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cvariadic-functions
bjorn93
bjorn93

Reputation: 213

Variadic function f() vs. f(...) in C

What is the difference between the following two variadic function definitions?

int f()
{
    /* function definition */
}

int f(...)
{
    /* function definition */
}

f() is actually defined as a variadic function. I'm also assuming <stdarg.h> can be included and used.

Upvotes: 2

Views: 107

Answers (1)

John Bode
John Bode

Reputation: 123558

An empty parameter list in a function declaration indicates that the function takes an unspecified number of arguments (which is not the same as a variable number of arguments). An empty parameter list in a function definition (such as in the first definition of f) indicates that the function takes no arguments. This is an obsolescent style and should not be used - to indicate that a function takes no parameters, use void as the identifier list.

As of C89, a variadic function declarator must have at least one fixed parameter, followed by the .... So the second definition of f won't work either.

int f( void ) // f takes no arguments
{
  // do something
}

int f( T fixed, ... ) // one fixed parameter of some type, additional parameters as needed
{
  // do something
}

printf is as good an example of a variadic function in the standard library as any - its prototype is

int printf( const char *fmt, ... );

Upvotes: 5

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