C - stack smashing detected

Question

I need to implement a pretty easy in-place LU-decomposition of matrix A. I'm using Gaussian elimination and I want to test it with a 3x3 matrix. The problem is, I keep getting stack smashing error and I don't have any idea why. I don't see any problems in my code, which could do this. Do you have any idea?

The problem is probably in the Factorization block.

###My code:###
#include <stdio.h>

int main() {
    int n = 3; // matrix size

    int A[3][3] = {
        {1, 4, 7},
        {2, 5, 8},
        {3, 6, 10}
    };

    printf("Matrix A:\n");

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

            if ( j % 2 == 0 && j != 0 ) {
                printf("\n");
            }
        }
    }

    // FACTORIZATION    
    int k;
    int rows;
    for( k = 0; k < n; k++ ) {
        rows = k + k+1;
        A[rows][k] = A[rows][k]/A[k][k];
        A[rows][rows] = A[rows][rows] - A[rows][k] * A[k][rows];
        printf("k: %d\n", k);
    }

    printf("Matrix after decomp:\n");
    for( int i=0; i < n; i++ ) {
        for( int j=0; j < n; j++ ) {
            printf("%d ", A[i][j]);

            if ( j % 3 == 0 && j != 0 ) {
                printf("\n");
            }
        }
    }

    return 0;
}

What is this supposed to do: `rows = k + k+1;`? Right after that you use `A[rows][k]` which goes out of bounds. — JS1, Jan 27 '15 at 23:53
It was supposed to be the same as this code in MatLab: http://pastebin.com/UAdm2sdj — Eenoku, Jan 27 '15 at 23:57
The Matlab code uses the syntax `1:n`, which creates the vector `1, 2, ..., n`. It then goes on to add the scalar `k` to the vector, which is entirely different from what your C code is doing. — Martin Törnwall, Jan 28 '15 at 00:17
In the future, also consider a tool like [valgrind](http://valgrind.org/). It detects when you read or write memory out of bounds. — Matt Kline, Jan 28 '15 at 00:24
@MartinTörnwall could you please write a little piece of C code to explain this? I'm probably too tired today, but I still don't understand it :-( — Eenoku, Jan 28 '15 at 00:30

Dolda2000 · Answer 1 · 2015-01-28T01:00:17.887

Your error is most likely here:

rows = k + k+1;
A[rows][k] = A[rows][k]/A[k][k];
A[rows][rows] = A[rows][rows] - A[rows][k] * A[k][rows];

This means that rows goes through the values 1, 3, 5; and is then used to access an array with only three elements. That would, indeed, overflow, as the only valid offset among those is 1.

EDIT: Looking at your Matlab code, it is doing something completely different, as rows = k + 1:n sets rows to a small vector, which it then uses the splice the matrix, something C does not support as a primitive. You would need to reimplement both that and the matrix multiplication A(rows, k) * A(k, rows) using explicit loops.

M.M · Accepted Answer · 2015-01-28T01:43:52.463

Your original Matlab code was (Matlab has 1-based indexing):

for k = 1:n - 1
    rows = k + 1:n
    A(rows, k) = A(rows, k) / A(k, k)
    A(rows, rows) = A(rows, rows) - A(rows, k) * A(k, rows)
end

What rows = k + 1:n this does is that it sets rows to represent a range. The expression A(rows, k) is actually a reference to a vector-shaped slice of the matrix, and Matlab can divide a vector by a scalar.

On the last line, A(rows, rows) is a matrix-shaped slice , and A(rows, k) * A(k, rows) is a matrix multiplication, e.g. multiplying matrices of dimension (1,3) and (3,1) to get one of (3,3).

In C you can't do that using the builtin = and / operators.

The C equivalent is:

for ( int k = 0; k < n - 1; ++k )
{
// A(rows, k) = A(rows, k) / A(k, k)
    for ( int row = k + 1; row < n; ++row )
        A[row][k] /= A[k][k];

// A(rows, rows) = A(rows, rows) - A(rows, k) * A(k, rows)
     for ( int row = k + 1; row < n; ++row )
        for ( int col = k + 1; col < n; ++col )
            A[row][col] -= A[row][k] * A[k][col];
}

(disclaimer: untested!)

The first part is straightforward: every value in a vector is being divided by a scalar.

However, the second line is more complicated. The Matlab code includes a matrix multiplication and a matrix subtraction ; and also the operation of extracting a sub-matrix from a matrix. If we tried to write a direct translation of that to C, it is very complicated.

We need to use two nested loops to iterate over the rows and columns to perform this operation on the square matrix.

I must admit I don't quite understand how you draw the conclusion that only values on the main diagonal are set. Rather the opposite, isn't it true by the definition of `A` that, at least on the first iteration, values in all positions will be non-zero? — Dolda2000, Jan 28 '15 at 01:33
@Dolda2000 hmm you're right, I am not sure what I was thinking. So the correct C translation will be quite a lot more complicated. — M.M, Jan 28 '15 at 01:39
I'm glad I was not just completely confused. :) I should add, however, that your code appears to make another mistake in that `A[k][k]` would need to be buffered before doing the divisions lest it be overridden during the loop and thus modified for the later operations. — Dolda2000, Jan 28 '15 at 01:40
@Dolda2000 I don't think it does because `row` starts at `k+1` — M.M, Jan 28 '15 at 01:44
Ah yes, you're right; I was a bit confused by Matlab's indexing starting at 1. Sorry. :) — Dolda2000, Jan 28 '15 at 01:45

C - stack smashing detected

2 Answers2