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(libc.info)Receiving Arguments

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Receiving the Argument Values
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   Ordinary fixed arguments have individual names, and you can use these
names to access their values.  But optional arguments have no
names--nothing but `...'.  How can you access them?

   The only way to access them is sequentially, in the order they were
written, and you must use special macros from `stdarg.h' in the
following three step process:

  1. You initialize an argument pointer variable of type `va_list' using
     `va_start'.  The argument pointer when initialized points to the
     first optional argument.

  2. You access the optional arguments by successive calls to `va_arg'.
     The first call to `va_arg' gives you the first optional argument,
     the next call gives you the second, and so on.

     You can stop at any time if you wish to ignore any remaining
     optional arguments.  It is perfectly all right for a function to
     access fewer arguments than were supplied in the call, but you
     will get garbage values if you try to access too many arguments.

  3. You indicate that you are finished with the argument pointer
     variable by calling `va_end'.

     (In practice, with most C compilers, calling `va_end' does nothing.
     This is always true in the GNU C compiler.  But you might as well
     call `va_end' just in case your program is someday compiled with a
     peculiar compiler.)

   Note: Argument Macros, for the full definitions of `va_start',
`va_arg' and `va_end'.

   Steps 1 and 3 must be performed in the function that accepts the
optional arguments.  However, you can pass the `va_list' variable as an
argument to another function and perform all or part of step 2 there.

   You can perform the entire sequence of three steps multiple times
within a single function invocation.  If you want to ignore the optional
arguments, you can do these steps zero times.

   You can have more than one argument pointer variable if you like.
You can initialize each variable with `va_start' when you wish, and
then you can fetch arguments with each argument pointer as you wish.
Each argument pointer variable will sequence through the same set of
argument values, but at its own pace.

   *Portability note:* With some compilers, once you pass an argument
pointer value to a subroutine, you must not keep using the same
argument pointer value after that subroutine returns.  For full
portability, you should just pass it to `va_end'.  This is actually an
ISO C requirement, but most ANSI C compilers work happily regardless.