Old-Style Versus Modern Function Parameter Declarations

Early versions of C used a different parameter declaration method than does either
Standard C or Standard C++. This early approach is sometimes called the classic form.
This book uses a declaration approach called the modern form. Standard C supports
both forms, but strongly recommends the modern form. Standard C++ only supports
the modern parameter declaration method. However, you should know the old-style
form because many older C programs still use it.
The old-style function parameter declaration consists of two parts: a parameter
list, which goes inside the parentheses that follow the function name, and the actual
parameter declarations, which go between the closing parentheses and the function's
opening curly brace. The general form of the old-style parameter definition is
type func_name(parm1, parm2, . . .parmN)
type parm1;
type parm2;
.
..
type parmN;
{
function code
}
For example, this modern declaration:
float f(int a, int b, char ch)
{
/* ... */
}
will look like this in its old-style form:
float f(a, b, ch)
int a, b;
char ch;
{
/* ... */
}
Notice that the old-style form allows the declaration of more than one parameter in a
list after the type name.
 
Note :The old-style form of parameter declaration is designated as obsolete by the C
language and is not supported by C++.

Declaring Variable-Length Parameter Lists Of function in C, C++

You can specify a function that has a variable number of parameters. The most
common example is printf() . To tell the compiler that an unknown number of
arguments may be passed to a function, you must end the declaration of its
parameters using three periods. For example, this prototype specifies that func()
will have at least two integer parameters and an unknown number (including 0)
of parameters after that.
int func(int a, int b, ...);
This form of declaration is also used by a function's definition.
Any function that uses a variable number of parameters must have at least one
actual parameter. For example, this is incorrect:
int func(...); /* illegal */

Old-Style Versus Modern Function Parameter Declarations

Early versions of C used a different parameter declaration method than does either
Standard C or Standard C++. This early approach is sometimes called the classic form.
This book uses a declaration approach called the modern form. Standard C supports
both forms, but strongly recommends the modern form. Standard C++ only supports
the modern parameter declaration method. However, you should know the old-style
form because many older C programs still use it.
The old-style function parameter declaration consists of two parts: a parameter
list, which goes inside the parentheses that follow the function name, and the actual
parameter declarations, which go between the closing parentheses and the function's
opening curly brace. The general form of the old-style parameter definition is
type func_name(parm1, parm2, . . .parmN)
type parm1;
type parm2;
.
..
type parmN;
{
function code
}

For example, this modern declaration:
float f(int a, int b, char ch)
{
/* ... */
}
will look like this in its old-style form:
float f(a, b, ch)
int a, b;
char ch;
{
/* ... */
}
Notice that the old-style form allows the declaration of more than one parameter in a
list after the type name.
The old-style form of parameter declaration is designated as obsolete by the C
language and is not supported by C++.

Declaring Variable-Length Parameter Lists

You can specify a function that has a variable number of parameters. The most
common example is printf() . To tell the compiler that an unknown number of
arguments may be passed to a function, you must end the declaration of its
parameters using three periods. For example, this prototype specifies that func()
will have at least two integer parameters and an unknown number (including 0)
of parameters after that.
int func(int a, int b, ...);
This form of declaration is also used by a function's definition.
Any function that uses a variable number of parameters must have at least one
actual parameter. For example, this is incorrect:
int func(...); /* illegal */