The POSIX threads library, or pthreads, is a standardized interface for using lightweight threads in C programs on POSIX (Portable Operating System Interface based on Unix) platforms.

Note that C libraries can easily be used in programs written in other compiled languages, so pthreads can be used directly in C++ and Fortran programs. There are also well-developed interfaces for many other languages.

The pthreads system addresses the same basic needs as OpenMP, but using a different approach. Threads are created manually using pthread_create() in much the same way as we would use fork() to create a new process.

/*
 *  Example from Pthreads tutorial at:
 *  https://computing.llnl.gov/tutorials/pthreads/
 */

#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   printf("Hello World! It's me, thread #%ld!\n", tid);
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   for(t=0; t<NUM_THREADS; t++){
      printf("In main: creating thread %ld\n", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d\n", rc);
         exit(-1);
      }
   }
   
   /*
    *   Code here will be run simultaneously by multiple threads.
    */

   /* Last thing that main() should do */
   pthread_exit(NULL);
}