View source for Pthreads

{{short description|Execution model which allows for parallel computing}}{{lowercase}}
In [[computing]], '''POSIX Threads''', commonly known as '''pthreads''' (after its header '''{{mono|<pthread.h>}}'''), is an [[execution model]] that exists independently from a [[programming language]], as well as a [[Parallel computing|parallel execution]] model.  It allows a [[Computer program|program]] to control multiple different flows of work that overlap in time.  Each flow of work is referred to as a ''[[thread (computing)|thread]]'', and creation and control over these flows is achieved by making calls to the POSIX Threads [[API]]. POSIX Threads is an API defined by the [[Institute of Electrical and Electronics Engineers]] (IEEE) standard ''[[POSIX]].1c, Threads extensions (IEEE Std 1003.1c-1995)''.

Implementations of the API are available on many [[Unix-like]] POSIX-conformant [[operating system]]s such as [[FreeBSD]], [[NetBSD]], [[OpenBSD]], [[Linux]], [[macOS]], [[Android (operating system)|Android]],<ref>{{cite web|url=https://android.googlesource.com/platform/bionic/+/10ce969/libc/bionic/pthread.c|title=libc/bionic/pthread.c - platform/bionic - Git at Google|website=android.googlesource.com}}</ref> [[Solaris (operating system)|Solaris]], [[Redox (operating system)|Redox]], and [[AUTOSAR]] Adaptive, typically bundled as a library '''libpthread'''. [[DR-DOS]] and [[Microsoft Windows]] implementations also exist: within the [[Windows Services for UNIX|SFU/SUA]] subsystem which provides a native implementation of a number of POSIX APIs, and also within [[third-party software component|third-party]] packages such as ''pthreads-w32'',<ref>{{cite web|url=http://sources.redhat.com/pthreads-win32/conformance.html|title=Pthread Win-32: Level of standards conformance|date=2006-12-22|access-date=2010-08-29|archive-date=2010-06-11|archive-url=https://web.archive.org/web/20100611172430/http://sources.redhat.com/pthreads-win32/conformance.html|url-status=dead}}</ref> which implements pthreads on top of existing [[Windows API]].

==Contents==
pthreads defines a set of [[C (programming language)|C]] programming language [[data type|types]], [[function (computer science)|functions]] and constants. It is implemented with a <code>pthread.h</code> header and a thread [[Library (computing)|library]].

There are around 100 threads procedures, all prefixed <code>pthread_</code> and they can be categorized into five groups:

*Thread management – creating, joining threads etc.
*[[Mutex]]es
*[[Condition variable]]s
*[[synchronization (computer science)|Synchronization]] between threads using [[Readers–writer lock|read write locks]] and [[Barrier (computer science)|barriers]]
*[[Spinlock]]s<ref>{{cite web |title=pthread.h(0p) — Linux manual page |url=https://www.man7.org/linux/man-pages/man0/pthread.h.0p.html |access-date=18 December 2022}}</ref>

The POSIX [[semaphore (programming)|semaphore]] API works with POSIX threads but is not part of the 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 <code>sem_</code> instead of <code>pthread_</code>.

==Example==
An example illustrating the use of pthreads in C:

<syntaxhighlight lang="c">
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

#define NUM_THREADS 5

void* perform_work(void* arguments){
    int index = *((int*)arguments);
    int sleep_time = 1 + rand() % NUM_THREADS;
    printf("Thread %d: Started.\n", index);
    printf("Thread %d: Will be sleeping for %d seconds.\n", index, sleep_time);
    sleep(sleep_time);
    printf("Thread %d: Ended.\n", index);
    return NULL;
}

int main(void) {
    pthread_t threads[NUM_THREADS];
    int thread_args[NUM_THREADS];
    int result_code;
  
    //create all threads one by one
    for (int i = 0; i < NUM_THREADS; i++) {
        printf("In main: Creating thread %d.\n", i);
        thread_args[i] = i;
        result_code = pthread_create(&threads[i], NULL, perform_work, &thread_args[i]);
        assert(!result_code);
    }

    printf("In main: All threads are created.\n");

    // wait for each thread to complete
    for (int i = 0; i < NUM_THREADS; i++) {
        result_code = pthread_join(threads[i], NULL);
        assert(!result_code);
        printf("In main: Thread %d has ended.\n", i);
    }

