Question

I have written a C program that uses two threads for reading and writing. I have declared the variable which are accessed by both the threads as Global. How to avoid the use of global variables in this case.

Answer 1

HOw to avoid the use of global variables in this case.

There is no need to avoid global variables. Only thing you have to consider is valid data by some lock mechanism.

Putting all global variables in to a struct is for readability and code control when your project grows.

I suggest you to use mutex lock.. Here is an modified sample code.

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


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;


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;      

   /* 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);             

   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], NULL, dotprod, (void *)i);
   }  

   /* 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);
}