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Here is an example of ANSI C code that will generally cause a segmentation fault on platforms with memory protection. It attempts to modify a string literal, which is undefined behavior according to the ANSI C standard. Most compilers will not catch this at compile time, and instead compile this to executable code that will crash:
On x86 there exists an older memory management mechanism known as segmentation.If the application loads a segment register with the selector of a non-present segment (which under POSIX-compliant OSes can only be done with assembly language), the exception is generated.
Illegal accesses and invalid page faults can result in a segmentation fault or bus error, resulting in an app or OS crash. Software bugs are often the causes of these problems, but hardware memory errors, such as those caused by overclocking, may corrupt pointers and cause valid code to fail.
In many languages (e.g., the C programming language) deleting an object from memory explicitly or by destroying the stack frame on return does not alter associated pointers. The pointer still points to the same location in memory even though that location may now be used for other purposes. A straightforward example is shown below:
The description above is given for what is now called a serially concatenated code. Turbo codes, as described first in 1993, implemented a parallel concatenation of two convolutional codes, with an interleaver between the two codes and an iterative decoder that passes information forth and back between the codes. [6]
A low code-rate close to zero implies a strong code that uses many redundant bits to achieve a good performance, while a large code-rate close to 1 implies a weak code. The redundant bits that protect the information have to be transferred using the same communication resources that they are trying to protect.
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[7] [8] For example, a stack buffer overflow can overwrite the return address of a subroutine with an invalid value, which will cause, e.g., a segmentation fault, when the subroutine returns. However, if an exploit overwrites the return address with a valid value, the code in that address will be executed.