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matrix addition using shared memory

Posted on 2010-11-23
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Last Modified: 2012-05-10
I want to add 2 matrices using shared memory and parallel processes, in linux, but my code doesnt run correctly .
Where is the problem?
#include <stdio.h>

#include <sys/shm.h>

#include <sys/stat.h>

#include <stdlib.h>



int main ()

{



int idA,idB,idC, child_id;

int *A,*B, *C;//the 3 arrays

int* child_memory;

int *pA, *pB, *pC;

const int size = 4096;

int i;

//allocate shared memory for an array of 5 integers

idA = shmget(IPC_PRIVATE, 5*sizeof(int), S_IRUSR | S_IWUSR);

idB = shmget(IPC_PRIVATE, 5*sizeof(int), S_IRUSR | S_IWUSR);

idC = shmget(IPC_PRIVATE, 5*sizeof(int), S_IRUSR | S_IWUSR);



//attach shared memory to parent

A = (int *) shmat(idA, NULL, 0);

B = (int *) shmat(idB, NULL, 0);

C = (int *) shmat(idC, NULL, 0);

for(i=0; i<5; i++){

         A[i]=1;

         B[i]=2;

         C[i]=0;

         }

for (i=0; i<5; i++){

    printf("A[%d]=%d B[%d]=%d  C[%d]=%d\n",i,A[i],i,B[i],i,C[i]);

}

printf("\n"); 

for(i=0;i<5; i++){

   child_id=fork();

   if (child_id==0){

       A = (int*) shmat(idA, NULL, 0);

       B = (int*) shmat(idB, NULL, 0);

       C = (int*) shmat(idC, NULL, 0);             

       C[i] = A[i] + B[i];

       

   }

   else if (child_id>0){

    wait(0);

        

   }

   else {perror("fork");exit(1);}

}   

printf("C[%d] = %d ", i,C[i]);       



shmdt(A);shmdt(B);shmdt(C);



shmctl(idA, IPC_RMID, NULL);

shmctl(idB, IPC_RMID, NULL);

shmctl(idC, IPC_RMID, NULL);

return 0;

}

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Question by:dimi67
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4 Comments
 
LVL 53

Expert Comment

by:Infinity08
ID: 34196336
Could you elaborate a bit on how it doesn't run correctly ? What do you observe ? What did you expect instead ?
0
 

Author Comment

by:dimi67
ID: 34197385
.if you run the code you will see that t values of array C don't change at all...
0
 
LVL 53

Accepted Solution

by:
Infinity08 earned 2000 total points
ID: 34197828
>> printf("C[%d] = %d ", i,C[ i]);  

At this point, i = 5. That'll show an out-of-range value from the array. You probably intended to put this in a loop for i from 0 to 4.

You probably also want to end the children (ie. exit) once they've done their job.

And you probably also want to let the parent wait until the children have done their job.
0
 

Author Closing Comment

by:dimi67
ID: 34198875
ok men, I fixed it with exit(0) at the end of the child...
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