Integer Literals

Integer literals may be expressed in a variety of formats. The sections below describe the rules for each format and provide examples.

Decimal (base 10)

Rules

  • Cannot start with 0 (except for 0 itself), that would make it octal.
  • Cannot include a decimal point - that would make it floating point, even if the fractional part is 0.
  • Cannot include commas or spaces.
  • Must use only the digits zero-nine.
  • May be preceded by a minus sign "-".

Examples of Valid Decimal Integers

0 5 127 -1021 65535

Examples of Invalid Decimal Integers

32,767 25.0 1 024 0552 7A

Hexadecimal (base 16)

Rules

  • Must begin with 0x or 0X.
  • May use digits zero-nine and the letters A-F or a-f (not case sensitive).
  • Cannot include a decimal point.
  • Cannot include commas or spaces.
  • May be preceded by a minus sign "-".

Examples of Valid Hexadecimal Integers

0x 0x1 0x0A2B 0x12 0xBEEF

Yes, 0x is legal and represents a value of 0. However, nobody uses 0x alone, except perhaps in The International Obfuscated C Code Contest.

Examples of Invalid Hexadecimal Integers

0x5.3 0EA12 0xEG 53h

Octal (base 8)

While octal is still part of the ANSI/ISO C specification, almost no one uses it anymore.

Rules

  • Must begin with 0.
  • Must use only the digits zero-seven.
  • Cannot include a decimal point.
  • Cannot include commas or spaces.
  • May be preceded by a minus sign "-".

Examples of Valid Octal Integers

01 012 073125

Examples of Invalid Octal Integers

05.3 0o12 080 53o

Binary (base 2)

​

ANSI C does not specify a format for binary integer literals. However, this quasi-standard notation is supported by many compilers for embedded microcontrollers. If your code must be ANSI compliant, do not use this notation.

Rules

  • Must begin with 0b or 0B.
  • Must use only the digits zero-one.
  • Cannot include a decimal point.
  • Cannot include commas or spaces.
  • May be preceded by a minus sign "-".

Examples of Valid Binary Integers

0b 0b1 0b0101001100001111

Examples of Invalid Binary Integers

0b1.0 01100 0b12 10b

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