fclose function bugging program, or it is just my computer?

[stolnjak]

Hi, i use live version of ubuntu, and gcc 4.6.3

Twice happend to me, that fclose bug my program
First i just move on, commeting fclose, and leave to program automatically close opened files at the end of executation

First it happens, that fclose mess my memory, so when i use printf on same array, my output is not same... ( fclose is in between two printfs )

Second it happens, that fclose cause segmentation fault

Here is second code, and there is two issues
First, this fclose cause segmentation fault
Second, when i want to print same array it won't yield same result:

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

#define inputChannel "ulaz4.txt"
#define outputChannel "izlaz4.txt"


void main() {

    FILE *in = fopen(inputChannel, "r");

    int n;
    fscanf(in, "%d", &n);

    int **matrix = (int **)  malloc(sizeof(int) * n );
        
        

    int i,j;
    for ( i = 0; i < n; i++ ){
        
        matrix[i] = (int *) malloc(sizeof(int) * n);

        for ( j = 0; j < n; j++ ){
            fscanf(in, "%d", matrix+(i*n + j) );
            printf("%d,", *(matrix+(i*n+j)) );}

    }

    //fclose(in); // here is segmentation fault
    printf("\n");

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

        for ( j = 0; j < n; j++ )
            printf("%d,", *(matrix+(i*n+j)) );  // Here is not same result as above
    }

    
    
}

Is that just becase i use live version of ubuntu, or i'm doing something wrong?

Here is my output:
1,2,3,4,
1,158605712,3,4,

And here is my input:
2
1 2
3 4

[jimblumberg]

Are you sure the file is even opening? Perhaps you should check?

Are you sure one of your malloc() calls is not failing? Perhaps you should check?

Why are you casting the return value from malloc(), that's not a good practice in C?

What is the value of n?

Why the void main(), main should be defined to return an int.

Your printf and fscanf() look wrong, you probably should be getting some warnings (if you're using a properly configured compiler with adequate warning levels). The variable types don't seem to match the format specifiers which invokes undefined behavior.

Here are the warnings my compiler generates (after fixing the problem with main()).

||=== Build: Debug in c_homework (compiler: gcc 6.1.0) ===|
main.c||In function ‘main’:|
main.c|24|warning: format ‘%d’ expects argument of type ‘int *’, but argument 3 has type ‘int **’ [-Wformat=]|
main.c|25|warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=]|
main.c|36|warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=]|
||=== Build finished: 0 error(s), 3 warning(s) (0 minute(s), 1 second(s)) ===|

Where are you freeing all that memory you're allocating?

Jim

[algorism]

You are allocating a two-dimensional array but are indexing it as if it was a one-dimensional array simulating a two-dimensional array. You need to make up your mind.

Here's an example using a 1-d array to simulate a 2-d array.

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

#define INPUT "ulaz4.txt"

int main() {
    FILE *in = fopen(INPUT, "r");

    int n = 0;
    fscanf(in, "%d", &n);

    int *matrix = malloc(n * n * sizeof *matrix);

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            fscanf(in, "%d", &matrix[i * n + j]);
            printf("%3d ", matrix[i * n + j]);
        }
        printf("\n");
    }
 
    fclose(in);

    printf("\n");
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++)
            printf("%3d ", matrix[i * n + j]);
        printf("\n");
    }

    free(matrix);

    return 0;
}

Here's an example allocating a 2-d array with non-contiguous data (i.e., each row is separately allocated).

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

#define INPUT "ulaz4.txt"

int main() {
    FILE *in = fopen(INPUT, "r");

    int n = 0;
    fscanf(in, "%d", &n);

    int **matrix = malloc(n * sizeof *matrix);

    for (int i = 0; i < n; i++) {
        matrix[i] = malloc(n * sizeof **matrix);
        for (int j = 0; j < n; j++) {
            fscanf(in, "%d", &matrix[i][j]);
            printf("%3d ", matrix[i][j]);
        }
        printf("\n");
    }
 
    fclose(in);

    printf("\n");
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++)
            printf("%3d ", matrix[i][j]);
        printf("\n");
    }

    for (int i = 0; i < n; i++)
        free(matrix[i]);
    free(matrix);

    return 0;
}

Here's an example allocating a 2-d array with contiguous data (i.e., all rows are allocated together so that they are contiguous).

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

#define INPUT "ulaz4.txt"

int main() {
    FILE *in = fopen(INPUT, "r");

    int n = 0;
    fscanf(in, "%d", &n);

    // allocate space for matrix row pointers
    int **matrix = malloc(n * sizeof *matrix);

    // allocate space for matrix data
    matrix[0] = malloc(n * n * sizeof **matrix);

    // set the row pointers
    for (int i = 1; i < n; i++)
        matrix[i] = matrix[i - 1] + n;

    // fill the matrix with data
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            fscanf(in, "%d", &matrix[i][j]);
            printf("%3d ", matrix[i][j]);
        }
        printf("\n");
    }

    fclose(in);
 
    printf("\n");
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++)
            printf("%3d ", matrix[i][j]);
        printf("\n");
    }

    free(matrix[0]);  // free matrix data
    free(matrix);     // free matrix row pointers

    return 0;
}