Newline
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In computing, a newline (also known as a line break or end-of-line / EOL character) is a special character or sequence of characters signifying the end of a line of text. The name comes from the fact that the next character after the newline will appear on a new line—that is, on the next line below the text immediately preceding the newline. The actual codes representing a newline vary across hardware platforms and operating systems, which can be a problem when exchanging data between systems with different representations. There is also some confusion as to whether newlines terminate or separate lines. If a newline is considered a separator, there will be no newline after the last line of a file. The general convention on most systems is to add a newline even after the last line, i.e., to treat newline as a line terminator. Some programs have problems processing the last line of a file if it isn't newline terminated. Conversely, programs that expect newline to be used as a separator will interpret a final newline as starting a new (empty) line. This can result in a different line count being reported for the file, but is otherwise generally harmless. Due to the word wrap feature newline code typically only needs to be stored if a line break is required independent of whether the next word would fit on the same line, such as between paragraphs and in vertical lists, see hard return and soft return.
RepresentationsSoftware applications and operating systems usually represent a newline with one or two control characters:
Most textual Internet protocols (including HTTP, SMTP, FTP, IRC and many others) mandate the use of ASCII CR+LF (0Dh 0Ah) on the protocol level, but recommend that tolerant applications recognize lone LF as well. In practice, there are many applications that erroneously use the C newline character '\n' instead (see section Newline in programming languages below). This leads to problems when trying to communicate with systems adhering to a stricter interpretation of the standards; one such system is the qmail MTA that actively refuses to accept messages from systems that send bare LF instead of the required CR+LF.[1] UnicodeThe Unicode standard addresses the problem by defining a large number of characters that conforming applications should recognize as line terminators: LF: Line Feed, U+000A This may seem overly complicated compared to an approach such as converting all line terminators to a single character, for example LF. The simple approach breaks down, however, when trying to convert a text file from an encoding like EBCDIC to Unicode and back. When converting to Unicode, NEL would have to be replaced by LF, but when converting back it would be impossible to decide if an LF should be mapped to an EBCDIC LF or NEL. The approach taken in the Unicode standard allows this transformation to be information-preserving while still enabling applications to recognize all possible types of line terminators. HistoryASCII was developed simultaneously by the ISO and the ASA, the predecessor organization to ANSI. During the period of 1963–1968, the ISO draft standards supported the use of either CR+LF or LF alone as a newline, while the ASA drafts supported only CR+LF. The Multics operating system began development in 1964 and used LF alone as its newline. Unix followed the Multics practice, and later systems followed Unix. The sequence CR+LF was in common use on many early computer systems that had adopted teletype machines, typically an ASR33, as a console device, because this sequence was required to position those printers at the start of a new line. On these systems, text was often routinely composed to be compatible with these printers, since the concept of device drivers hiding such hardware details from the application was not yet well developed; applications had to talk directly to the teletype machine and follow its conventions. The separation of the two functions concealed the fact that the print head could not return from the far right to the beginning of the next line in one-character time. That is why the sequence was always sent with the CR first. In fact, it was often necessary to send extra characters (extraneous CRs or NULs, which are ignored) to give the print head time to move to the left margin. Even after teletypes were replaced by computer terminals with higher baud rates, many operating systems still supported automatic sending of these fill characters, for compatibility with cheaper terminals that required multiple character times to scroll the display. MS-DOS, built upon a CP/M clone called 86-DOS (which Microsoft purchased and renamed), adopted CP/M's CR+LF; CP/M's use of CR+LF made sense for using computer terminals via serial lines. This convention was inherited by Microsoft's later Windows operating system. Newline in programming languagesTo facilitate the creation of portable programs, programming languages provide some abstractions to deal with the different types of newline sequences used in different environments. The C programming language provides the escape sequences '\n' (newline) and '\r' (carriage return). However, contrary to popular belief, these are in fact not required to be equivalent to the ASCII LF and CR control characters. The C standard only guarantees two things:
On Unix platforms, where C originated, the native newline sequence is ASCII LF (0x0A), so '\n' was simply defined to be that value. With the internal and external representation being identical, the translation performed in text mode effectively turns into a no-op, making text mode and binary mode behave the same. This has caused many programmers who developed their software on Unix systems to simply ignore the distinction completely, resulting in code that is not portable to different platforms. Another common problem is the use of '\n' when communicating using an Internet protocol that mandates the use of ASCII CR+LF for ending lines. Writing '\n' to a text mode stream works correctly on Windows systems, but produces only LF on Unix, and something completely different on more exotic systems. Using "\r\n" in binary mode is slightly better, as it works on many ASCII-compatible systems, but still fails in the general case. One approach is to use binary mode and specify the numeric values of the control sequence directly, "\x0D\x0A". Perl[2] and C++ provide the same interpretation of '\n' as C. C++ also provides std::endl, which is a function that emits the underlying system's representation of a newline, then flushes the stream. Java also provides '\n' and '\r' escape sequences. In contrast to C, these are guaranteed to represent the values U+000A and U+000D, respectively. The Java I/O libraries do not transparently translate these into platform-dependent newline sequences on input or output. Instead, they provide functions for writing a full line that automatically add the native newline sequence, and functions for reading lines that accept any of CR, LF, or CR+LF as a line terminator (see BufferedReader.readLine()). The System.getProperties method can be used to retrieve the underlying line separator. Like std::endl for C++ mentioned above, some languages have created special variables, constants and subroutines to facilitate newlines during program execution. Another example is the PHP constant PHP_EOL, which will produce either '\r\n' or '\n' appropriate to the operating system the program is executed on.[3] Though special newline handling facilities can aid execution during runtime, they do not ensure the validity of newlines for the source code itself. Common problemsThe different newline conventions often cause text files that have been transferred between systems of different types to be displayed incorrectly. For example, files originating on Unix or Apple Macintosh systems may appear as a single long line on a Windows system. Conversely, when viewing a file from a Windows computer on a Unix system, the extra CR may be displayed as ^M at the end of each line or as a second line break. The problem can be hard to spot if some programs handle the foreign newlines properly while others don't. For example, a compiler may fail with obscure syntax errors even though the source file looks correct when displayed on the console or in an editor. Modern text editors generally recognize all flavours of CR / LF newlines and allow the user to convert between the different standards. Web browsers are usually also capable of displaying text files of different types. The File Transfer Protocol can automatically convert newlines in files being transferred between systems with different newline representations when the transfer is done in "ASCII mode". However, transferring binary files in this mode usually has disastrous results: Any occurrence of the newline byte sequence—which does not have line terminator semantics in this context, but is just part of a normal sequence of bytes—will be translated to whatever newline representation the other system uses, effectively corrupting the file. FTP clients often employ some heuristics (for example, inspection of filename extensions) to automatically select either binary or ASCII mode, but in the end it is up to the user to make sure his or her files are transferred in the correct mode. If there is any doubt as to the correct mode, binary mode should be used, as then no files will be altered by FTP, though they may display incorrectly. See alsoExternal links
de:Zeilenvorschub es:Nueva línea fr:CRLF ja:改行コード pl:End-of-line pt:Newline |
