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,[1] which implements pthreads on top of existing Windows API.
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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:
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".
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.
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.[2] With some additional patches the last version 2.8.0 is compatible with 64-bit Windows systems.[3][4]
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.[5]
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