Null character

For other uses, see Null symbol.

The null character (also null terminator) is a control character with the value zero.[1][2][3][4] It is present in many character sets, including those defined by the Baudot and ITA2 codes, ISO/IEC 646 (or ASCII), the C0 control code, the Universal Coded Character Set (or Unicode), and EBCDIC. It is available in nearly all mainstream programming languages.[5] It is often abbreviated as NUL (or NULL, though in some contexts that term is used for the null pointer). In 8-bit codes, it is known as a null byte.

The original meaning of this character was like NOP—when sent to a printer or a terminal, it has no effect (some terminals, however, incorrectly display it as space). When electromechanical teleprinters were used as computer output devices, one or more null characters were sent at the end of each printed line to allow time for the mechanism to return to the first printing position on the next line.[citation needed] On punched tape, the character is represented with no holes at all, so a new unpunched tape is initially filled with null characters, and often text could be inserted at a reserved space of null characters by punching the new characters into the tape over the nulls.

Today the character has much more significance in the programming language C and its derivatives and in many data formats, where it serves as a reserved character used to signify the end of a string,[6] often called a null-terminated string.[7] This allows the string to be any length with only the overhead of one byte; the alternative of storing a count requires either a string length limit of 255 or an overhead of more than one byte (there are other advantages/disadvantages described in the null-terminated string article).

  1. ^ ASCII format for Network Interchange. IETF. sec. 5.2. doi:10.17487/RFC0020. RFC 20. NUL (Null): The all-zeros character which may serve to accomplish time fill and media fill.
  2. ^ "The set of control characters of the ISO 646" (PDF). Secretariat ISO/TC 97/SC 2. 1975-12-01. p. 4.4. Archived from the original (PDF) on 2014-05-12. Position: 0/0, Name: Null, Abbreviation: Nul
  3. ^ "Unicode Character 'NULL' (U+0000)". Retrieved 2018-10-20.
  4. ^ "C0 Controls and Basic Latin" (PDF). Unicode Consortium. 2018. Retrieved 2018-10-20.
  5. ^ "A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string literal." — ANSI/ISO 9899:1990 (the ANSI C standard), section 5.2.1
  6. ^ "A string is a contiguous sequence of characters terminated by and including the first null character" — ANSI/ISO 9899:1990 (the ANSI C standard), section 7.1.1
  7. ^ Working Draft, Standard for Programming Language C++ (PDF) (ISO 14882 standard working draft), ISO/IEC, 28 February 2011, p. 427, N3242=11-0012, retrieved 27 February 2013, A null-terminated byte string, or NTBS, is a character sequence whose highest-addressed element with defined content has the value zero (the terminating null character); no other element in the sequence has the value zero.