Constants

Programming Tools Guide
Chapter 3, C language compiler

Constants

Constants in the C language include:

The first three types, integral constants, floating point constants, and character constants, are explained in this subsection. Enumeration constants are explained in ``Enumerations''.

Integral constants

Integral constants may be:

Decimal:
- Digits: ``0-9''.
- The first digit may not be ``0'' (zero).
Octal:
- Digits: ``0-7''.
- The first digit must be ``0'' (zero).
Hexadecimal:
- Digits: ``0-9'' plus letters ``a-f'' or ``A-F''. Letters correspond to decimal values 10-15.
- Prefixed by ``0x'' or ``0'' (digit zero).

Suffixes:

All of the above can be suffixed to indicate type, as follows:

 --------------------------------------------------
 Suffix   Type
 --------------------------------------------------

 u or U   unsigned
 l or L   long
 both     unsigned long

Floating point constants

Consist of integer part, decimal point, fraction part, an e or E, an optionally signed integer exponent, and a type suffix, one of f, F, l, or L. Each of these elements is optional; however one of the following must be present for the constant to be a floating point constant:
- A decimal point (preceded or followed by a number).
- An e with an exponent.
- Any combination of the above. Examples:
```
   xxx  e exp
   xxx.
   .xxx
```
Type determined by suffix; f or F indicates float, l or L indicateslong double, otherwise type is double.

Character constants

One or more characters enclosed in single quotes, as in `x'.
All character constants have type int.
The value of a character constant is the numeric value of the character in the ASCII character set.
A multiple-character constant that is not an escape sequence (see below) has a value derived from the numeric values of each character. For example, the constant `123' has a value of
```
 -----------------------
|  0 |  '1'|  '2'|  '3'|
|----|-----|-----|-----|
```
or 0x313233.
Character constants may not contain the character ' or new-line. To represent these characters, and some others that may not be contained in the source character set, the compiler provides the following escape sequences:

Table 3-1 Escape sequences

 new-line          NL (LF)   \n|  audible alert        BEL   \a
 horizontal tab    HT        \t|  question mark        ?     \?
 vertical tab      VT        \v|  double quote         "     \"
 backspace         BS        \b|  octal escape         ooo   \ooo 
 carriage return   CR        \r|  hexadecimal escape   hh    \xhh 
 formfeed          FF        \f|  backslash            \     \\
 single quote      '         \'|

If the character following a backslash is not one of those specified, the compiler issues a warning and treat the backslash-character sequence as the character itself. Thus, '\\q' is treated as 'q'. However, if you represent a character this way, you run the risk that the character may be made into an escape sequence in the future, with unpredictable results. An explicit new-line character is invalid in a character constant and causes an error message.

The octal escape consists of one to three octal digits.
The hexadecimal escape consists of one or more hexadecimal digits

Wide characters and multibyte characters

A wide character constant is a character constant prefixed by the letter L.
A wide character has an external encoding as a multibyte character and an internal representation as the integral type wchar_t, defined in stddef.h.
A wide character constant has the integral value for the multibyte character between single quote characters, as defined by the locale-dependent mapping function mbtowc.