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http://pages.cs.wisc.edu/~travitch/pthreads_primer.html

Introduction

POSIX threads (pthreads) are a standardized interface onoperating system threads.

Compiling a pthreads Program

Relevant headers aside (they are discussed below), a program wishingto use pthreads must link against the pthreads library. Here is anexample invocation of gcc
demonstrating this:

gcc -pedantic -Wall -o theaded_program src.c -lpthread

The -l flag specifies the name of a library to linkagainst (pthread, in our case); since pthreads is a systemlibrary,gcc knows where to find it.

Creating Threads

Any program using pthreads will need to include pthread.h.Below is the Hello World of pthreads programs:

#include <pthread.h>
#include <stdio.h>

void * entry_point(void *arg)
{
    printf("Hello world!\n");

    return NULL;
}

int main(int argc, char **argv)
{
    pthread_t thr;
    if(pthread_create(&thr, NULL, &entry_point, NULL))
    {
        printf("Could not create thread\n");
        return -1;
    }

    if(pthread_join(thr, NULL))
    {
        printf("Could not join thread\n");
        return -1;
    }
    return 0;
}

In this example, note that "Hello again?" is neverprinted. The thread exits inother_function andentry_point never returns. The argument topthread_exit is a value to be returned to the joiningthread.

Synchronization

The pthreads specification provides many synchronizationprimitives; we will cover three in this primer:

• Barriers
• Mutexes
• Semaphores

Implementing barrier in Pthreads

Barriers

Some parallel computations need to "meet up" at certainpoints before continuing. This can, of course, be accomplishedwith semaphores, but another construct is often more convenient:the barrier (the pthreads librarypthread_barrier_t).As a motivating
example, take this program:

#define _XOPEN_SOURCE 600

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>


#define ROWS 10000
#define COLS 10000
#define THREADS 10

double initial_matrix[ROWS][COLS];
double final_matrix[ROWS][COLS];
// Barrier variable
pthread_barrier_t barr;

extern void DotProduct(int row, int col,
                       double source[ROWS][COLS],
                       double destination[ROWS][COLS]);
extern double determinant(double matrix[ROWS][COLS]);

void * entry_point(void *arg)
{
    int rank = (int)arg;
    for(int row = rank * ROWS / THREADS; row < (rank + 1) * THREADS; ++row)
        for(int col = 0; col < COLS; ++col)
            DotProduct(row, col, initial_matrix, final_matrix);

    // Synchronization point
    int rc = pthread_barrier_wait(&barr);
    if(rc != 0 && rc != PTHREAD_BARRIER_SERIAL_THREAD)
    {
        printf("Could not wait on barrier\n");
        exit(-1);
    }

    for(int row = rank * ROWS / THREADS; row < (rank + 1) * THREADS; ++row)
        for(int col = 0; col < COLS; ++col)
            DotProduct(row, col, final_matrix, initial_matrix);
}

int main(int argc, char **argv)
{
    pthread_t thr[THREADS];

    // Barrier initialization
    if(pthread_barrier_init(&barr, NULL, THREADS))
    {
        printf("Could not create a barrier\n");
        return -1;
    }

    for(int i = 0; i < THREADS; ++i)
    {
        if(pthread_create(&thr[i], NULL, &entry_point, (void*)i))
        {
            printf("Could not create thread %d\n", i);
            return -1;
        }
    }

    for(int i = 0; i < THREADS; ++i)
    {
        if(pthread_join(thr[i], NULL))
        {
            printf("Could not join thread %d\n", i);
            return -1;
        }
    }

    double det = Determinant(initial_matrix);
    printf("The determinant of M^4 = %f\n", det);

    return 0;
}

This program spawns a number of threads, assigning each tocomputepart of a matrix multiplication. Each threadthen uses the result of that computation in the next phase: anothermatrix multiplication.

There are a few things to note here:

1. The barrier declaration at the top
2. The barrier initialization in main
3. The point where each thread waits for its peers to finish.

NOTE

The preprocessor definition of _XOPEN_SOURCE at the topof the program is important; without it, the barrier prototypesare not defined inpthread.h. The definition must comebefore any headers are included.

