Escape sequences in C

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In the C programming language, an escape sequence is specially delimited text in a character or string literal that represents one or more other characters to the compiler. It allows a programmer to specify characters that are otherwise difficult or impossible to specify in a literal.

An escape sequence starts with a backslash (\) called the escape character and subsequent characters define the meaning of the escape sequence. For example, \n denotes a newline character.

The same or similar escape sequences are used in other, related languages such C++, C#, Java and PHP.

Value

To demonstrate the value of the escape sequence feature, to output the text Foo on one line and Bar on the next line, the code must output a newline between the two words.

The following code achieves the goal via text formatting and a hard-coded ASCII character value for newline (0x0A). This behaves as desired with the words on sequential lines, but an escape sequence has advantages.

#include <stdio.h>
int main() {
    printf("Foo%cBar", 0x0A);
    return 0;
}

The \n escape sequence allows for shorter code by specifying the newline in the string literal, and for faster runtime by eliminating the text formatting operation. Also, the compiler can map the escape sequence to a character encoding system other than ASCII and thus make the code more portable.

#include <stdio.h>
int main() {
    printf("Foo\nBar");
    return 0;
}

How it works

An escape sequence changes how the compiler interprets character data in a literal. For example, \n does not represent a backslash followed by the letter n. The backslash escapes the compiler's normal, literal way of interpreting character data. After a backslash, the compiler expects subsequent characters to complete one of the defined escape sequences, and then translates the escape sequence into the characters it represents.

This syntax does require special handling to encode a backslash character – since it is a metacharacter that changes literal interpretation behavior; not the literal backslash character. The issue is solved by using two backslashes (\\) to mean one.

Escape sequences

The following table includes escape sequences defined in standard C as well as some non-standard sequences. The C standard requires an escape sequence that does not match a defined sequence to be diagnosed – i.e., the compiler must print an error message. Regardless, some compilers define additional escape sequences.

The table shows the ASCII value a sequence maps to, however, it may map to different values based on encoding.

Escape

^ The non-standard sequence \e represents the escape character in GCC,^[2] clang and tcc. It was not added to the C standard because it has no meaningful equivalent in some character sets (such as EBCDIC).^[1]

Newline

Sequence \n maps to one byte, despite the fact that the platform may use more than one byte to denote a newline, such as the DOS/Windows CRLF sequence, 0x0D 0x0A. The translation from 0x0A to 0x0D 0x0A on DOS and Windows occurs when the byte is written out to a file or to the console, and the inverse translation is done when text files are read.

Hex

A hex escape sequence must have at least one hex digit following \x, with no upper bound; it continues for as many hex digits as there are. Thus, for example, \xABCDEFG denotes the byte with the numerical value ABCDEF₁₆, followed by the letter G, which is not a hex digit. However, if the resulting integer value is too large to fit in a single byte, the actual numerical value assigned is implementation-defined. Most platforms have 8-bit char types, which limits a useful hex escape sequence to two hex digits. However, hex escape sequences longer than two hex digits might be useful inside a wide character or wide string literal (prefixed with L):

// single char with value 0x12 (18 decimal)
char s1[] = "\x12";
// single char with implementation-defined value, unless char is long enough
char s1[] = "\x1234";
// single wchar_t with value 0x1234, provided wchar_t is long enough (16 bits suffices)
wchar_t s2[] = L"\x1234";

Octal

^ An octal escape sequence consists of a backslash followed by one to three octal digits. The octal escape sequence ends when it either contains three octal digits, or the next character is not an octal digit. For example, \11 is an octal escape sequence denoting a byte with decimal value 9 (11 in octal). However, \1111 is the octal escape sequence \111 followed by the digit 1. In order to denote the byte with numerical value 1, followed by the digit 1, one could use "\1""1", since C concatenates adjacent string literals.

Some three-digit octal escape sequences are too large to fit in a single byte. This results in an implementation-defined value for the resulting byte.

The escape sequence \0 is a commonly used octal escape sequence, which denotes the null character, with value zero in ASCII and most encoding systems.

Universal character names

^ ^ Since the C99 standard, C supports escape sequences that denote Unicode code points, called universal character names. They have the form \uhhhh or \Uhhhhhhhh, where h stands for a hex digit. Unlike other escape sequences, a universal character name may expand into more than one code unit.

The sequence \uhhhh denotes the code point hhhh, interpreted as a hexadecimal number. The sequence \Uhhhhhhhh denotes the code point hhhhhhhh, interpreted as a hexadecimal number. Code points located at U+10000 or higher must be denoted with the \U syntax, whereas lower code points may use \u or \U. The code point is converted into a sequence of code units in the encoding of the destination type on the target system. For example, where the encoding is UTF-8, and UTF-16 for wchar_t:

// A single byte with the value 0xC0; not valid UTF-8
char s1[] = "\xC0";
// Two bytes with values 0xC3, 0x80; the UTF-8 encoding of U+00C0
char s2[] = "\u00C0";
// A single wchar_t with the value 0x00C0
wchar_t s3[] = L"\xC0";
// A single wchar_t with the value 0x00C0
wchar_t s4[] = L"\u00C0";

A value greater than \U0000FFFF may be represented by a single wchar_t if the UTF-32 encoding is used, or two if UTF-16 is used.

Importantly, the universal character name \u00C0 always denotes the character "À", regardless of what kind of string literal it is used in, or the encoding in use. The octal and hex escape sequences always denote certain sequences of numerical values, regardless of encoding. Therefore, universal character names are complementary to octal and hex escape sequences; while octal and hex escape sequences represent code units, universal character names represent code points, which may be thought of as "logical" characters.

Alternatives

Some languages provide different mechanisms for coding behavior that the escape sequence provide. For example, the following Pascal code writes the two words on sequential lines:

writeln('Foo');
write('Bar');

writeln outputs a newline after the parameter text, while write does not.

See also

• Escape sequence
• Digraph

References

Further reading

• ISO/IEC 9899:1999, Programming languages — C
• Kernighan, Brian W.; Ritchie, Dennis M. (2003) [1988]. The C Programming Language (2 ed.). Prentice Hall. ISBN 978-0-13308621-8.
• Lafore, Robert (2001). Object-Oriented Programming in Turbo C++ (1 ed.). Galgotia Publications. ISBN 978-8-18562322-1.