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Before ISO C, programmers used a slightly different facility for writing variadic functions. The GNU C compiler still supports it; currently, it is more portable than the ISO C facility, since support for ISO C is still not universal. The header file which defines the old-fashioned variadic facility is called varargs.h.
Using varargs.h is almost the same as using stdarg.h. There is no difference in how you call a variadic function; see Calling Variadics. The only difference is in how you define them. First of all, you must use old-style non-prototype syntax, like this:
tree
build (va_alist)
va_dcl
{
Secondly, you must give va_start only one argument, like this:
va_list p;
va_start (p);
These are the special macros used for defining old-style variadic functions:
This macro, as defined in varargs.h, initializes the argument pointer variable ap to point to the first argument of the current function.
The other argument macros, va_arg and va_end, are the same
in varargs.h as in stdarg.h; see Argument Macros, for
details.
It does not work to include both varargs.h and stdarg.h in
the same compilation; they define va_start in conflicting ways.