Question

I’ve been having trouble with this parallel matrix multiplication code, I keep getting an error when trying to access a data member in my structure.

This is my main function:

struct arg_struct
{
  int* arg1;
  int* arg2;
  int arg3;
  int* arg4;
};


int main()
{
  pthread_t allthreads[4];
  int A [N*N];
  int B [N*N];
  int C [N*N];
  randomMatrix(A);
  randomMatrix(B);
  printMatrix(A);
  printMatrix(B);
  struct arg_struct *args = (arg_struct*)malloc(sizeof(struct arg_struct));
  args.arg1 = A;
  args.arg2 = B;
  int x;
  for (int i = 0; i < 4; i++)
  {
     args.arg3 = i;
     args.arg4 = C;
     x = pthread_create(&allthreads[i], NULL, &matrixMultiplication, (void*)args); 
     if(x!=0)
     exit(1);
  }

  return 0;
}

and the matrixMultiplication method used from another C file:

void *matrixMultiplication(void* arguments)
{
     struct arg_struct* args = (struct arg_struct*) arguments;
     int block = args.arg3;
     int* A = args.arg1;
     int* B = args.arg2;
     int* C = args->arg4;
     free(args);
     int startln = getStartLineFromBlock(block);
     int startcol = getStartColumnFromBlock(block);
     for (int i = startln; i < startln+(N/2); i++)
     {
        for (int j = startcol; j < startcol+(N/2); j++)
        {
          setMatrixValue(C,0,i,j);
          for(int k = 0; k < N; k++)
          {
             C[i*N+j] += (getMatrixValue(A,i,k) * getMatrixValue(B,k,j));
             usleep(1);
          } 
        }
     }
}

Another error I am getting is when creating the thread: “invalid conversion from ‘void ()(int, int*, int, int*)’ to ‘void* ()(void)’ [-fpermissive]
“

Can anyone please tell me what I’m doing wrong?

Answer 1

Big Boss is right in the sense that he has identified the problem, but to add to/augment the reply he made.

Option 1:
Just create an arg_struct in the loop and set the members, then pass it through:

for(...)
{
    struct arg_struct *args = (arg_struct*)malloc(sizeof(struct arg_struct)); 
    args->arg1 = A;
    args->arg2 = B;    //set up args as now...
    ...
    x = pthread_create(&allthreads[i], NULL, &matrixMultiplication, (void*)args);
    ....
}

keep the free call in the thread, but now you could then use the passed struct directly rather than creating locals in your thread.

Option 2:
It looks like you want to copy the params from the struct internally to the thread anyway so you don’t need to dynamically allocate.

Just create an arg_struct and set the members, then pass it through:

arg_struct args;
//set up args as now...
for(...)
{
   ...
   x = pthread_create(&allthreads[i], NULL, &matrixMultiplication, (void*)&args);
}

Then remove the free call.

However as James pointed out you would need to synchronize in the thread/parent on the structure to make sure that it wasn’t changed. That would mean a Mutex or some other mechanism. So probably the move of the allocation to the for loop is easier to begin with.

Part 2:

I’m working on windows (so I can’t experiment currently), but pthread_create param 3 is referring to the thread function matrixMultiplication which is defined as void* matrixMultiplication( void* ); – it looks correct to me (signature wise) from the man pages online, void* fn (void* )

I think I’ll have to defer to someone else on your second error. Made this post a comunnity wiki entry so answer can be put into this if desired.