Null-terminated string: Difference between revisions

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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 null-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>{{cite conference | first = Dennis M. | last = Ritchie (| date = April 1993). [| ___location = Cambridge, MA | title = The development of the C language]. Proc.| 2ndconference = Second History of Programming Languages Confconference | url = https://www.bell-labs.com/usr/dmr/www/chist.html }}</ref><ref>{{cite book | first = Dennis M. | last = Ritchie | article = The development of the C language | title =

History of Programming Languages | edition = 2 | editor-first1= Thomas J. | editor-last1= Bergin, Jr. | editor-first2 = Richard G. | editor-last2 = Gibson, Jr. | publisher = ACM Press | ___location = New York | via = Addison-Wesley (Reading, Mass) | year = 1996 | isbn = 0-201-89502-1 }}</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 null-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.