A segmentation fault occurs when a program tries to
access a memory location forbidden by the operating system.
Common Causes
Trying to access an array element that is beyond the length of the
array.
int array[10];
for(i = 1; i <= 10; i++)
{
array[i] = i;
}
This code is trying to write to the 11th element of the array, but
space was allocated for only 10 elements. As buggy as this code is,
it may or may not cause a segmentation fault. If the address of the
non-existent 11th element is not within your memory space, there will
be a segmentation fault. If that address is within your memory space,
this code will either write over one of your other variables, or
will run fine if that address isn't being used. No matter what the
result, the code is buggy and needs to be fixed.
Being careless about the edges of a 2-dimensional array
Checking neighboring elements in a 2 dimensional array must be done
carefully so that you are not accessing memory that is beyond the edges
of the array (the same problem, just enlarged).
for(i = 0, i < ROWS; i++)
{
for(j = 0; j < COLS; j++)
{
if(board[i][j] == 'X' && board[i][j-1] == 'X'
&& board[i][j+1] == 'X')
{
vertical++;
}
Oops, we looked off of the edge !
Buggy ? Definitely
Seg Fault ? maybe, maybe not
Uninitialized Pointers
int x, *xPtr;
*xPtr = 5;
Dereferencing a NULL pointer
Buggy Code :
while(target != curr -> data && curr != NULL)
{
prev = curr;
curr = curr -> next;
}
Although we want to traverse the list until we find the
target or reach the end of the list, if the target value
is not in the list, curr will be NULL when the end of the
list is reached. At that time, when we try to look at data
in the node that curr's pointing to, we'll get a seg fault
because curr is NULL.
Corrected Code :
while(curr != NULL && target != curr -> data)
{
prev = curr;
curr = curr -> next;
}
This does NOT cause a segmentation fault, because C uses lazy
evaluation. If curr is NULL, the code following the && is
never executed.
Trying to free() memory that you didn't dynamically allocate
int x, *xPtr;
xPtr = &x;
free(xPtr);