I'm looking at some linux specific code which is outputting the likes of:
\r\x1b[J>
to the std io.
I understand that <ESC>[J represents deleting the contents of the screen from the current line down, but what does \r do here?
I'm also seeing the following:
>user_input\n\r>
where user_input is the text entered by the user. But what is the purpose of the \r here?
The character '\r' is carriage return. It returns the cursor to the start of the line.
It is often used in Internet protocols conjunction with newline ('\n') to mark the end of a line (most standards specifies it as "\r\n", but some allows the wrong way around). On Windows the carriage-return newline pair is also used as end-of-line. On the old Macintosh operating system (before OSX) a single carriage-return was used instead of newline as end-of-line, while UNIX and UNIX-like systems (like Linux and OSX) uses a single newline.
Control character \r moves caret (a.k.a text cursor) to the leftmost position within current line.
From 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,
'\r\n', 0x0D0A). These characters are based on printer commands: The
line feed indicated that one line of paper should feed out of the
printer thus instructed the printer to advance the paper one line, and
a carriage return indicated that the printer carriage should return to
the beginning of the current line. Some rare systems, such as QNX
before version 4, used the ASCII RS (record separator, 0x1E, 30 in
decimal) character as the newline character.
FWIW - this is a part of carriage control - from mainframe control words to Windows/UNIX/FORTRAN carriage control. Carriage control can be implemented at a language level like FORTRAN does, or system-wide like UNIX and Windows do.
\n arose from limitations of early PDP user "interfaces" - the tty terminal. Go to a museum if you want see one.
A very simple point: The difference between \n \r is explained above. But all of these explanations are really saying that carriage control is implementation dependent.
The [J is part of ANSI escape sequences and what they do on a "standards conforming tty terminal".
DOS used to have ANSI.SYS to provide: colors, underline, bold using those sequences.
http://ascii-table.com/ansi-escape-sequences.php
Is a good reference for the question: what does some odd looking string in the output do?
\r is carriage return. Similarly \n is linefeed.
Related
We know that Windows uses a CR + LF pair as its new line, Unix (including Linux and OS X) uses a single LF, while MacOS uses a single CR.
Does that mean that the interpretation of a newline in C and C++ depends upon the execution environment, even though K&R (section 1.5.3 Line Counting) states the following very categorically?
so '\n' stands for the value of the newline character, which is 10 in ASCII.
We know that Windows uses a CR + LF pair as its new line,…
The page you link to does not say Windows uses “CR + LF” as its new line character. It says Windows marks the end of a line in a text file with a carriage-return character and a line-feed character. That does not mean those characters are a new-line character or vice-versa.
Does that mean that the interpretation of a newline…
The new-line character is a new-line character. In C, it is intended to mark a new line. When ASCII is used, ASCII’s line-feed character (code 10) is typically used as C’s new-line character ('\n').
If a C program reads a Windows-style text file using a binary stream, it will see a carriage-return character and a line-feed marking the ends of lines. If a C program reads a Windows-style text file using a text stream (in an environment that supports this), the Windows line-ending indications (carriage-return character and line-feed character) will be automatically translated to C new-line characters.
Conversely, if a C program writes to a Windows-style text file using a text stream, the new-line characters it writes will be translated to Windows line-ending indications. If it writes using a binary stream, it must write the carriage-return characters and the line-feed characters itself.
Does that mean that the interpretation of a newline in C and C++ depends upon the execution environment
No, it does not depend. The interpretation depends the tool that reading the file which is platform suggested but can differ. A robust text tool will tolerate various encodings and will
handle change.
Further, text files originating on one system are accessed/edited by other planforms with different rules.
No, \n always means LF.
On Windows there is LF <-> CR-LF conversion that's performed by the IO streams (FILE *, std::??stream), if the stream is opened in text mode (as opposed to binary mode).
Does that mean that the interpretation of a newline in C and C++ depends upon the execution environment?
The interpretation of the file contents does indeed depend on the execution environment, so that the C programmer does not have to handle the different conventions explicitly:
if the stream is open as binary "rb", no translation is performed and each byte of the file contents is returned directly by getchar(). Unix systems handle text files and binary files identically, so no translation occurs for text files either.
on other systems, streams open in text mode "rt" or just "r" are handled in a system specific way to translate line ending patterns to the single byte '\n', which in ASCII has the value 10. On Windows and MS/DOS systems, this translation converts CR/LF pairs to single bytes '\n', which can be implemented as simply removing CR bytes. This convention was inherited from previous microcomputer operating systems such as Gary Kildall's CP/M, whose APIs were emulated in QDOS, Seattle Computer Products' original 8086 OS that later became MS/DOS.
older Mac systems (before OS/X) used to represent line endings with a single CR byte, but Apple changed this when they adopted a Unix kernel for their OS/X system. No translation is performed anymore on macOS.
