A Simple Parallel Pthread Example and Test

a11c_pthread.cpp

@(@\newcommand{\W}[1]{ \; #1 \; } \newcommand{\R}[1]{ {\rm #1} } \newcommand{\B}[1]{ {\bf #1} } \newcommand{\D}[2]{ \frac{\partial #1}{\partial #2} } \newcommand{\DD}[3]{ \frac{\partial^2 #1}{\partial #2 \partial #3} } \newcommand{\Dpow}[2]{ \frac{\partial^{#1}}{\partial {#2}^{#1}} } \newcommand{\dpow}[2]{ \frac{ {\rm d}^{#1}}{{\rm d}\, {#2}^{#1}} }@)@This is cppad-20221105 documentation. Here is a link to its current documentation . A Simple Parallel Pthread Example and Test
Purpose
This example just demonstrates pthreads and does not use CppAD at all.

Source Code


# include <pthread.h>
# include <limits>
# include <cmath>
# include <cassert>
// for size_t
# include <cstddef>

# define NUMBER_THREADS 4

# ifdef NDEBUG
# define CHECK_ZERO(expression) expression
# else
# define CHECK_ZERO(expression) assert( expression == 0 );
# endif
namespace {
    // Beginning of Example A.1.1.1c of OpenMP 2.5 standard document ---------
    void a1(int n, float *a, float *b)
    {   int i;
        // for some reason this function is missing on some systems
        // assert( pthread_is_multithreaded_np() > 0 );
        for(i = 1; i < n; i++)
            b[i] = (a[i] + a[i-1]) / 2.0f;
        return;
    }
    // End of Example A.1.1.1c of OpenMP 2.5 standard document ---------------
    struct start_arg { int  n; float* a; float* b; };
    void* start_routine(void* arg_vptr)
    {   start_arg* arg = static_cast<start_arg*>( arg_vptr );
        a1(arg->n, arg->a, arg->b);

        void* no_status = nullptr;
        pthread_exit(no_status);

        return no_status;
    }
}

bool a11c(void)
{   bool ok = true;

    // Test setup
    int i, j, n_total = 10;
    float *a = new float[size_t(n_total)];
    float *b = new float[size_t(n_total)];
    for(i = 0; i < n_total; i++)
        a[i] = float(i);

    // number of threads
    int n_thread = NUMBER_THREADS;
    // the threads
    pthread_t thread[NUMBER_THREADS];
    // arguments to start_routine
    struct start_arg arg[NUMBER_THREADS];
    // attr
    pthread_attr_t attr;
    CHECK_ZERO( pthread_attr_init( &attr ) );
    CHECK_ZERO( pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE) );
    //
    // Break the work up into sub work for each thread
    int n = n_total / n_thread;
    arg[0].n = n;
    arg[0].a = a;
    arg[0].b = b;
    for(j = 1; j < n_thread; j++)
    {   arg[j].n = n + 1;
        arg[j].a = arg[j-1].a + n - 1;
        arg[j].b = arg[j-1].b + n - 1;
        if( j == (n_thread - 1) )
            arg[j].n = n_total - j * n + 1;
    }
    for(j = 0; j < n_thread; j++)
    {   // inform each thread of which block it is working on
        void* arg_vptr = static_cast<void*>( &arg[j] );
        CHECK_ZERO( pthread_create(
            &thread[j], &attr, start_routine, arg_vptr
        ) );
    }
    for(j = 0; j < n_thread; j++)
    {   void* no_status = nullptr;
        CHECK_ZERO( pthread_join(thread[j], &no_status) );
    }

    // check the result
    float eps = 100.0f * std::numeric_limits<float>::epsilon();
    for(i = 1; i < n ; i++)
        ok &= std::fabs( (2. * b[i] - a[i] - a[i-1]) / b[i] ) <= eps;

    delete [] a;
    delete [] b;

    return ok;
}

Input File: example/multi_thread/pthread/a11c_pthread.cpp