Node:Old Varargs, Previous:Argument Macros, Up:How Variadic

Old-Style Variadic Functions

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:

va_alist

Macro

This macro stands for the argument name list required in a variadic function.

va_dcl

Macro

This macro declares the implicit argument or arguments for a variadic function.

void va_start (va_list ap)

Macro

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.