Antique systems used to have even more cumbersome representations for text files, such as fixed length records and the stream implementation was inserting extra '\n' bytes to simulate unix line endings when read such streams in text mode.
It is important to understand that this translation process is system specific and is not designed to handle files copied from other systems that use a different convention. Advanced Text tools such as the QEmacs programmers' editor can detect different line endings and perform the appropriate translation regardless of the current execution environment, preserving the convention used in the file, or converting it to another convention under user control.
I understand that QString::split should be used to get a QStringList from a multiline QString. But if I have a file and I don't know if it comes from Mac, Windows or Unix, I'm not sure if QString.split("\n") would work well in all the cases. What is the best way to handle this situation?
If it's acceptable to remove blank lines, you can try:
QString.split(QRegExp("[\r\n]"),QString::SkipEmptyParts);
This splits the string whenever any of the newline character (either line feed or carriage return) is found. Any consecutive line breaks (e.g. \r\n\r\n or \n\n) will be considered multiple delimiters with empty parts between them, which will be skipped.
Emanuele Bezzi's answer misses a couple of points.
In most cases, a string read from a text file will have been read using a text stream, which automatically translates the OS's end-of-line representation to a single '\n' character. So if you're dealing with native text files, '\n' should be the only delimiter you need to worry about. For example, if your program is running on a Windows system, reading input in text mode, line endings will be marked in memory with single \n characters; you'll never see the "\r\n" pairs that exist in the file.
But sometimes you do need to deal with "foreign" text files.
Ideally, you should probably translate any such files to the local format before reading them, which avoids the issue. Only the translation utility needs to be aware of variant line endings; everything else just deals with text.
But that's not always possible; sometimes you might want your program to handle Windows text files when running on a POSIX system (Linux, UNIX, etc.), or vice versa.
A Windows-format text file on a POSIX system will appear to have an extra '\r' character at the end of each line.
A POSIX-format text file on a Windows system will appear to consist of one very long line with embedded '\n' characters.
The most general approach is to read the file in binary mode and deal with the line endings explicitly.
I'm not familiar with QString.split, but I suspect that this:
QString.split(QRegExp("[\r\n]"),QString::SkipEmptyParts);
will ignore empty lines, which will appear either as "\n\n" or as "\r\n\r\n", depending on the format. Empty lines are perfectly valid text data; you shouldn't ignore them unless you're certain that it makes sense to do so.
If you need to deal with text input delimited either by "\n", "\r\n", or "\r", then I think something like this:
QString.split(QRegExp("\n|\r\n|\r"));
would do the job. (Thanks to parsley72's comment for helping me with the regular expression syntax.)
Another point: you're probably not likely to encounter text files that use just '\r' to delimit lines. That's the format used by MacOS up to version 9. MaxOS X is based on UNIX, and it uses standard UNIX-style '\n' line endings (though it probably tolerates '\r' line endings as well).
What are difference between:
\r\n - Line feed followed by carriage return.
\n - Line feed.
\r - Carriage Return.
They are the line terminator used by different systems:
\r\n = Windows
\n = UNIX and Mac OS X
\r = Old Mac
You should use std::endl to abstract it, if you want to write one out to a file:
std::cout << "Hello World" << std::endl;
In general, an \r character moves the cursor to the beginning of the line, while an \n moves the cursor down one line. However, different platforms interpret this in different ways, leading to annoying compatibility issues, especially between Windows and UNIX. This is because Windows requires an \r\n to move down one line and move the cursor to the start of the line, whereas on UNIX a single \n suffices.
Also, obligatory Jeff Atwood link: http://www.codinghorror.com/blog/2010/01/the-great-newline-schism.html
Historical info
The terminology comes from typewriters. Back in the day, when people used typewriters to write, when you got to the end of a line you'd press a key on the typewriter that would mechanically return the carriage to the left side of the page and feed the page up a line. The terminology was adopted by computers and represented as the ascii control codes 0xa, for the linefeed, and 0xd for the carriage return. Different operating systems use them differently, which leads to problems when editing a text file written on a Unix machine on a Windows machine and vice-versa.
Pragmatic info
On Unix based machines in text files a newline is represented by the linefeed character, 0xa. On Windows both a linefeed and carriage return are used. For example when you write some code on Linux that has the following in it where the file was opened in text mode:
fprintf(f, "\n");
the underlying runtime will insert only a linefeed character 0xa to the file. On Windows it will translate the \n and insert 0xd0xa. Same code but different results depending on the operating system used. However this changes if the file is opened in binary mode on Windows. In this case the insertion is done literally and only a linefeed character is inserted. This means that the sequence you asked about \r\n will have a different representation on Windows if output to a binary or text stream. In the case of it being a text stream you'll see the following in the file: 0xd0xd0xa. If the file was in binary mode then you'll see: 0xd0xa.