    printf("Main program has ended.\n");
    return 0;
}

</syntaxhighlight>

This program creates five threads, each executing the function ''perform_work'' 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]]. This program can be compiled using the [[GNU Compiler Collection|gcc]] compiler with the following command:
 gcc pthreads_demo.c -pthread -o pthreads_demo
Here is one of the many possible outputs from running this program.
<syntaxhighlight lang="output">
In main: Creating thread 0.
In main: Creating thread 1.
In main: Creating thread 2.
In main: Creating thread 3.
Thread 0: Started.
In main: Creating thread 4.
Thread 3: Started.
Thread 2: Started.
Thread 0: Will be sleeping for 3 seconds.
Thread 1: Started.
Thread 1: Will be sleeping for 5 seconds.
Thread 2: Will be sleeping for 4 seconds.
Thread 4: Started.
Thread 4: Will be sleeping for 1 seconds.
In main: All threads are created.
Thread 3: Will be sleeping for 4 seconds.
Thread 4: Ended.
Thread 0: Ended.
In main: Thread 0 has ended.
Thread 2: Ended.
Thread 3: Ended.
Thread 1: Ended.
In main: Thread 1 has ended.
In main: Thread 2 has ended.
In main: Thread 3 has ended.
In main: Thread 4 has ended.
Main program has ended.
</syntaxhighlight>

== POSIX Threads for Windows ==
Windows does not support the pthreads standard natively, therefore the Pthreads4w project seeks to provide a portable and open-source [[Wrapper library|wrapper]] implementation. It can also be used to port [[Unix]] software (which uses pthreads) with little or no modification to the Windows platform.<ref>{{cite web |url=http://world.std.com/~jmhart/opensource.htm |title=Experiments with the Open Source Pthreads Library and Some Comments |last=Hart |first=Johnson M. |date=2004-11-21 |access-date=2010-08-29 |url-status=dead |archive-url=https://web.archive.org/web/20100830235615/http://world.std.com/~jmhart/opensource.htm |archive-date=2010-08-30 }}</ref> Pthreads4w version 3.0.0<ref>[https://sourceforge.net/projects/pthreads4w/files/ File: pthreads4w-code-v3.0.0.zip – Source for pthreads4w v3.0.0]</ref> or later, released under the Apache Public License v2.0, is compatible with 64-bit or 32-bit Windows systems. Version 2.11.0,<ref>[https://sourceforge.net/projects/pthreads4w/files/ File: pthreads4w-code-v2.11.0.zip – Source for pthreads4w v2.11.0]</ref> released under the LGPLv3 license, is also 64-bit or 32-bit compatible.

The [[Mingw-w64]] project also contains a wrapper implementation of 'pthreads, '''winpthreads''', which tries to use more native system calls than the Pthreads4w project.<ref>see http://locklessinc.com/articles/pthreads_on_windows which is where it was originally derived from</ref>

[[Interix]] environment subsystem available in the [[Windows Services for UNIX|Windows Services for UNIX/Subsystem for UNIX-based Applications]] package provides a native port of the pthreads API, i.e. not mapped on Win32 API but built directly on the operating system [[syscall]] interface.<ref>{{cite web |url=https://technet.microsoft.com/en-us/library/bb496994.aspx |title=Chapter 1: Introduction to Windows Services for UNIX 3.5|date=5 December 2007 }}</ref>

==See also==
* [[Runtime system]]
* [[OpenMP]]
* [[Cilk]]/[[Cilk Plus]]
* [[Threading Building Blocks]] (TBB)
* [[Native POSIX Thread Library]] (NPTL)
* [[DCEThreads]]
* [[clone (Linux system call)]]
* [[Spurious wakeup]]
* [[Thread-local storage]]
* [[GNU Portable Threads]]
* [[Grand Central Dispatch]] (Apple's thread library)
* [[State Threads]], an event driven approach to threading

==References==
{{reflist|2}}

==Further reading==
*{{cite book |author=David R. Butenhof|title =Programming with POSIX Threads| publisher= Addison-Wesley | isbn= 978-0-201-63392-4|year =1997}}
*{{cite book |author1=Bradford Nichols |author2=Dick Buttlar |author3=Jacqueline Proulx Farell |title=Pthreads Programming |publisher=O'Reilly & Associates |isbn=978-1-56592-115-3 |date=September 1996 |url-access=registration |url=https://archive.org/details/pthreadsprogramm00nich }}
*{{cite book | author= Charles J. Northrup | title= Programming with UNIX Threads | publisher= John Wiley & Sons | isbn= 978-0-471-13751-1 | date= 1996-01-25 | url-access= registration | url= https://archive.org/details/programmingwithu0000nort }}
*{{cite book  |author1=Kay A. Robbins  |author2=Steven Robbins  |name-list-style=amp  |title=UNIX Systems Programming  |publisher=Prentice-Hall  |isbn=978-0-13-042411-2  |year=2003  |url-access=registration  |url=https://archive.org/details/unixsystemsprogr0000robb  }}

== External links ==
* [http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pthread.h.html The Open Group Base Specifications Issue 7, IEEE Std 1003.1]
{{Parallel Computing}}

{{DEFAULTSORT:Posix Threads}}
[[Category:C POSIX library]]
[[Category:Parallel computing]]
[[Category:Threads (computing)]]