Editing Segmentation fault (section)

=== Null pointer dereference ===
In C and C-like languages, [[null pointer]]s are used to mean "pointer to no object" and as an error indicator, and [[dereferencing]] a null pointer (a read or write through a null pointer) is a very common program error. The C standard does not say that the null pointer is the same as the pointer to [[memory address]]&nbsp;0, though that may be the case in practice. Most operating systems map the null pointer's address such that accessing it causes a segmentation fault. This behavior is not guaranteed by the C standard. Dereferencing a null pointer is [[undefined behavior]] in C, and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null.

<syntaxhighlight lang=c>
int *ptr = NULL;
printf("%d", *ptr);
</syntaxhighlight>

This sample code creates a [[null pointer]], and then tries to access its value (read the value). Doing so causes a segmentation fault at runtime on many operating systems.

Dereferencing a null pointer and then assigning to it (writing a value to a non-existent target) also usually causes a segmentation fault:
<syntaxhighlight lang=c>
int *ptr = NULL;
*ptr = 1;
</syntaxhighlight>

The following code includes a null pointer dereference, but when compiled will often not result in a segmentation fault, as the value is unused and thus the dereference will often be optimized away by [[dead code elimination]]:
<syntaxhighlight lang=c>
int *ptr = NULL;
*ptr;
</syntaxhighlight>