"Program has triggered a breakpoint" while free memory malloced - malloc

Question

When I free memory, the error "Program has triggered a breakpoint" occur. Check the code below, I wonder where is wrong ?
int SSavep(char *visited, int t, int n, int m)
{
int* map = (int*)malloc(m*n * sizeof(int));
int* q = (int*)malloc(m*n * sizeof(int));
int count = 0, cur = 0;
int begin = 0, end = 0;
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
//set value for map
}
}
..........
if (t >= map[end]) {
free(map);
free(q);
return 0;
}
else{
free(map);
free(q);
return -1;
}
}

Answer 1

the entire code is as below:
static int dir[4][2] = { {0,1},{0,-1},{1,0},{-1,0} };
int SSavep(char *visited, int t, int n, int m)
{
int* map = (int*)malloc(m*n * sizeof(int));
int* q = (int*)malloc(m*n * sizeof(int));
int count = 0, cur = 0;
int begin = 0, end = 0;
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (visited[i*n + j] == '.')
map[i*n + j] = 0;
else if (visited[i*n + j] == '*')
map[i*n + j] = -1;
else if (visited[i*n + j] == 'p') {
map[i*n + j] = -12;
end = i*n + j;
}
else {
map[i*n + j] = -9;
begin = i*n + j;
}
}
}
q[count++] = begin;
while (cur < count && q[cur] != end) {
int i = q[cur] / n;
int j = q[cur] % n;
for (int k = 0; k < 4; k++) {
int ni = i + dir[k][0];
int nj = j + dir[k][1];
if (ni < 0 || ni >= m || nj < 0 || nj >= n || map[ni*n + nj]>0 || map[ni*n + nj] == -1)
continue;
map[ni*n + nj] = map[i*n + j] + 1;
q[count++] = ni*n + nj;
}
cur++;
}
if (map[end] > 0 && t >= map[end]) {
free(map);
free(q);
return 0;
}
else{
free(map);
free(q);
return -1;
}
}

Answer 2

You are getting error on >> free(q);
for m=n=4
int* q = (int*)malloc(m*n * sizeof(int));
q == [m*n*sizeof(int)] == 4*4*4 == 64 bytes == int[16];
Because you wrote beyond the address space reserved for pointer variable 'q'.
check 'count' variable before free(q). I've got 1208, called with:
char* visited = new char[100 * 100];
memset(visited, 0, 10000);
int res = SSavep(visited, 0, 4, 4);
Btw this algo looks alot like some path finding examining neighboring cells on map and assigning weights, right? If so there are many opensource solutions, why not using them instead of reinventing the wheel? There are links to opensource solutions on wiki Path Finding page:
https://en.wikipedia.org/wiki/Pathfinding
check the bottom of the page for links.