|The C Preprocessor: Variadic Macros|
A macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:
#define eprintf(…) fprintf (stderr, __VA_ARGS__)
This kind of macro is called variadic. When the macro is invoked,
all the tokens in its argument list after the last named argument (this
macro has none), including any commas, become the variable
argument. This sequence of tokens replaces the identifier
__VA_ARGS__ in the macro body wherever it appears. Thus, we
have this expansion:
eprintf ("%s:%d: ", input_file, lineno) → fprintf (stderr, "%s:%d: ", input_file, lineno)
The variable argument is completely macro-expanded before it is inserted into the macro expansion, just like an ordinary argument. You may use the ‘#’ and ‘##’ operators to stringize the variable argument or to paste its leading or trailing token with another token. (But see below for an important special case for ‘##’.)
If your macro is complicated, you may want a more descriptive name for
the variable argument than
__VA_ARGS__. CPP permits
this, as an extension. You may write an argument name immediately
before the ‘…’; that name is used for the variable argument.
eprintf macro above could be written
#define eprintf(args…) fprintf (stderr, args)
using this extension. You cannot use
__VA_ARGS__ and this
extension in the same macro.
You can have named arguments as well as variable arguments in a variadic
macro. We could define
eprintf like this, instead:
#define eprintf(format, …) fprintf (stderr, format, __VA_ARGS__)
This formulation looks more descriptive, but unfortunately it is less flexible: you must now supply at least one argument after the format string. In standard C, you cannot omit the comma separating the named argument from the variable arguments. Furthermore, if you leave the variable argument empty, you will get a syntax error, because there will be an extra comma after the format string.
eprintf("success!\n", ); → fprintf(stderr, "success!\n", );
GNU CPP has a pair of extensions which deal with this problem. First, you are allowed to leave the variable argument out entirely:
eprintf ("success!\n") → fprintf(stderr, "success!\n", );
Second, the ‘##’ token paste operator has a special meaning when placed between a comma and a variable argument. If you write
#define eprintf(format, …) fprintf (stderr, format, ##__VA_ARGS__)
and the variable argument is left out when the
eprintf macro is
used, then the comma before the ‘##’ will be deleted. This does
not happen if you pass an empty argument, nor does it happen if
the token preceding ‘##’ is anything other than a comma.
eprintf ("success!\n") → fprintf(stderr, "success!\n");
The above explanation is ambiguous about the case where the only macro parameter is a variable arguments parameter, as it is meaningless to try to distinguish whether no argument at all is an empty argument or a missing argument. CPP retains the comma when conforming to a specific C standard. Otherwise the comma is dropped as an extension to the standard.
The C standard
mandates that the only place the identifier
can appear is in the replacement list of a variadic macro. It may not
be used as a macro name, macro argument name, or within a different type
of macro. It may also be forbidden in open text; the standard is
ambiguous. We recommend you avoid using it except for its defined
Variadic macros became a standard part of the C language with C99.
GNU CPP previously supported them
with a named variable argument
(‘args…’, not ‘…’ and
is still supported for backward compatibility.