Segmentation fault when freeing my matrix - c

Question

I'm using code::blocks.
Code sends a seg fault when freeing the matrix after 2-3 iterations in dealloc_mat.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int **_mat;
int _lines, _columns;
void alloc_mat();
void dealloc_mat();
int main(int argc, char *argv[])
{
_lines = 31, _columns = 22;
alloc_mat();
dealloc_mat();
return 0;
}
void alloc_mat()
{
int i, row, col;
_mat = malloc(sizeof(int *) * _lines);
for(i = 0 ; i < _lines ; i++)
{
int *tmpMatrix = malloc(sizeof(int) * _columns);
_mat[i] = &tmpMatrix[i];
}
for(row = 0 ; row < _lines ; row++)
{
for(col = 0 ; col < _columns ; col++)
{
_mat[row][col] = 0;
}
}
}
void dealloc_mat()
{
int row;
for(row = 0; row < _lines; row++)
{
free(_mat[row]);
}
free(_mat);
}

Answer 1

Here's the bug:
_mat[i] = &tmpMatrix[i];
Should be
_mat[i] = &tmpMatrix[0];
or better
_mat[i] = tmpMatrix;

Answer 2

The problem is that you're not allocating it correctly. This:
for(i = 0 ; i < _lines ; i++)
{
int *tmpMatrix = malloc(sizeof(int) * _columns);
_mat[i] = &tmpMatrix[i];
}
should be this:
for(i = 0 ; i < _lines ; i++)
{
_mat[i] = malloc(sizeof(int) * _columns);
}
Further, _mat, _lines and _columns are reserved identifiers in C, and you shouldn't use them. Any identifier beginning with an underscore with file scope in the ordinary (i.e. _mat) or tag (i.e. struct _mat) namespaces is reserved.

Answer 3

Here are a couple of functions used to allocate memory for strings, arrays of strings actually, you can easily modify them for your purposes:
char **strings; // created with global scope (before main())
void allocMemory(int numStrings, int max)
{
int i;
strings = malloc(sizeof(char*)*(numStrings+1));
for(i=0;i<numStrings; i++)
strings[i] = malloc(sizeof(char)*max + 1);
}
void freeMemory(int numStrings)
{
int i;
for(i=0;i<numStrings; i++)
if(strings[i]) free(strings[i]);
free(strings);
}
Here is how the above would be modified (and used) for ints: (note, it is really just recognizing the differences in sizeof(type))
Note also: using malloc() does not initialize values. If you want to guarantee an initial value for each element (eg. 0), you can use calloc() instead.
void allocMemoryInt(int rows, int cols);
void freeMemoryInt(int numStrings);
int **array;
int main(void)
{
allocMemoryInt(10, 3);
freeMemoryInt(10);
return 0;
}
void allocMemoryInt(int rows, int cols)
{
int i;
array = malloc(sizeof(int *)*(rows)); //create memory for row pointers
for(i=0;i<rows; i++)
array[i] = malloc(sizeof(int)*cols + 1); //create memory for (row * col) elements
}
void freeMemoryInt(int rows)
{
int i;
for(i=0;i<rows; i++)
if(array[i]) free(array[i]);//call free for each row
free(array); //free pointer array(will clean up everything allocated)
}