Question

I have few lines of C++ code which has simple purpose – to store a matrix in two-dimensional array and not consume more memory, than is needed. This means I have to allocate memory for each number in matrix before putting it in matrix.

#include <cstdlib>
#include <cstdio>

using namespace std;

int main() {
    int ** matrix;

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

    // Fill in the matrix
    matrix[0] = (int *) malloc(3 * sizeof(int));
    matrix[0][0] = 5;
    matrix[0][1] = 10;
    matrix[0][2] = 15;

    matrix[1] = (int *) malloc(3 * sizeof(int));
    matrix[1][0] = 2;
    matrix[1][1] = 4;
    matrix[1][2] = 6;

    int i, n;

    // Print the whole matrix
    for (n = 0; n < 3; n++) {
        for (i = 0; i < 3; i++) {
            printf("%i\t", matrix[n][i]);
        }
        printf("\n");
    }

    return 0;
}

When i compile the code above and runs it, it crashes when printing the matrix:

3838 Segmentation fault (core dumped) sh “${SHFILE}”

The stackdump looks like this:

Exception: STATUS_ACCESS_VIOLATION at eip=0040126A
eax=00000000 ebx=00B0021C ecx=00000000 edx=00000000 esi=6123DBAA edi=61179FC3
ebp=0028CD18 esp=0028CCF0 program=C:\workspace\c\PA1_9\dist\Debug\Cygwin_4.x-Windows\pa1_9.exe, pid 3828, thread main
cs=0023 ds=002B es=002B fs=0053 gs=002B ss=002B
Stack trace:
Frame     Function  Args
0028CD18  0040126A  (6123DBAA, 61179FC3, 0028CD58, 61006CD3)
0028CD58  61006CD3  (00000000, 0028CD94, 61006570, 7EFDE000)
End of stack trace 

I guess there will be some troubles with pointers/values… but i don’t know where and why…

Answer 1

You’re code is almost correct, but the matrix allocation is wrong. You are allocating space to store a single int * while you try to initialize two elements (matrix[0] and matrix[1]) :

matrix = (int**)malloc(2 * sizeof(int *));

The outer printing loop is wrong : there are only 2 “lines” in the matrix (with 3 columns each). The program will crash when you attempt to access matrix[2] :

// Print the whole matrix
for (n = 0; n < 2; n++) {
    for (i = 0; i < 3; i++) {
        printf("%i\t", matrix[n][i]);
    }
    printf("\n");
}