memcpy problem

[caduardo21]

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

int main(void)
{
    int i, j;
    int lines, columns;
    int *matrix1, *matrix2;
    
    printf("lines: ");
    scanf("%d", &lines);
    
    printf("columns: ");
    scanf("%d", &columns);
    
    matrix1 = malloc(lines * columns);
    /*matrix2 = malloc(lines * columns);*/
    
    printf("\nvalues for matrix1\n");
    for (i = 0; i < lines; i++)
        for (j = 0; j < columns; j++) {
            printf("matrix1[%d][%d]: ", i, j);
            scanf("%d", &matrix1[i * columns + j]);
        }
    
    memcpy(matrix2, matrix1, sizeof(matrix1));
    
    for (i = 0; i < lines; i++)
        for (j = 0; j < columns; j++)
            printf("matrix2[%d][%d] = %d\n", i, j, matrix2[i * columns + j]);
    
    free(matrix1);
    free(matrix2);
    
    return 0;
}

When I run this program, this is what happens...

lines: 3
columns: 3

values for matrix1
matrix1[0][0]: 1
matrix1[0][1]: 2
matrix1[0][2]: 3
matrix1[1][0]: 4
matrix1[1][1]: 5
matrix1[1][2]: 6
matrix1[2][0]: 7
matrix1[2][1]: 8
matrix1[2][2]: 9
matrix2[0][0] = 1
matrix2[0][1] = 1073834432
matrix2[0][2] = 1073783856
matrix2[1][0] = 1074016346
matrix2[1][1] = 1074016362
matrix2[1][2] = 1074016378
matrix2[2][0] = 1074016394
matrix2[2][1] = 1074265552
matrix2[2][2] = 1074016426
*** glibc detected *** free(): invalid next size (fast): 0x080498b8 ***
Aborted

What is going on here? Thanks.

[Dave_Sinkula]

First, this method of dynamic allocation does not produce a contiguous array, so you cannot use memcpy.

[pianorain]

Perhaps you really didn't mean to comment this line out?

/*matrix2 = malloc(lines * columns);*/

You need some memory to be able to put that information somewhere.

[edit]

First, this method of dynamic allocation does not produce a contiguous array, so you cannot use memcpy.

Why not? It's a single malloc call. Looks pretty contiguous to me.

[edit again]

sizeof(matrix1)

I'm pretty sure that will always be the size of your int pointer, not the size of the array. You need to copy (lines*columns), since that's what you malloc'ed.

[Dave_Sinkula]

[edit]Why not? It's a single malloc call. Looks pretty contiguous to me.

Yes, I must be seeing things.

    matrix1 = malloc(lines * columns * sizeof *matrix1);
    matrix2 = malloc(lines * columns * sizeof *matrix2);
    /* ... */
    memcpy(matrix2, matrix1, lines * columns * sizeof *matrix2);

[caduardo21]

I changed the program to this...

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

int main(void)
{
    int i, j;
    int lines, columns;
    int *matrix1, *matrix2;
    
    printf("lines: ");
    scanf("%d", &lines);
    
    printf("columns: ");
    scanf("%d", &columns);
    
    matrix1 = malloc(lines * columns);
    matrix2 = malloc(lines * columns);
    
    printf("\nvalues for matrix1\n");
    for (i = 0; i < lines; i++)
        for (j = 0; j < columns; j++) {
            printf("matrix1[%d][%d]: ", i, j);
            scanf("%d", &matrix1[i * columns + j]);
        }
    
    memcpy(matrix2, matrix1, lines * columns);
    
    for (i = 0; i < lines; i++)
        for (j = 0; j < columns; j++)
            printf("matrix2[%d][%d] = %d\n", i, j, matrix2[i * columns + j]);
    
    free(matrix1);
    free(matrix2);
    
    return 0;
}

lines: 3
columns: 3

values for matrix1
matrix1[0][0]: 1
matrix1[0][1]: 2
matrix1[0][2]: 3
matrix1[1][0]: 4
matrix1[1][1]: 5
matrix1[1][2]: 6
matrix1[2][0]: 7
matrix1[2][1]: 8
matrix1[2][2]: 9
matrix2[0][0] = 1
matrix2[0][1] = 2
matrix2[0][2] = 3
matrix2[1][0] = 8
matrix2[1][1] = 9
matrix2[1][2] = 0
matrix2[2][0] = 0
matrix2[2][1] = 0
matrix2[2][2] = 0
*** glibc detected *** free(): invalid next size (fast): 0x080498d0 ***
Aborted

[itsme86]

matrix1 = malloc(lines * columns);
matrix2 = malloc(lines * columns);

The elements of matrix1 and matrix2 are of type int so you should actually be allocating lines * columns * sizeof(int) bytes.

memcpy(matrix2, matrix1, lines * columns);

The third argument to memcpy() should be multiplied by sizeof(int) as well.