Keywords

Keywords are preserved words that have special meaning in C language. The meaning of the keywords has already been described to the C compiler. These meaning cannot be changed. Thus, the keywords cannot be used as variable names because that would try to change the existing meaning of the keyword, which is not allowed. There are total 32 keywords in C language.

autodoubleintstruct
breakelselongswitch
caseenumregistertypedef
constexternreturnunion
charfloatshortunsigned
continueforsignedvolatile
defaultgotosizeofvoid
doifstaticwhile

Identifiers

In C language identifiers are the names given to variables, constants, functions and user-define data. These identifier are defined against a set of rules.


Rules for an Identifier

  1. An Identifier can only have alphanumeric characters( a-z , A-Z , 0-9 ) and underscore( _ ).
  2. The first character of an identifier can only contain alphabet( a-z , A-Z ) or underscore ( _ ).
  3. Identifiers are also case sensitive in C. For example name and Name are two different identifier in C.
  4. Keywords are not allowed to be used as Identifiers.
  5. No special characters, such as semicolon, period, whitespaces, slash or comma are permitted to be used in or as Identifier.

Character set

In C language characters are grouped into the following catagories,

  1. Letters(all alphabets a to z & A to Z).
  2. Digits (all digits 0 to 9).
  3. Special characters, ( such as colon :, semicolon ;, period ., underscore _, ampersand & etc).
  4. White spaces.