Declarations in C

By: Sam Chen in C Tutorials on 2007-09-20

All variables must be declared before use, although certain declarations can be made implicitly by content. A declaration specifies a type, and contains a list of one or more variables of that type, as in

   int  lower, upper, step;
   char c, line[1000];

Variables can be distributed among declarations in any fashion; the lists above could well be written as

   int  lower;
   int  upper;
   int  step;
   char c;
   char line[1000];

The latter form takes more space, but is convenient for adding a comment to each declaration for subsequent modifications.

A variable may also be initialized in its declaration. If the name is followed by an equals sign and an expression, the expression serves as an initializer, as in

   char  esc = '\\';
   int   i = 0;
   int   limit = MAXLINE+1;
   float eps = 1.0e-5;

If the variable in question is not automatic, the initialization is done once only, conceptionally before the program starts executing, and the initializer must be a constant expression. An explicitly initialized automatic variable is initialized each time the function or block it is in is entered; the initializer may be any expression. External and static variables are initialized to zero by default. Automatic variables for which is no explicit initializer have undefined (i.e., garbage) values.

The qualifier const can be applied to the declaration of any variable to specify that its value will not be changed. For an array, the const qualifier says that the elements will not be altered.

   const double e = 2.71828182845905;
   const char msg[] = "warning: ";

The const declaration can also be used with array arguments, to indicate that the function does not change that array:

   int strlen(const char[]);

The result is implementation-defined if an attempt is made to change a const.

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