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The C standard does not say that the null pointer is the same as the pointer to memory address 0, though that may be the case in practice. Dereferencing a null pointer is undefined behavior in C, [7] and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null.
Dereferencing a null pointer in C produces undefined behavior, [7] which could be catastrophic. However, most implementations [citation needed] simply halt execution of the program in question, usually with a segmentation fault. However, initializing pointers unnecessarily could hinder program analysis, thereby hiding bugs.
In C code, segmentation faults most often occur because of errors in pointer use, particularly in C dynamic memory allocation. Dereferencing a null pointer, which results in undefined behavior, will usually cause a segmentation fault. This is because a null pointer cannot be a valid memory address.
Another frequent source of dangling pointers is a jumbled combination of malloc() and free() library calls: a pointer becomes dangling when the block of memory it points to is freed. As with the previous example one way to avoid this is to make sure to reset the pointer to null after freeing its reference—as demonstrated below.
In C the use of any automatic variable before it has been initialized yields undefined behavior, as does integer division by zero, signed integer overflow, indexing an array outside of its defined bounds (see buffer overflow), or null pointer dereferencing. In general, any instance of undefined behavior leaves the abstract execution machine in ...
In C++, because dereferencing a null pointer is undefined behavior, compiler optimizations may cause other checks to be removed, leading to vulnerabilities elsewhere in the code. [29] [30] Some lists may also include race conditions (concurrent reads/writes to shared memory) as being part of memory safety (e.g., for access control).
This can be understood as taking a null pointer of type structure st, and then obtaining the address of member m within said structure. While this implementation works correctly in many compilers, it has generated some debate regarding whether this is undefined behavior according to the C standard, [2] since it appears to involve a dereference of a null pointer (although, according to the ...
"as the NULL pointer is implementation defined" This is a popular misconception: Yes, the integer value of a null pointer is implementation defined (i.e. what you get if you cast it to int, but the C standard says that void* p = 0; creates a null pointer (and that the macro NULL is just the integer 0). See C99 standard, section 6.3.2.3: "An ...