Because of the differences in how operating systems represent newlines in text files text editors have had to evolve to deal with them, although some, like Notepad, still don't know what to do. So in general if you're working on Windows and you're given a text file that was originally written on a Unix machine it's not a good idea to edit it in Notepad because it will insert the Windows style carriage return linefeed (0xd0xa) into the file when you really want a 0xa. This can cause problems for programs running on old Unix machines that read text files as input.
See http://en.wikipedia.org/wiki/Newline
Different operating systems have different conventions; Windows uses \r\n, Mac uses \r, and UNIX uses \n.
\r
This sends the cursor to the beginning column of the display
\n
This moves the cursor to the new line of the display, but the cursor stays in the same column as the previous line.
\r\n
Combine 1 and 2. The cursor is moved to the new line, and it is also moved to the first column of the display.
Some compilers prints both a new line and carriage return when you specify only \n.
I have a multi-line ASCII string coming from some (Windows/UNIX/...) system. Now, I know about differences in newline character in Windows and UNIX (CR-LF / LF) and I want to parse this string on both (CR and LF) characters to detect which newline character(s) is used in this string, so I need to know what "\n" in VS6 C++ means.
My question is if I write a peace of code in Visual Studio 6 for Windows:
bool FindNewline (string & inputString) {
size_t found;
found = inputString.find ("\n");
return (found != string::npos ? true : false);
}
does this searches for CR+LF or only LF? Should I put "\r\n" or compiler interprets "\n" like CR+LF?
inputString.find ("\n");
will search for the LF character (alone).
Library routines may 'translate' between CR/LF and '\n' when I/O is performed on a text stream, but inside the realm of your program code, '\n' is just a line-feed.
"\n" means "\n". Nothing else. So you search for LF only. However Microsoft CRT does some conversions for you when you read a file in text mode, so you can write simpler code, sometimes.
All translation between "\n" and "\r\n" happens during I/O. At all other times, "\n" is just that and nothing more.
Somehow: return (found != string::npos ? true : false); reminds me of another answer I wrote a while back.
Apart from the VS6 part (you really, really want to upgrade this, the compiler is way out of date and Microsoft doesn't really support it anymore), the answer to the question depends on how you are getting the string.
For example, if you read it from a file in text mode, the runtime library will translate \r\n into \n. So if all your text strings are read in text mode via the usual file-based APIs, your search for\n` (ie, newline only) would be sufficient.
If the strings originate in files that are read in binary mode on Windows and are known to contain the DOS/Windows line separator \r\n, the you're better off searching for that character sequence.
EDIT: If you do get it in binary form, yes, ideally you'd have to check for both \r\n and \n. However I would expect that they aren't mixed within one string and still carry the same meaning unless it's a really messed up data format. I would probably check for \r\n first and then \n second if the strings are short enough and scanning them twice doesn't make that much of a difference. If it does, I'd write some code that checks for both \r\n and single \n in a single pass.
I am writing code that runs in Windows and outputs a text file that later becomes the input to a program in Linux. This program behaves incorrectly when given files that have newlines that are CR+LF rather than just LF.
I know that I can use tools like dos2unix, but I'd like to skip the extra step. Is it possible to get a C++ program in Windows to use the Linux newline instead of the Windows one?
Yes, you have to open the file in "binary" mode to stop the newline translation.
How you do it depends on how you are opening the file.
Using fopen:
FILE* outfile = fopen( "filename", "wb" );
Using ofstream:
std::ofstream outfile( "filename", std::ios_base::binary | std::ios_base::out );
OK, so this is probably not what you want to hear, but here's my $0.02 based on my experience with this:
If you need to pass data between different platforms, in the long run you're probably better off using a format that doesn't care what line breaks look like. If it's text files, users will sometimes mess with them. If by messing the line endings up they cause your application to fail, this is going to be a support intensive application.
Been there, done that, switched to XML. Made the support guys a lot happier.
A much cleaner solution is to use the ASCII escape sequence for the LF character (decimal 10): '\012' or '\x0A' represents an explicit single line feed regardless of platform.
Note that this at least on some compilers does not work; for example, on MSVC 2019 16.11.6, both '\012' and '\x0A' get translated to carriage return and line feed. It also does not matter there whether a string literal ("\012") or a char literal ('\012') is used.
This method also avoids string length surprises, as '\n' can expand to two characters. But so can multibyte unicode characters, in UTF8, when written directly into a string literal in the source code.
Note also that '\r' is the platform-independent code for a single carriage return (decimal 13). The '\f' character is not the line feed, but rather the form feed (decimal 12), which is not a newline on any platform I am aware of. C does not offer a single-character backslash escape for the line feed, thus the need for the longer octal or hexadecimal escapes.