Sudoku demonstration in C

Question

I’m making a Sudoku that the computer plays, I already have all the logic programmed, but I can’t print the answer in the file and on the screen, if someone can take a look and tested in my code I would be very grateful.

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

#define N 9

void facil(void);
void medio(void);
void dificil(void);
int isValueInLineOrColumn(int val, char arr[N][N], int line, int column);

int conferencia;

int main()
{
    int opc=0;

    printf("1-facil\t2-medio\t3-dificil");
    scanf("%d", &opc);

    conferencia=opc;

    switch(opc)/*para a função do nivel escolhido*/
    {
        case 1:
            facil();
            break;

        case 2:
            medio();
        break;

        case 3:
            dificil();
            break;
    }

    return 0;
}

void facil(void)
{
    FILE *arquivo;
    char mesa[4][4];
    int i, j;

    arquivo=fopen("facil.txt","r");

    for(i=0; i<4; i++)
    {
        for(j=0; j<4; j++)
        {
            while((mesa[i][j]=fgetc(arquivo))!=EOF)
            {
                fflush(stdin);
                printf( "%c", mesa[i][j]);
            }
        }
    }

    fclose(arquivo);

    arquivo=fopen("facil.txt","a");

    for(i=0; i<N; i++)
    {
        for(j=0; j<N; j++)
        {
            int val = 1+rand()%N;

            while((mesa[i][j]=0))
            {
                if(isValueInLineOrColumn(val, mesa, i, j))
                    fprintf(arquivo ,"%c", mesa[i][j]=0);
                else
                    fprintf(arquivo ,"%c", mesa[i][j]=val);
            }   
        }
    }
}

void medio(void)
{
    FILE *arquivo;
    char mesa[N][N];
    int i, j;

    arquivo=fopen("medio.txt","r");

    for(i=0; i<N; i++)
    {
        for(j=0; j<N; j++)
        {
            while((mesa[i][j]=fgetc(arquivo))!=EOF)
            {
                   fflush(stdin);
                   printf( "%c", mesa[i][j]);
            }
        }
    }

    fclose(arquivo);

    arquivo=fopen("medio.txt","a");

    for(i=0; i<N; i++)
    {
        for(j=0; j<N; j++)
        {
            int val = 1+rand()%N;

            while((mesa[i][j]=0))
            {
                if(isValueInLineOrColumn(val, mesa, i, j))
                    fprintf(arquivo ,"%c", mesa[i][j]=0);
                else
                    fprintf(arquivo ,"%c", mesa[i][j]=val);
            }   
        }
    }
}

void dificil(void)
{
    FILE *arquivo;
    char mesa[N][N];
    int i, j;

    srand( (unsigned)time(NULL) );

    arquivo=fopen("dificil.txt","r");

    for(i=0; i<N; i++)
    {
        for(j=0; j<N; j++)
        {
            while((mesa[i][j]=fgetc(arquivo))!=EOF)
                {
                    fflush(stdin);
                    printf( "%c", mesa[i][j]);
                }
        }
    }

    fclose(arquivo);

    arquivo=fopen("dificil.txt","a");

    for(i=0; i<N; i++)
    {
        for(j=0; j<N; j++)
        {
            int val = 1+rand()%N;

            while((mesa[i][j]=0))
            {
                if(isValueInLineOrColumn(val, mesa, i, j))
                    fprintf(arquivo ,"%c", mesa[i][j]=0);
                else
                    fprintf(arquivo ,"%c", mesa[i][j]=val);
            }   
        }
    }


}

int isValueInLineOrColumn(int val, char arr[N][N], int line, int column)
{
    int i;

    for (i=0; i < N; i++)
    {
        if(arr[line][i] == val)
            return 1;

        if (arr[i][column] == val)
            return 1;
    }

    return 0;
}

easy file:

---------
|0|2|4|0|
|1|0|0|3|
|4|0|0|2|
|0|1|3|0|
---------

middle file:

-------------------
|0|2|0|0|3|9|0|0|6|
|0|4|0|1|0|0|5|0|0|
|0|3|2|0|1|0|4|0|0|
|8|0|0|2|0|0|0|0|5|
|0|0|7|0|0|0|9|6|0|
|3|0|0|0|5|0|0|0|4|
|4|0|0|7|8|0|0|5|0|
-------------------

hard file:

-------------------
|8|0|0|4|0|6|0|0|7|
|0|0|0|0|0|0|4|0|0|
|5|0|9|0|3|0|7|8|0|
|0|0|0|0|7|0|0|0|0|
|0|4|8|0|2|0|1|0|3|
|0|5|2|0|0|0|0|9|0|
|0|0|1|0|0|0|0|0|0|
|3|0|0|9|0|2|0|0|5|
-------------------