Completely blank output for C openMP program

I have written the following program for matrix multiplication:

#include <stdio.h>
#include <omp.h>
#include <time.h>
#define NRA 500
#define NCA 500
#define NCB 500

int mat_mul() ;
int i , j , k ;
int main(){
    double start , end ;
    start  =  omp_get_wtime() ;
    mat_mul() ;
    end = omp_get_wtime() ; 
    printf("Time taken : \n %lf " , (end - start) );
    return 0;
}

int mat_mul(){
    int mat1[NRA][NCA] , mat2[NCA][NCB] , mat3[NRA][NCB] ;
    //double start , end ;
    //start  =  omp_get_wtime() ;
    #pragma omp parallel private( i , j , k ) shared(mat1 , mat2 , mat3)
    #pragma omp for

    for( i = 0 ; i < NRA ; i++){
        for( j = 0 ; j < NCB ; j++){
            mat1[i][j] = mat2[i][j] = rand() ;
            for ( k = 0 ; k <  NCB ; k++){
                mat3[i][k] += mat1[i][k] * mat2[k][j] ;
            }
        }
    }
    //end = omp_get_wtime() ; 
    printf("REsult : \n");
    for( i = 0 ; i < NRA ; i ++ ) {
        for( j =0 ; j < k ; j ++)
            printf("%lf" , (double)mat3[i][j]);
        printf("\n") ;
    //printf("Time taken : \n %lf " , (end - start) );
    }
    return 0 ;
}

Everything's fine(almost): It compiles, it executes, even terminates :-D (and it ACTUALLY provides a speed-up(Compare.

But unfortunately, the output appears somethinf like this:

[tejas@localhost Documents]$ gcc -pedantic -Wall  -std=c99 -fopenmp  par.c
par.c: In function ‘mat_mul’:
par.c:28:30: warning: implicit declaration of function ‘rand’ [-Wimplicit-function-declaration]
    mat1[i][j] = mat2[i][j] = rand() ;
                              ^~~~
[tejas@localhost Documents]$ ./a.out
REsult :
...
(LOTS of blank spaces later)
...
Time taken : 
0.177506 [tejas@localhost Documents]$ 

What am I doing wrong? I'm using Fedora 27 , GCC

Thank you in advance.