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It seems like just putting a linefeed is good enough, but I know it is supposed to be carriage return + line feed. Does anything horrible happen if you don't put the carriage return and only use line feeds?

This is in ANSI C and not going to be redirected to a file or anything else. Just a normal console app.

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The Windows console follows the same line ending convention that is assumed for files, or for that matter for actual, physical terminals. It needs to see both CR and LF to properly move to the next line.

That said, there is a lot of software infrastructure between an ANSI C program and that console. In particular, any standard C library I/O function is going to try to do the right thing, assuming you've allowed it the chance. This is why fopen()'s t and b modifiers for the mode parameter were defined.

With t (the default for most streams, and in particular for stdin and stdout) then any \n printed is converted to a CRLF sequence, and the reverse happens for reads. To turn off that behavior, use the b modifier.

Incidentally, the terminals traditionally hooked to *nix boxes including the DEC VT100 emulated by XTerm also needs both CR and LF. However, in the *nix world, the conversion from a newline character to a CRLF sequence is handled in the tty device driver so most programs don't need to know about it, and the t and b modifiers are both ignored. On those platforms, if you need to send and receive characters on a tty without that modification, you need to look up stty(1) or the system calls it depends on.

If your otherwise ANSI C program is avoiding C library I/O to the console (perhaps because you need access to the console's character color and other attributes) then whether you need to send CR or not will depend on which Win32 API calls you are using to send the characters.

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If you're in a *nix environment \n (Linefeed) is probably ok. If you're in Windows and aren't redirecting (now) a linefeed is also ok, but if someone at somepoint redirects, :-(

If you're doing Windows though, there could be issues if the output is redirected to a text file and then another process tries to consume the data.

The console knows what to show, but consumers might not be happy...

If you are using C# You might try the Environment.NewLine "constant".


If you're really in vanilla c, you're stuck with \r\n. :-)

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Actually, not entirely true. The C runtime library is very careful to do sensible translations for ANSI C file I/O. That is exactly why the t and b modifiers to fopen() were invented. The standard streams are opened in text mode, unless you go to some effort to change them, and piping and redirection work as a Windows user would expect. The story is different if you call the Win32 API directly, but then you aren't doing ANSI C I/O any more. –  RBerteig Mar 10 '11 at 8:47
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It depends on what you're using them for. Some programs will not display newlines properly if you don't put both \r and \n.

If you try to only write \n some programs that consume your text file (or output) may display your text as a single line instead of multiple lines.

There are also some file formats and protocols that will completely be invalid without using both \r and \n.

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I haven't tried it in so long that I'm not sure I remember what happens... but doesn't a linefeed by itself move down a line without returning to the left column?

Depending on your compiler, the standard output might be opened in text mode, in which case a single linefeed will be translated to \r\n before being written out.

Edit: I just tried a quick test, and in XP a file without returns displays normally. I still don't know if any compilers insert the returns for you.

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In C, files (called "streams") come in two flavors - binary or text.

The meaning of this distinction is left implementation/platform dependent, but on Windows (with common implementations that I've seen) when writing to text streams '\n' is automatically translated to "\r\n", and when reading from text streams "\r\n" is automatically translated to '\n'.

The "console" is actually "standard output", which is a stream opened by default as a text stream. So, in practice on Windows, writing "Hello, world!\n" should be quite sufficient - and portable.

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