C Pocket Reference

Function Calls

A function call is an expression whose value and type are those of the function’s return value.

The number and the type of the arguments in a function call must agree with the number and type of the parameters in the function definition. Any expression, including constants and arithmetic expressions, may be specified as an argument in a function call. When the function is called, the value of the argument is copied to the corresponding parameter of the function! For example:

double x=0.5, y, pow();    // Declaration
y = pow( 1.0 + x, 2.5 );   // Call to pow() yields 
                           // the double value (1.0+x)2.5

In other words, the arguments are passed to the function by value. The function itself cannot modify the values of the arguments in the calling function: it can only access its local copy of the values.

In order for a function to modify the value of a variable directly, the caller must give the function the address of the variable as an argument. In other words, the variable must be passed to the function by reference. Examples of functions that accept arguments by reference include scanf(), time(), and all functions that have an array as one of their parameters. For example:

double swap( double *px, double *py ) // Exchange values
                                      // of two variables
{ double z = *px; *px = *py; *py = z; }

The arguments of a function are subject to implicit type conversion:

• If the function was declared in prototype form (as is usually the case), each argument is converted to the type of the corresponding ...