Incrementing shared variable not returning expected output after mutex lock

Question

I'm invoking 100 threads, and each threads should increment a shared variable 1000 times. So the expected output should be 100000. Of course when multiple threads try to increment one shared variable, you can also get outputs that are not 100000 (it's not very likely that you can get 100000).

To deal with this, I put a lock in the method that increments the variable so that everything runs synchronously.

However, I'm still getting numbers like 99000, 98000, and sometimes 100000. But it should always be 100000 because I have a lock right?

This is what I have

volatile unsigned int count = 0;

void *increment(void *vargp);
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

int main() {
    fprintf(stdout, "Before: count = %d\n", count);

    int j;
    // run 10 times to test output
    for (j = 0; j < 10; j++) {
        // reset count every time
        count = 0;

        int i;
        // create 100 theads
        for (i = 0; i < 100; i++) {
            pthread_t thread;

            Pthread_create(&thread, NULL, increment, NULL);
        }

        fprintf(stdout, "After: count = %d\n", count);
    }

    return 0;
}          

void *increment(void *vargp) {
    int c;

    // protected by mutex lock
    pthread_mutex_lock(&mutex);

    // increment count 1000 times
    for (c = 0; c < 1000; c++) {
        count++;
    }

    pthread_mutex_unlock(&mutex);

    return NULL;
}

always check the returned value from a call to pthread_create() to assure the operation was successful — user3629249
– user3629249, Commented Aug 24, 2015 at 6:24
when returning/exiting from a thread, use 'pthread_exit( *status );' note return( NULL ); — user3629249
– user3629249, Commented Aug 24, 2015 at 6:31
the code is missing (in the main() function) the 100 calls to 'pthread_join()' and has only 1 entry for the thread ids. it needs 100 entries for the ids, so the call(s) to pthread-join() work correctly. — user3629249
– user3629249, Commented Aug 24, 2015 at 6:34
I noticed the spelling of pthread_create() is 'Pthread_create()' that is probably incorrect spelling — user3629249
– user3629249, Commented Aug 24, 2015 at 6:36
for every thread started, the main() function needs a call to pthread_join() to wait for the thread to complete — user3629249
– user3629249, Commented Aug 24, 2015 at 6:42

Community · Accepted Answer · 2017-05-23 12:29:44Z

5

Are you waiting for all the threads to finish? Doesn't look like you are. Try something like How can I wait for any/all pthreads to complete?

edited May 23, 2017 at 12:29

CommunityBot

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answered Aug 22, 2015 at 4:18

David Neiss

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user3629249 · Accepted Answer · 2015-08-24 06:47:14Z

the following code, run on ubuntu linux 14.04 compiles cleanly and performs correctly.

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

#define UNUSED(x)      (void)(x)
#define NUM_TESTS      (10)
#define NUM_INCREMENTS (1000)
#define NUM_THREADS    (100)

volatile unsigned int count = 0;

void *increment(void *vargp);

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

int main( void )
{
    fprintf(stdout, "Before: count = %d\n", count);

    pthread_t thread[NUM_THREADS];
    int j;

    // run 10 times to test output
    for (j = 0; j < NUM_TESTS; j++)
    {
        // reset count every time
        count = 0;

        int i;
        // create 100 theads
        for (i = 0; i < NUM_THREADS; i++)
        {
            pthread_create(&thread[i], NULL, increment, NULL);
        }

        for( i=0; i < NUM_THREADS; i++ )
        {
            void *retval;
            pthread_join( thread[i], &retval);
        }

        fprintf(stdout, "After: count = %d\n", count);
    }

    return 0;
}

void *increment(void *vargp)
{
    UNUSED(vargp);
    int c;

    // protected by mutex lock
    pthread_mutex_lock(&mutex);

    // increment count 1000 times
    for (c = 0; c < NUM_INCREMENTS; c++)
    {
        count++;
    }

    pthread_mutex_unlock(&mutex);

    pthread_exit( NULL );
}

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