Example: Using Mutexes

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

 /*   
 The following structure contains the necessary information  
 to allow the function "dotprod" to access its input data and 
 place its output into the structure.  
 */

 typedef struct 
  {
    double      *a;
    double      *b;
    double     sum; 
    int     veclen; 
  } DOTDATA;

 /* Define globally accessible variables and a mutex */

 #define NUMTHRDS 4
 #define VECLEN 100
    DOTDATA dotstr; 
    pthread_t callThd[NUMTHRDS];
    pthread_mutex_t mutexsum;

 /*
 The function dotprod is activated when the thread is created.
 All input to this routine is obtained from a structure 
 of type DOTDATA and all output from this function is written into
 this structure. The benefit of this approach is apparent for the 
 multi-threaded program: when a thread is created we pass a single
 argument to the activated function - typically this argument
 is a thread number. All  the other information required by the 
 function is accessed from the globally accessible structure. 
 */  

 void *dotprod(void *arg)
 {

    /* Define and use local variables for convenience */

    int i, start, end, len ;
    long offset;
    double mysum, *x, *y;
    offset = (long)arg;
     
    len = dotstr.veclen;
    start = offset*len;
    end   = start + len;
    x = dotstr.a;
    y = dotstr.b;

    /*
    Perform the dot product and assign result
    to the appropriate variable in the structure. 
    */

    mysum = 0;
    for (i=start; i<end ; i++) 
     {
       mysum += (x[i] * y[i]);
     }

    /*
    Lock a mutex prior to updating the value in the shared
    structure, and unlock it upon updating.
    */
    pthread_mutex_lock (&mutexsum);
    dotstr.sum += mysum;
    pthread_mutex_unlock (&mutexsum);

    pthread_exit((void*) 0);
 }

 /* 
 The main program creates threads which do all the work and then 
 print out result upon completion. Before creating the threads,
 the input data is created. Since all threads update a shared structure, 
 we need a mutex for mutual exclusion. The main thread needs to wait for
 all threads to complete, it waits for each one of the threads. We specify
 a thread attribute value that allow the main thread to join with the
 threads it creates. Note also that we free up handles when they are
 no longer needed.
 */

 int main (int argc, char *argv[])
 {
    long i;
    double *a, *b;
    void *status;
    pthread_attr_t attr;  

    /* Assign storage and initialize values */
    a = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
    b = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
   
    for (i=0; i<VECLEN*NUMTHRDS; i++)
      {
      a[i]=1.0;
      b[i]=a[i];
      }

    dotstr.veclen = VECLEN; 
    dotstr.a = a; 
    dotstr.b = b; 
    dotstr.sum=0;

    pthread_mutex_init(&mutexsum, NULL);
         
    /* Create threads to perform the dotproduct  */
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    for(i=0; i<NUMTHRDS; i++)
    {
    /* 
    Each thread works on a different set of data. The offset is specified 
    by 'i'. The size of the data for each thread is indicated by VECLEN.
    */
    pthread_create(&callThd[i], &attr, dotprod, (void *)i);
    }

    pthread_attr_destroy(&attr);

    /* Wait on the other threads */
    for(i=0; i<NUMTHRDS; i++)
       {
       pthread_join(callThd[i], &status);
       }

    /* After joining, print out the results and cleanup */
    printf ("Sum =  %f \n", dotstr.sum);
    free (a);
    free (b);
    pthread_mutex_destroy(&mutexsum);
    pthread_exit(NULL);
 }