Mutexes

The pthreads library provides a basic synchronization primitive:pthread_mutex_t. The declarations required to usepthreads mutexes are included inpthread.h. This is astandard mutex with lock and unlock operations; see this example:

#include <pthread.h>
#include <stdio.h>
#include <math.h>

#define ITERATIONS 10000

// A shared mutex
pthread_mutex_t mutex;
double target;

void* opponent(void *arg)
{
    for(int i = 0; i < ITERATIONS; ++i)
    {
        // Lock the mutex
        pthread_mutex_lock(&mutex);
        target -= target * 2 + tan(target);
        // Unlock the mutex
        pthread_mutex_unlock(&mutex);
    }

    return NULL;
}

int main(int argc, char **argv)
{
    pthread_t other;

    target = 5.0;

    // Initialize the mutex
    if(pthread_mutex_init(&mutex, NULL))
    {
        printf("Unable to initialize a mutex\n");
        return -1;
    }

    if(pthread_create(&other, NULL, &opponent, NULL))
    {
        printf("Unable to spawn thread\n");
        return -1;
    }


    for(int i = 0; i < ITERATIONS; ++i)
    {
        pthread_mutex_lock(&mutex);
        target += target * 2 + tan(target);
        pthread_mutex_unlock(&mutex);
    }

    if(pthread_join(other, NULL))
    {
        printf("Could not join thread\n");
        return -1;
    }

    // Clean up the mutex
    pthread_mutex_destroy(&mutex);

    printf("Result: %f\n", target);

    return 0;
}

The important functions for managing mutexes are:

• pthread_mutex_init:Initialize a new mutex.
• pthread_mutex_destroy:Clean up a mutex that is no longer needed.
• pthread_mutex_lock:Acquire a mutex (blocking if it is not available).
• pthread_mutex_unlock:Release a mutex that you previously locked.

Semaphores

The pthreads library itself does not provide a semaphore;however, a separate POSIX standard does define them. Thenecessary declarations to use these semaphores are containedinsemaphore.h.

NOTE: Do not confuse these with SystemV semaphoreswhich are insys/sem.h.

#include <semaphore.h>
#include <pthread.h>
#include <stdio.h>

#define THREADS 20

sem_t OKToBuyMilk;
int milkAvailable;

void* buyer(void *arg)
{
    // P()
    sem_wait(&OKToBuyMilk);
    if(!milkAvailable)
    {
        // Buy some milk
        ++milkAvailable;
    }
    // V()
    sem_post(&OKToBuyMilk);

    return NULL;
}

int main(int argc, char **argv)
{
    pthread_t threads[THREADS];

    milkAvailable = 0;

    // Initialize the semaphore with a value of 1.
    // Note the second argument: passing zero denotes
    // that the semaphore is shared between threads (and
    // not processes).
    if(sem_init(&OKToBuyMilk, 0, 1))
    {
        printf("Could not initialize a semaphore\n");
        return -1;
    }

    for(int i = 0; i < THREADS; ++i)
    {
        if(pthread_create(&threads[i], NULL, &buyer, NULL))
        {
            printf("Could not create thread %d\n", i);
            return -1;
        }
    }

    for(int i = 0; i < THREADS; ++i)
    {
        if(pthread_join(threads[i], NULL))
        {
            printf("Could not join thread %d\n", i);
            return -1;
        }
    }

    sem_destroy(&OKToBuyMilk);

    // Make sure we don't have too much milk.
    printf("Total milk: %d\n", milkAvailable);

    return 0;
}

The semaphore API has several functions of note:

• sem_init:Initialize a new semaphore. Note, the second argumentdenoteshow the semaphore will be shared. Passingzero denotes that it will be shared amongthreadsrather than processes. The final argument is
  the initialvalue of the semaphore.
• sem_destroy:Deallocate an existing semaphore.
• sem_wait:This is the P() operation.
• sem_post:This is the V() operation.

Relevant Man Pages

Man pages for all of the necessary library functionsshould be available on every CSL Linux system:

Resources

Below are some additional resources:

• Lawrence Livermore National Labs Pthreads Tutorial
• IBM developerWorks POSIX Threads explained (I)
• IBM developerWorks POSIX Threads explained (II)
• IBM developerWorks POSIX Threads explained (III)

• Implementing barrier in Pthreads