MPI matrix multification compile err: undeclared with code - matrix

Question

I coded a mpi matrix multification program, which use scanf("%d", &size), designate matrix size, then I defined int matrix[size*size], but when I complied it, it reported that matrix is undeclared. Please tell me why, or what my problem is!
According Ed's suggestion, I changed the matrix definition to if(myid == 0) block, but got the same err! Now I post my code, please help me find out where I made mistakes! thank you!
int size;
int main(int argc, char* argv[]) {
int myid, numprocs;
int *p;
MPI_Status status;
int i,j,k;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD,&myid);
MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
if(myid == 0)
{
scanf("%d", &size);
int matrix1[size*size];
int matrix2[size*size];
int matrix3[size*size];
int section = size/numprocs;
int tail = size % numprocs;
srand((unsigned)time(NULL));
for( i=0; i<size; i++)
for( j=0; j<size; j++)
{
matrix1[i*size+j]=rand()%9;
matrix3[i*size+j]= 0;
matrix2[i*size+j]=rand()%9;
}
printf("Matrix1 is: \n");
for( i=0; i<size; i++)
{
for( j=0; j<size; j++)
{
printf("%3d", matrix1[i*size+j]);
}
printf("\n");
}
printf("\n");
printf("Matrix2 is: \n");

Answer 1

Reformatted code would be nice...
One problem is that you haven't declared the size variable. Another problem is that the [size] notation for declaring arrays is only good for sizes that are known at compile time. You want to use malloc() instead.

Answer 2

You don't actually need to define a MAX_SIZE if you use dynamic memory allocation.
#include <stdio.h>
#include <stdlib.h>
...
scanf("%d", &size);
int *matrix1 = (int *) malloc(size*size*sizeof(int));
int *matrix2 = (int *) malloc(size*size*sizeof(int));
int *matrix3 = (int *) malloc(size*size*sizeof(int));
...