3.1. Integer Constants

An integer constant can be expressed as an ordinary decimal numeral, or as a numeral in octal or hexadecimal notation. You must specify the intended notation by a prefix.

A decimal constant begins with a nonzero digit. For example, 255 is the decimal constant for the base-10 value 255.

A number that begins with a leading zero is interpreted as an octal constant. Octal (or base eight) notation uses only the digits from 0 to 7. For example, 047 is a valid octal constant representing 4 x 8 + 7, and is equivalent with the decimal constant 39. The decimal constant 255 is equal to the octal constant 0377.

A hexadecimal constant begins with the prefix 0x or 0X. The hexadecimal digits A to F can be upper- or lowercase. For example, 0xff, 0Xff, 0xFF, and 0XFF represent the same hexadecimal constant, which is equivalent to the decimal constant 255.

Because the integer constants you define will eventually be used in expressions and declarations, their type is important. The type of a constant is determined at the same time as its value is defined. Integer constants such as the examples just mentioned usually have the type int. However, if the value of an integer constant is outside the range of the type int, then it must have a bigger type. In this case, the compiler assigns it the first type in a hierarchy that is large enough to represent the value. For decimal constants, the type hierarchy is:

    int, long, long long

For octal and hexadecimal constants , the type hierarchy is:

    int, unsigned int, long, unsigned long, long long, unsigned long long

For example, on a 16-bit system, the decimal constant 50000 has the type long, since the greatest possible int value is 32,767, or 2¹⁵ - 1.

You can also influence the types of constants in your programs explicitly by using suffixes . A constant with the suffix l or L has the type long (or a larger type if necessary, in accordance with the hierarchies just mentioned). Similarly, a constant with the suffix ll or LL has at least the type long long. The suffix u or U can be used to ensure that the constant has an unsigned type. The long and unsigned suffixes can be combined. Table 3-1 gives a few examples.

Table 3-1. Examples of constants with suffixes

Integer constant	Type
0x200	int
512U	unsigned int
0L	long
0Xf0fUL	unsigned long
0777LL	long long
0xAAAllu	unsigned long long