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C++ String: What is the difference between '\n' and '\r\n'?

Sun, 18/02/2007 - 17:51 — admin

Q: What is the difference between '\n' and '\r\n'?

A:
Background
There are a few characters which can indicate a new line. The usual ones are these two:

  • '\n' or '0x0A' (10 in decimal) -> This character is called "Line Feed" (LF).
  • '\r' or '0x0D' (13 in decimal) -> This one is called "Carriage return" (CR).

Different Operating Systems handle newlines in a different way. Here is a short list of the most common ones:
  • DOS and Windows

    They expect a newline to be the combination of two characters, namely '\r\n' (or 13 followed by 10).

  • Unix (and hence Linux as well)

    Unix uses a single '\n' to indicate a new line.

  • Mac

    Macs use a single '\r'.

This difference gives rise to a number of problems. For example, a file created under Unix (so with newlines as a single LF) will not open correctly under Window's Notepad. Any Windows program that expects newlines to be CRLF will not work correctly with these files.

To unify things a bit, so that writing portable C/C++ programs is possible, file streams have both a "translated" and an "untranslated" mode. If you open a file in translated mode, the runtime library will convert a '\n' to the appropriate newline character(s). If the following program is compiled under Unix, the file will contain a single LF to indicate the newline. If it's compiled under windows, it will contain a CRLF.

#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp = fopen("testfile.txt", "w");
fprintf(fp, "Hello World\n");
fclose(fp);
return 0;
}

If you look at the generated file with a hex editor, you will see that the windows version has generated the following:

H e l l o W o r l d CR LF

0x48 0x65 0x6C 0x6C 0x6F 0x20 0x57 0x6F 0x72 0x6C 0x64 0x0D 0x0A

So file streams are handled in a transparent way, provided of course that you only handle files compatible with your operating system. But many times you have to pass multi-line strings directly to some system functions.

In practice
In Windows you have to pass multi-line strings with '\r\n', otherwise the system functions don't recognize them correctly as multi-line. This is true e.g. for setting the text of Edit controls, Labels, Windows etc. Also, when you read multi-line text from a file that initially contains '\r\n' in translated mode, the string in memory will contain only a single '\n'. See for example the documentation on MSDN about 'fread()':

Originally Posted by MSDN
The fread function reads up to count items of size bytes from the input stream and stores them in buffer. The file pointer associated with stream (if there is one) is increased by the number of bytes actually read. If the given stream is opened in text mode, carriage return




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