character constant

Syntax

' c-char ' (1)
u ' c-char ' (since C11) (2)
U ' c-char ' (since C11) (3)
L ' c-char ' (4)
' c-char-sequence ' (5)

where.

  • c-char is either
    • a character from the basic source character set minus single-quote ('), backslash (\), or the newline character.
    • escape sequence: one of special character escapes \' \" \? \\ \a \b \f \n \r \t \v, hex escapes \x... or octal escapes \... as defined in escape sequences.
(since C99)
  • c-char-sequence is a sequence of two or more c-chars.
1) single-byte integer character constant, e.g. 'a' or '\n' or '\13'. Such constant has type int and a value equal to the representation of c-char in the execution character set as a value of type char mapped to int. If c-char is not representable as a single byte in the execution character set, the value is implementation-defined.
2) 16-bit wide character constant, e.g. u'貓', but not u'????' (u'\U0001f34c'). Such constant has type char16_t and a value equal to the value of c-char in the 16-bit encoding produced by mbrtoc16 (normally UTF-16). If c-char is not representable or maps to more than one 16-bit character, the behavior is implementation-defined.
3) 32-bit wide character constant, e.g. U'貓' or U'????'. Such constant has type char32_t and a value equal to the value of c-char in in the 32-bit encoding produced by mbrtoc32 (normally UTF-32). If c-char is not representable or maps to more than one 32-bit character, the behavior is implementation-defined.
4) wide character constant, e.g. L'β' or L'貓. Such constant has type wchar_t and a value equal to the value of c-char in the execution wide character set (that is, the value that would be produced by mbtowc). If c-char is not representable or maps to more than one wide character (e.g. a non-BMP value on Windows where wchar_t is 16-bit), the behavior is implementation-defined .
5) multicharacter constant, e.g. 'AB', has type int and implementation-defined value.

Notes

Multicharacter constants were inherited by C from the B programming language. Although not specified by the C standard, compilers implement multicharacter constants as specified in B: the values of each char in the constant initialize successive bytes of the resulting integer, in big-endian zero-padded right-adjusted order, e.g. the value of '\1' is 0x00000001 and the value of '\1\2\3\4' is 0x01020304.

In C++, ordinary character constants have type char, rather than int.

Unlike integer constants, a character constant may have a negative value if char is signed: on such implementations '\xFF' is an int with the value -1.

When used in a controlling expression of #if or #elif, character constants may be interpreted in terms of the source character set, the execution character set, or some other implementation-defined character set.

Example

#include <stddef.h>
#include <stdio.h>
#include <uchar.h>
 
int main (void)
{
    printf("constant    value     \n");
    printf("--------    ----------\n");
 
    // integer character constants,
    int c1='a'; printf("'a':        %#010x\n", c1);
    int c2='????'; printf("'????':       %#010x\n\n", c2); // implementation-defined
 
    // multicharacter constant
    int c3='ab'; printf("'ab':       %#010x\n\n", c3); // implementation-defined
 
    // 16-bit wide character constants
    char16_t uc1 = u'a'; printf("'a':        %#010x\n", (int)uc1);
    char16_t uc2 = u'¢'; printf("'¢':        %#010x\n", (int)uc2);
    char16_t uc3 = u'猫'; printf("'猫':       %#010x\n", (int)uc3);
    // implementation-defined (???? maps to two 16-bit characters)
    char16_t uc4 = u'????'; printf("'????':       %#010x\n\n", (int)uc4);
 
    // 32-bit wide character constants
    char32_t Uc1 = U'a'; printf("'a':        %#010x\n", (int)Uc1);
    char32_t Uc2 = U'¢'; printf("'¢':        %#010x\n", (int)Uc2);
    char32_t Uc3 = U'猫'; printf("'猫':       %#010x\n", (int)Uc3);
    char32_t Uc4 = U'????'; printf("'????':       %#010x\n\n", (int)Uc4);
 
    // wide character constants
    wchar_t wc1 = L'a'; printf("'a':        %#010x\n", (int)wc1);
    wchar_t wc2 = L'¢'; printf("'¢':        %#010x\n", (int)wc2);
    wchar_t wc3 = L'猫'; printf("'猫':       %#010x\n", (int)wc3);
    wchar_t wc4 = L'????'; printf("'????':       %#010x\n\n", (int)wc4);
}

Possible output:

constant    value     
--------    ----------
'a':        0x00000061
'????':       0xf09f8d8c
 
'ab':       0x00006162
 
'a':        0x00000061
'¢':        0x000000a2
'猫':       0x0000732b
'????':       0x0000df4c
 
'a':        0x00000061
'¢':        0x000000a2
'猫':       0x0000732b
'????':       0x0001f34c
 
'a':        0x00000061
'¢':        0x000000a2
'猫':       0x0000732b
'????':       0x0001f34c

References

  • C11 standard (ISO/IEC 9899:2011):
    • 6.4.4.4 Character constants (p: 67-70)
  • C99 standard (ISO/IEC 9899:1999):
    • 6.4.4.4 Character constants (p: 59-61)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • 3.1.3.4 Character constants

See also

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