What are carriage return, linefeed, and form feed?

问题:

What is the meaning of the following control characters:

1. Carriage return

2. Line feed

3. Form feed

回答1:

Carriage return means to return to the beginning of the current line without advancing downward. The name comes from a printer\'s carriage, as monitors were rare when the name was coined. This is commonly escaped as \"\\r\", abbreviated CR, and has ASCII value 13 or 0x0D.

Linefeed means to advance downward to the next line; however, it has been repurposed and renamed. Used as \"newline\", it terminates lines (commonly confused with separating lines). This is commonly escaped as \"\\n\", abbreviated LF or NL, and has ASCII value 10 or 0x0A. CRLF (but not CRNL) is used for the pair \"\\r\\n\".

Form feed means advance downward to the next \"page\". It was commonly used as page separators, but now is also used as section separators. (It\'s uncommonly used in source code to divide logically independent functions or groups of functions.) Text editors can use this character when you \"insert a page break\". This is commonly escaped as \"\\f\", abbreviated FF, and has ASCII value 12 or 0x0C.

As control characters, they may be interpreted in various ways.

The most common difference (and probably the only one worth worrying about) is lines end with CRLF on Windows, NL on Unix-likes, and CR on older Macs (the situation has changed with OS X to be like Unix). Note the shift in meaning from LF to NL, for the exact same character, gives the differences between Windows and Unix. (Windows is, of course, newer than Unix, so it didn\'t adopt this semantic shift. I don\'t know the history of Macs using CR.) Many text editors can read files in any of these three formats and convert between them, but not all utilities can.

Form feed is a bit more interesting (even though less commonly used directly), and with the usual definition of page separator, it can only come between lines (e.g. after the newline sequence of NL, CRLF, or CR) or at the start or end of the file.

回答2:

\\r is carriage return and moves the cursor back like if i will do-

printf(\"stackoverflow\\rnine\")
ninekoverflow

means it has shifted the cursor to the beginning of \"stackoverflow\" and overwrites the starting four characters since \"nine\" is four character long.

\\n is new line character which changes the line and takes the cursor to the beginning of a new line like-

printf(\"stackoverflow\\nnine\")
stackoverflow
nine

\\f is form feed, its use has become obsolete but it is used for giving indentation like

printf(\"stackoverflow\\fnine\")
stackoverflow
             nine

if i will write like-

printf(\"stackoverflow\\fnine\\fgreat\")
stackoverflow
             nine
                 great

回答3:

\\f is used for page break. You cannot see any effect in the console. But when you use this character constant in your file then you can see the difference.

Other example is that if you can redirect your output to a file then you don\'t have to write a file or use file handling.

For ex:

Write this code in c++

void main()    
{
    clrscr();
    cout<<\"helloooooo\" ;

    cout<<\"\\f\";
    cout<<\"hiiiii\" ;

}

and when you compile this it generate an exe(for ex. abc.exe)

then you can redirect your output to a file using this:

abc > xyz.doc

then open the file xyz.doc you can see the actual page break between hellooo and hiiii....

回答4:

Have a look at Wikipedia:

Systems based on ASCII or a compatible character set use either LF (Line feed, \'\\n\', 0x0A, 10 in decimal) or CR (Carriage return, \'\\r\', 0x0D, 13 in decimal) individually, or CR followed by LF (CR+LF, 0x0D 0x0A). These characters are based on printer commands: The line feed indicated that one line of paper should feed out of the printer, and a carriage return indicated that the printer carriage should return to the beginning of the current line.

回答5:

Apart from above information, there is still an interesting history of LF (\\n) and CR (\\r). [Original author : 阮一峰 Source : http://www.ruanyifeng.com/blog/2006/04/post_213.html] Before computer came out, there was a type of teleprinter called Teletype Model 33. It can print 10 characters each second. But there is one problem with this, after finishing printing each line, it will take 0.2 second to move to next line, which is time of printing 2 characters. If a new characters is transferred during this 0.2 second, then this new character will be lost.

So scientists found a way to solve this problem, they add two ending characters after each line, one is \'Carriage return\', which is to tell the printer to bring the print head to the left.; the other one is \'Line feed\', it tells the printer to move the paper up 1 line.

