Integers literals can be expressed in several numeration bases.
Decimal numbers use the usual syntax [1-9][0-9]*. For
example, 2345.
Octal numbers are expressed using a prefix 0o (or
0O) followed by one or more digits in the range [0-7].
Examples are 0o0, 0o100 and 0o777.
Hexadecimal numbers are expressed using a prefix 0x (or
0X) followed by one or more hexadecimal digits in the range
[0-f]. Examples are 0x0 and 0xfe00ffff. Note
that both the x in the prefix and the letters in the
hexadecimal number are case insensitive. Thus, 0XdeadBEEF is a
valid (but ugly as hell) literal.
Binary numbers are expressed using a prefix 0b (or
0B) followed by one or more binary digits in the range
[0-1]. Examples of binary literals are 0b0 and
0B010.
Negative numbers, of any numeration base, are constructed using the
minus operator as explained below. Therefore the minus symbol
- in negative numbers is not part of the literal themselves.
_The character _ can appear anywhere in a numeric literal
except as the first character. It is ignored, and its purpose is to
make it easier for programmers to read them:
0xf000_0000_0000_0000 0b0000_0001_0000_0001
The type of a numeric literal is the smallest signed integer capable of holding it, starting with 32 bits, in steps of powers of two and up to 64 bits.11
So, for example, the value 2 has type int<32>, but the
value 0xffff_ffff has type int<64>, because it is out of
the range of signed 32-bit numbers.
A set of suffixes can be used to construct integer literals of certain
types explicitly. L or l is for 64-bit integers.
H or h is for 16-bit integers (also known as
halves), B or b is for 8-bit integers (also known
as bytes) n or N is for 4-bit integers (also known
as nibbles) and t or T is for 1-bit integers (also
known as bits).
Thus, 10L is a 64-bit integer with value
0x0000_0000_0000_000A, 10H is a 16-bit integer with
value 0x000A and 10b is a 8-bit integer with value
0x0A.
Similarly, the signed or unsigned attribute of an integer can be
explicitly specified using the suffix u or U (the
default are signed types). For example 0xffff_ffffU has type
uint<32> and 0ub has type uint<8>. It is
possible to combine width-indicating suffixes with signedness
suffixes: 10UL denotes the same literal as 10LU. Note
that bit constants (suffix t or T) are always unsigned
and do not require U.
The above rules guarantee that it is always possible to determine the width and signedness of an integer constant just by looking at it, with no ambiguity.
Rationale: the width of a C “int” is 32 bits in most currently used architectures, and binary data formats are usually modelled after C.