Null-terminated string: Difference between revisions

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{{see also|String (computer science)#Null-terminated}}

In [[computer programming]], a '''null-terminated string''' is a [[character string]] stored as an [[Array data structure|array]] containing the characters and terminated with a [[null character]] (a character with a value of zero, called NUL in this article). Alternative names are '''[[C string]]''', which refers to the [[C (programming language)|C programming language]] and '''ASCIIZ''' (although C can use encodings other than ASCII).

The length of a C string is found by searching for the (first) NUL byte. This can be slow as it takes O(''n'') ([[linear time]]) with respect to the string length. It also means that a string cannot contain a NUL character (there is a NUL in memory, but it is after the last character, not "in" the string).

== History ==

Null-terminated strings were produced by the <code>.ASCIZ</code> directive of the [[PDP-11]] [[assembly language]]s and the <code>ASCIZ</code> directive of the [[MACRO-10]] macro assembly language for the [[PDP-10]]. These predate the development of the C programming language, but other forms of strings were often used.

At the time C (and the languages that it was derived from) was developed, memory was extremely limited, so using only one byte of overhead to store the length of a string was attractive. The only popular alternative at that time, usually called a "Pascal string" (a more modern term is "[[String (computer science)#Length-prefixed|length-prefixed]]"), used a leading ''byte'' to store the length of the string. This allows the string to contain NUL and made finding the length need only one memory access (O(1) [[constant time|(constant) time]]), but limited string length to 255 characters (on a machine using 8-bit bytes). C designer [[Dennis Ritchie]] chose to follow the convention of NULnull-termination, already established in [[BCPL]], to avoid the limitation on the length of a string and because maintaining the count seemed, in his experience, less convenient than using a terminator.<ref>Dennis M. Ritchie (1993). [The development of the C language]. Proc. 2nd History of Programming Languages Conf.</ref>

This had some influence on CPU [[instruction set]] design. Some CPUs in the 1970s and 1980s, such as the [[Zilog Z80]] and the [[Digital Equipment Corporation|DEC]] [[VAX]], had dedicated instructions for handling length-prefixed strings. However, as the NULnull-terminated string gained traction, CPU designers began to take it into account, as seen for example in IBM's decision to add the "Logical String Assist" instructions to the [[IBM ES/9000 family|ES/9000]] 520 in 1992.

[[FreeBSD]] developer [[Poul-Henning Kamp]], writing in ''[[ACM Queue]]'', would later refer to the victory of null-terminated strings over a 2-byte (not one-byte) length as "the most expensive one-byte mistake" ever.<ref>{{citation |last=Kamp |first=Poul-Henning |date=25 July 2011 |title=The Most Expensive One-byte Mistake |journal=ACM Queue |volume=9 |number=7 |issn=1542-7730 |accessdate=2 August 2011 |url=http://queue.acm.org/detail.cfm?id=2010365 }}</ref>

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While simple to implement, this representation has been prone to errors and performance problems.

The NUL Null-termination has historically created [[computer insecurity|security problems]].<ref>{{cite journal|url= http://insecure.org/news/P55-07.txt |author=Rain Forest Puppy |title=Perl CGI problems |work=Phrack Magazine |publisher=artofhacking.com |date=9 September 1999 |volume=9 |issue=55 |page=7 |accessdate=3 January 2016}}</ref> A NUL byte inserted into the middle of a string will truncate it unexpectedly.<ref>https://security.stackexchange.com/questions/48187/null-byte-injection-on-php</ref> A common bug was to not allocate the additional space for the NUL, so it was written over adjacent memory. Another was to not write the NUL at all, which was often not detected during testing because athe NUL was already there by chance from previous useblock of thememory samealready blockcontained of memoryzeros. Due to the expense of finding the length, many programs did not bother before copying a string to a fixed-size [[Data buffer|buffer]], causing a [[buffer overflow]] if it was too long.

The inability to store a NULzero requires that string datatext and binary data be kept distinct and handled by different functions (with the latter requiring the length of the data to also be supplied). This can lead to code redundancy and errors when the wrong function is used.

The speed problems with finding the length can usually be mitigated by combining it with another operation that is O(''n'') anyway, such as in <code>[[strlcpy]]</code>. However, this does not always result in an intuitive [[API]].

== Character encodings ==

Null-terminated strings require that the encoding does not use a zero byte (0x00) anywhere, therefore it is not possible to store every possible [[ASCII]] or [[UTF-8]] string.<ref>{{cite web|title=UTF-8, a transformation format of ISO 10646|url=http://tools.ietf.org/html/rfc3629#section-3|accessdate=19 September 2013}}</ref><ref><!-- This is the encoding table provided as a resource by the Unicode consortium: http://www.unicode.org/resources/utf8.html -->{{cite web|title=Unicode/UTF-8-character table|url=http://www.utf8-chartable.de/|accessdate=13 September 2013}}</ref><ref>{{cite web|last=Kuhn|first=Markus|title=UTF-8 and Unicode FAQ|url=http://www.cl.cam.ac.uk/~mgk25/unicode.html#utf-8|accessdate=13 September 2013}}</ref> However, it is common to store the subset of ASCII or UTF-8 – every character except the NUL character – in null-terminated strings. Some systems use "[[modified UTF-8]]" which encodes the NUL character as two non-zero bytes (0xC0, 0x80) and thus allow all possible strings to be stored. This is not allowed by the UTF-8 standard, because it is an [[UTF-8#Overlong_encodings|overlong encoding]], and it is seen as a security risk. Some other byte may be used as end of string instead, like 0xFE or 0xFF, which are not used in UTF-8.

[[UTF-16]] uses 2-byte integers and as either byte may be zero (and in fact ''every other'' byte is, when representing ASCII text), cannot be stored in a null-terminated byte string. However, some languages implement a string of 16-bit [[UTF-16]] characters, terminated by a 16-bit NUL character. (Again the NUL character, which encodes as a single zero code unit, is the only character that cannot be stored. UTF-16 does not have any alternative encoding of zero).

== Improvements ==