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For example, in x86 real mode, the address 0000:0000 is readable and also usually writable, and dereferencing a pointer to that address is a perfectly valid but typically unwanted action that may lead to undefined but non-crashing behavior in the application; if a null pointer is represented as a pointer to that address, dereferencing it will ...
Null pointer dereference – A null pointer dereference will often cause an exception or program termination in most environments, but can cause corruption in operating system kernels or systems without memory protection or when use of the null pointer involves a large or negative offset.
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.
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.
Dereferencing any of these variables could cause a segmentation fault: dereferencing the null pointer generally will cause a segfault, while reading from the wild pointer may instead result in random data but no segfault, and reading from the dangling pointer may result in valid data for a while, and then random data as it is overwritten.
load onto the stack a reference from an array aastore 53 0101 0011 arrayref, index, value → store a reference in an array aconst_null 01 0000 0001 → null push a null reference onto the stack aload 19 0001 1001 1: index → objectref load a reference onto the stack from a local variable #index: aload_0 2a 0010 1010 → objectref
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 ...
A Fortran reference is best thought of as an alias of another object, such as a scalar variable or a row or column of an array. There is no syntax to dereference the reference or manipulate the contents of the referent directly. Fortran references can be null.