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From: Richard Stanton <stanton AT haas DOT berkeley DOT edu>
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Date: Sun, 3 Aug 2008 22:03:20 -0700
Subject: cygwin gcc: Different numerical results in thread vs in main()
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The following program performs exactly the same calculation 5 times, once inside main(), and again in 4 thread functions. Though the calculations are identical, the results are not. When I compile and run the same program on my Mac, all the results are identical. By the way, this is using the latest gcc 3.4.4.

Richard Stanton

------------------

[c:\projects\threads]thread1
Main:        t/1+t =    0.0005609048868329022342546
Thread 0: t/1+t =    0.0005609048868329021258344
Thread 1: t/1+t =    0.0005609048868329021258344
Thread 2: t/1+t =    0.0005609048868329021258344
Thread 3: t/1+t =    0.0005609048868329021258344


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

#define NUM_THREADS 4

double t =  0.0005612196776927068104374;

void * thread_function (void *arg)
{
    int i;
    int id = *((int *)arg);

    printf("Thread %d: t/1+t = %30.25lf\n", id, t/(1.0+t));

    return NULL;
}


int main()
{
        int i;
        int arg[NUM_THREADS] = {0,1,2,3};
        pthread_t thread[NUM_THREADS];
        pthread_attr_t attr;

        printf("Main:     t/1+t = %30.25lf\n", t/(1.0+t));

        /* initialize and set the thread attributes */
        pthread_attr_init( &attr );
        pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE );

        /* creating threads */
        for ( i=0; i<NUM_THREADS; i++ ) {
            if ( pthread_create( &thread[i], &attr, thread_function, (void *)&arg[i] )) {
                fprintf(stderr,"Creating thread %d failed!",i);
                return 1;
            }
        }

        /* joining threads (== waiting for them to exit) */
        for ( i=0; i<NUM_THREADS; i++ ) {
            if ( pthread_join ( thread[i], NULL ) ) {
                fprintf(stderr,"Joing thread %d failed!",i);
                return 1;
            }
        }

        return 0;
}

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