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pthreads

Implementations of the API are available on many Unix-like POSIX-conformant operating systems such as FreeBSD, NetBSD, OpenBSD, Linux, macOS, Android,^[1] Solaris, Redox, and AUTOSAR Adaptive, typically bundled as a library libpthread. 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,^[2] which implements pthreads on top of existing Windows API.

Contents[edit]

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 threads 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
• Spinlocks^[3]

The POSIX 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 sem_ instead of pthread_.

Example[edit]

An example illustrating the use of pthreads in C:

#include <stdio.h>
#include <stdlib.h>
#include <assert.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 i;
  int result_code;
  
  //create all threads one by one
  for (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 (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;
}

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 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.

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.

POSIX Threads for Windows[edit]

Windows does not support the pthreads standard natively, therefore the Pthreads4w project seeks to provide a portable and open-source wrapper implementation. It can also be used to port Unix software (which uses pthreads) with little or no modification to the Windows platform.^[4] Pthreads4w version 3.0.0^[5] or later, released under the Apache Public License v2.0, is compatible with 64-bit or 32-bit Windows systems. Version 2.11.0,^[6] 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.^[7]

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 API but built directly on the operating system syscall interface.^[8]

See also[edit]

• 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)
• Beginthread (a subroutine within Windows for creating a new thread and Unix thread)
• State Threads, an event driven approach to threading

References[edit]

Further reading[edit]

• David R. Butenhof (1997). Programming with POSIX Threads. Addison-Wesley. ISBN 978-0-201-63392-4.
• Bradford Nichols; Dick Buttlar; Jacqueline Proulx Farell (September 1996). Pthreads Programming. O'Reilly & Associates. ISBN 978-1-56592-115-3.
• Charles J. Northrup (1996-01-25). Programming with UNIX Threads. John Wiley & Sons. ISBN 978-0-471-13751-1.
• Kay A. Robbins & Steven Robbins (2003). UNIX Systems Programming. Prentice-Hall. ISBN 978-0-13-042411-2.

External links[edit]

• The Open Group Base Specifications Issue 7, IEEE Std 1003.1