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Macros with Variable Numbers of Arguments
In GNU C, a macro can accept a variable number of arguments, much as
a function can. The syntax for defining the macro looks much like that
used for a function. Here is an example:
#define eprintf(format, args...) \
fprintf (stderr, format , ## args)
Here `args' is a "rest argument": it takes in zero or more
arguments, as many as the call contains. All of them plus the commas
between them form the value of `args', which is substituted into the
macro body where `args' is used. Thus, we have this expansion:
eprintf ("%s:%d: ", input_file_name, line_number)
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
Note that the comma after the string constant comes from the definition
of `eprintf', whereas the last comma comes from the value of `args'.
The reason for using `##' is to handle the case when `args' matches
no arguments at all. In this case, `args' has an empty value. In this
case, the second comma in the definition becomes an embarrassment: if
it got through to the expansion of the macro, we would get something
fprintf (stderr, "success!\n" , )
which is invalid C syntax. `##' gets rid of the comma, so we get the
fprintf (stderr, "success!\n")
This is a special feature of the GNU C preprocessor: `##' before a
rest argument that is empty discards the preceding sequence of
non-whitespace characters from the macro definition. (If another macro
argument precedes, none of it is discarded.)
It might be better to discard the last preprocessor token instead of
the last preceding sequence of non-whitespace characters; in fact, we
may someday change this feature to do so. We advise you to write the
macro definition so that the preceding sequence of non-whitespace
characters is just a single token, so that the meaning will not change
if we change the definition of this feature.