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A string is defined as a contiguous sequence of code units terminated by the first zero code unit (often called the NUL code unit). [1] This means a string cannot contain the zero code unit, as the first one seen marks the end of the string. The length of a string is the number of code units before the zero code unit. [1]
The ability to represent a null character does not always mean the resulting string will be correctly interpreted, as many programs will consider the null to be the end of the string. Thus the ability to type it (in case of unchecked user input) creates a vulnerability known as null byte injection and can lead to security exploits. [10]
For example, the null character (U+0000 NULL) is used in C-programming application environments to indicate the end of a string of characters. In this way, these programs only require a single starting memory address for a string (as opposed to a starting address and a length), since the string ends once the program reads the null character.
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The code 127 10 is also a control character. [1] [2] Extended ASCII sets defined by ISO 8859 added the codes 128 10 through 159 10 as control characters. This was primarily done so that if the high bit was stripped, it would not change a printing character to a C0 control code. This second set is called the C1 set.
This allows the string to contain NUL and made finding the length need only one memory access (O(1) (constant) time), but limited string length to 255 characters. C designer Dennis Ritchie chose to follow the convention of null-termination to avoid the limitation on the length of a string and because maintaining the count seemed, in his ...
This representation of an n-character string takes n + 1 space (1 for the terminator), and is thus an implicit data structure. In terminated strings, the terminating code is not an allowable character in any string. Strings with length field do not have this limitation and can also store arbitrary binary data.
Current Windows versions and all back to Windows XP and prior Windows NT (3.x, 4.0) are shipped with system libraries that support string encoding of two types: 16-bit "Unicode" (UTF-16 since Windows 2000) and a (sometimes multibyte) encoding called the "code page" (or incorrectly referred to as ANSI code page). 16-bit functions have names suffixed with 'W' (from "wide") such as SetWindowTextW.