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FUNCTIONS, PROTOTYPES, PARAMETERS and RETURN VALUES.
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A function in the C language is equivalent to a subroutine or function in
Fortran, or a procedure or function in Pascal. In C, a function may
return a value to the calling statement, as a function does in Pascal, or
it may not, as with a procedure in Pascal.
These notes refer to the ANSI C standard, which has been developed since
1983 and the style of the function declaration is referred to as the
modern format. Pre-ANSI compilers may not conform and certainly do not
include function prototypes. The pre-ANSI format is called classical.
There are many standard functions, like printf(), but a user may
incorporate functions of his own. A function definition has the form:
return-type function-name(parameter declarations, if any)
{
declarations
statements
}
A typical example illustrates this for the raising of a base value to
the power n:
int power(int base, int n) /* ANSI C function prototyping */
{
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p; /* Value returned by function */
}
The above example is taken from Kernighan and Ritchie. Another example
is given on pages 177 and 178 of the User's Guide (version 2.0).
A function is used in the simple program EXAMPLE.C which is included in
the tutorial menu.
In ANSI C, the function declaration or prototype specifies the type of
the return value from the function as well as the type and number of
parameters. This allows error-checking of type and number of parameters.
Turbo C supports both the classical and modern styles, but the modern
style using function prototypes and prototype-style function definitions
is recommended. Problems can then be prevented, especially when
compiling libraries of C routines. It is recommended that a separate
file, called a 'header' file (with a .h extension) is used and is
included with the user program with the directive '#include'. Error
checking can then take place at compile time.
If a function does not return a value then the term 'void' is used in the
function declaration or prototype and similarly if there are no
parameters, again 'void' is used. In this context void means empty, not
invalid.
From the example in the User's Guide, it is observed that the variable
named as a parameter in the parenthesized list in a function definition
is different from the value used in a call to that function. This is
natural, as the parameter name is fixed and local to the function, but
may need to represent a variety of different variables for each call to
that function. The terminology attempts to highlight this point by
referring to the parenthesized list in a function definition as either a
'parameter' or a 'formal argument', whilst the value used in the call of
the function is referred to as either an 'argument' or 'actual argument'.
An example from the User's Guide is:
/* Function declarations */
...
float get_ratio(float dividend, float divisor); /* parameters */
...
...
/* Main function */
main()
{
float a,b,ratio;
...
ratio = get_ratio(a,b); /* arguments */ ...
}
Many standard functions are contained in library files with extension
names '.lib' and the appropriate header files (with extension names '.h')
provide the function prototype declarations for these library functions.
The Turbo C Reference Guide (version 2.0) lists these header files on
pages 8 and 9, and the library routines are listed on pages 10-16.