Later, computer became popular, these two concepts are used on computers. At that time, the storage device was very expensive, so some scientists said that it was expensive to add two characters at the end of each line, one is enough, so there are some arguments about which one to use.

In UNIX/Mac and Linux, \'\\n\' is put at the end of each line, in Windows, \'\\r\\n\' is put at the end of each line. The consequence of this use is that files in UNIX/Mac will be displayed in one line if opened in Windows. While file in Windows will have one ^M at the end of each line if opened in UNIX or Mac.

回答6:

On old paper-printer terminals, advancing to the next line involved two actions: moving the print head back to the beginning of the horizontal scan range (carriage return) and advancing the roll of paper being printed on (line feed).

Since we no longer use paper-printer terminals, those actions aren\'t really relevant anymore, but the characters used to signal them have stuck around in various incarnations.

回答7:

Consider an IBM 1403 impact printer. CR moved the print head to the start of the line, but did NOT advance the paper. This allowed for \"overprinting\", placing multiple lines of output on one line. Things like underlining were achieved this way, as was BOLD print. LF advanced the paper one line. If there was no CR, the next line would print as a staggered-step because LF didn\'t move the print head. FF advanced the paper to the next page. It typically also moved the print head to the start of the first line on the new page, but you might need CR for that. To be sure, most programmers coded CRFF instead of CRLF at the end of the last line on a page because an extra CR created by FF wouldn\'t matter.

回答8:

As a supplement,

1, Carriage return: It\'s a printer terminology meaning changing the print location to the beginning of current line. In computer world, it means return to the beginning of current line in most cases but stands for new line rarely.

2, Line feed: It\'s a printer terminology meaning advancing the paper one line. So Carriage return and Line feed are used together to start to print at the beginning of a new line. In computer world, it generally has the same meaning as newline.

3, Form feed: It\'s a printer terminology, I like the explanation in this thread.

If you were programming for a 1980s-style printer, it would eject the paper and start a new page. You are virtually certain to never need it.

http://en.wikipedia.org/wiki/Form_feed

It\'s almost obsolete and you can refer to Escape sequence \\f - form feed - what exactly is it? for detailed explanation.

Note, we can use CR or LF or CRLF to stand for newline in some platforms but newline can\'t be stood by them in some other platforms. Refer to wiki Newline for details.

LF: Multics, Unix and Unix-like systems (Linux, OS X, FreeBSD, AIX, Xenix, etc.), BeOS, Amiga, RISC OS, and others

CR: Commodore 8-bit machines, Acorn BBC, ZX Spectrum, TRS-80, Apple II family, Oberon, the classic Mac OS up to version 9, MIT Lisp Machine and OS-9

RS: QNX pre-POSIX implementation

0x9B: Atari 8-bit machines using ATASCII variant of ASCII (155 in decimal)

CR+LF: Microsoft Windows, DOS (MS-DOS, PC DOS, etc.), DEC TOPS-10, RT-11, CP/M, MP/M, Atari TOS, OS/2, Symbian OS, Palm OS, Amstrad CPC, and most other early non-Unix and non-IBM OSes

LF+CR: Acorn BBC and RISC OS spooled text output.

回答9:

\"\\n\" is the linefeed character. It means end the present line and go to a new line for anyone who is reading it.

回答10:

Carriage return and line feed are also references to typewriters, in that the with a small push on the handle on the left side of the carriage (the place where the paper goes), the paper would rotate a small amount around the cylinder, advancing the document one line. If you had finished typing one line, and wanted to continue on to the next, you pushed harder, both advancing a line and sliding the carriage all the way to the right, then resuming typing left to right again as the carriage traveled with each keystroke. Needless to say, word-wrap was the default setting for all word processing of the era. P:D

回答11:

Those are non-printing characters, relating to the concept of \"new line\". \\n is linefeed. \\r is carriage return. On different platforms they have different meanings, relative to a valid new line. In windows, a new line is \\r\\n. In linux, \\n. In mac, \\r.

In practice, you put them in any string, and it will have effect on the print-out of the string.