Segmentation fault

**Kemaratu** · 11-29-2009

I'm probably just being stupid, but I can't figure out for the life of me why I'm still getting a segmentation fault on this one... any suggestions?

Code:

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

int ** MatrixMult( int **A, int **B, int n );

int main( int argc, char *argv[] ) {

  int **x, **y, n, i, j;
  FILE *fin;

  fin = fopen( argv[1], "r" );

  fscanf( fin, "%d", &n );

  x = ( int ** ) malloc( sizeof( int ) * n );
  y = ( int ** ) malloc( sizeof( int ) * n );

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

    x[i] = ( int * ) malloc( sizeof( int ) * n );
    y[i] = ( int * ) malloc( sizeof( int ) * n );

  }

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

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

	fscanf( fin, "%d", &n );
	x[i][j] = n;

      }

    }

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

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

	fscanf( fin, "%d", &n );
	y[i][j] = n;

      }

    }

  y = MatrixMult( x, y, n );

  fclose ( fin );

  return 0 ;
}

int ** MatrixMult ( int **A, int **B, int n ){

  int **z, i, j, k;

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

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

    z[i] = ( int * ) malloc( sizeof(int) *n );

  }

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

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

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

	z[i][j] += A[i][k] * B[k][j];

      }

    }

  }

  return z;

}

**laserlight** · 11-30-2009

This is wrong:

Code:

x = ( int ** ) malloc( sizeof( int ) * n );
y = ( int ** ) malloc( sizeof( int ) * n );

it should be:

Code:

x = ( int ** ) malloc( sizeof( int* ) * n );
y = ( int ** ) malloc( sizeof( int* ) * n );

but even better:

Code:

x = malloc( sizeof( x[0] ) * n );
y = malloc( sizeof( y[0] ) * n );

Likewise, although this is correct:

Code:

x[i] = ( int * ) malloc( sizeof( int ) * n );
y[i] = ( int * ) malloc( sizeof( int ) * n );

you should write it as:

Code:

x[i] = malloc( sizeof( x[i][0] ) * n );
y[i] = malloc( sizeof( x[i][0] ) * n );

**Kemaratu** · 11-30-2009

Awesome! Thanks!

Unfortunately that didn't fix my segementation fault...

**laserlight** · 11-30-2009

So, you applied the correct fix everywhere, and still have a segmentation fault? Time to use a debugger as I do not intend to debug this for you

**Kemaratu** · 11-30-2009

M'kay. Thanks anyway :-)

**laserlight** · 11-30-2009

Oh, and this looks wrong:

Code:

fscanf( fin, "%d", &n );
x[i][j] = n;

You should not be changing n. You probably should be passing the address of x[i][j] instead.

A few other things to note:

Check that argc > 1
Check that the file was opened successfully.
Free the memory allocated when you are done.
Make your indentation a little more consistent.

**Elysia** · 11-30-2009

I'll throw you a bone:
Warning 5 warning C6386: Buffer overrun: accessing 'y[i]', the writable size is 'sizeof(int)*n' bytes, but '12' bytes might be written: Lines: 8, 9, 11, 13, 15, 16, 18, 20, 21, 18, 20, 21, 18, 25, 27, 29, 30, 27, 25, 36, 38, 40, 41 41
Warning 6 warning C6386: Buffer overrun: accessing 'x[i]', the writable size is 'sizeof(int)*n' bytes, but '12' bytes might be written: Lines: 8, 9, 11, 13, 15, 16, 18, 20, 21, 18, 20, 21, 18, 25, 27, 29, 30 30
Warning 7 warning C6011: Dereferencing NULL pointer 'y[i]': Lines: 8, 9, 11, 13, 15, 16, 18, 20, 21, 18, 20, 21, 18, 25, 27, 29, 30, 27, 25, 36, 38, 40, 41 41
Warning 8 warning C6011: Dereferencing NULL pointer 'x[i]': Lines: 8, 9, 11, 13, 15, 16, 18, 20, 21, 18, 20, 21, 18, 25, 27, 29, 30 30
Warning 9 warning C6011: Dereferencing NULL pointer 'y': Lines: 8, 9, 11, 13, 15, 16, 18, 20, 21 21
Warning 10 warning C6011: Dereferencing NULL pointer 'x': Lines: 8, 9, 11, 13, 15, 16, 18, 20 20
Warning 11 warning C6011: Dereferencing NULL pointer 'z[i]': Lines: 56, 58, 60, 62, 60, 66, 68, 70, 72 72
Warning 12 warning C6011: Dereferencing NULL pointer 'z': Lines: 56, 58, 60, 62 62
The last number on the line is the line to where the warning was issued.
You should re-think the design to eliminate those warnings.

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