What is a segmentation fault?

Question

What is a segmentation fault? Is it different in C and C++? How are segmentation faults and dangling pointers related?

Answer 1

Segmentation fault is a specific kind of error caused by accessing memory that “does not belong to you.” It’s a helper mechanism that keeps you from corrupting the memory and introducing hard-to-debug memory bugs. Whenever you get a segfault you know you are doing something wrong with memory – accessing a variable that has already been freed, writing to a read-only portion of the memory, etc. Segmentation fault is essentially the same in most languages that let you mess with memory management, there is no principal difference between segfaults in C and C++.

There are many ways to get a segfault, at least in the lower-level languages such as C(++). A common way to get a segfault is to dereference a null pointer:

int *p = NULL;
*p = 1;

Another segfault happens when you try to write to a portion of memory that was marked as read-only:

char *str = "Foo"; // Compiler marks the constant string as read-only
*str = 'b'; // Which means this is illegal and results in a segfault

Dangling pointer points to a thing that does not exist anymore, like here:

char *p = NULL;
{
    char c;
    p = &c;
}
// Now p is dangling

The pointer p dangles because it points to the character variable c that ceased to exist after the block ended. And when you try to dereference dangling pointer (like *p='A'), you would probably get a segfault.