thread library.

There are around 100 Pthreads procedures, all prefixed "pthread_" and they can be categorized into four groups:

• Thread management - creating, joining threads etc.
• Mutexes
• Condition variables
• Synchronization between threads using read/write locks and barriers

The POSIX semaphore API works with POSIX threads but is not part of threads standard, having been defined in the POSIX.1b, Real-time extensions (IEEE Std 1003.1b-1993) standard. Consequently the semaphore procedures are prefixed by "sem_" instead of "pthread_".

Compiling this using the standard GCC compiler requires the use of the command line operator "-pthread" to compile, such that a line to compile might read: "gcc -pthread pthread_program.c".

Example

An example illustrating the use of Pthreads in C:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
 
#define NUM_THREADS     5
 
void *TaskCode(void *argument)
{
   int tid;
 
   tid = *((int *) argument);
   printf("Hello World! It's me, thread %d!\n", tid);
 
   /* optionally: insert more useful stuff here */
 
   return NULL;
}
 
int main(void)
{
   pthread_t threads[NUM_THREADS];
   int thread_args[NUM_THREADS];
   int rc, i;
 
   /* create all threads */
   for (i=0; i<NUM_THREADS; ++i) {
      thread_args[i] = i;
      printf("In main: creating thread %d\n", i);
      rc = pthread_create(&threads[i], NULL, TaskCode, (void *) &thread_args[i]);
      assert(0 == rc);
   }
 
   /* wait for all threads to complete */
   for (i=0; i<NUM_THREADS; ++i) {
      rc = pthread_join(threads[i], NULL);
      assert(0 == rc);
   }
 
   exit(EXIT_SUCCESS);
}

This program creates five threads, each executing the function TaskCode that prints the unique number of this thread to standard output. If a programmer wanted the threads to communicate with each other, this would require defining a variable outside of the scope of any of the functions, making it a global variable.

POSIX Threads for Windows

Windows does not support the pthreads standard natively, therefore the Pthreads-w32 project seeks to provide a portable and open-source implementation. It can be also used to port Unix software (which use pthreads) with little or no modification to the Windows platform. With some additional patches the last version 2.8.0 is compatible with 64-bit Windows systems.

Interix environment subsystem available in the Windows Services for UNIX/Subsystem for UNIX-based Applications package provides a native port of the pthreads API, i.e. not mapped on Win32/Win64 API but built directly on the operating system syscall interface.

See also

• OpenMP
• Native POSIX Thread Library (NPTL)
• DCEThreads
• clone (Linux system call)
• Spurious wakeup
• Thread-local storage
• GNU Portable Threads
• FSU Pthreads
• Grand Central Dispatch (Apple's thread library)
• Beginthread (a subroutine within Windows for creating a new thread and unix thread)

Further reading

• David R. Butenhof. Programming with POSIX Threads. Addison-Wesley. ISBN 0-201-63392-2. 
• Bradford Nichols, Dick Buttlar, Jacqueline Proulx Farell. Pthreads Programming. O'Reilly & Associates. ISBN 1-56592-115-1. 
• Charles J. Northrup. Programming with UNIX Threads. John Wiley & Sons. ISBN 0-471-13751-0. 
• Kay A. Robbins and Steven Robbins. UNIX Systems Programming. Prentice-Hall. ISBN 0-13-042411-0. 

External links

• Pthreads Tutorial
• C/C++ Tutorial: using Pthreads
• Article "POSIX threads explained" by Daniel Robbins (Gentoo Linux founder)
• Interview "Ten Questions with David Butenhof about Parallel Programming and POSIX Threads" by Michael Suess
• Open Source POSIX Threads for Win32
• The Open Group Base Specifications Issue 6, IEEE Std 1003.1
• Pthreads Presentation at 2007 OSCON (O'Reilly Open Source Convention) by Adrien Lamothe. An overview of Pthreads with current trends.
• Comparing programmability of Open MP and pthreads