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(libc.info)Argument Macros

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Argument Access Macros
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   Here are descriptions of the macros used to retrieve variable
arguments.  These macros are defined in the header file `stdarg.h'.

 - Data Type: va_list
     The type `va_list' is used for argument pointer variables.

 - Macro: void va_start (va_list AP, LAST-REQUIRED)
     This macro initializes the argument pointer variable AP to point
     to the first of the optional arguments of the current function;
     LAST-REQUIRED must be the last required argument to the function.

     Note: Old Varargs, for an alternate definition of `va_start'
     found in the header file `varargs.h'.

 - Macro: TYPE va_arg (va_list AP, TYPE)
     The `va_arg' macro returns the value of the next optional argument,
     and modifies the value of AP to point to the subsequent argument.
     Thus, successive uses of `va_arg' return successive optional
     arguments.

     The type of the value returned by `va_arg' is TYPE as specified in
     the call.  TYPE must be a self-promoting type (not `char' or
     `short int' or `float') that matches the type of the actual
     argument.

 - Macro: void va_end (va_list AP)
     This ends the use of AP.  After a `va_end' call, further `va_arg'
     calls with the same AP may not work.  You should invoke `va_end'
     before returning from the function in which `va_start' was invoked
     with the same AP argument.

     In the GNU C library, `va_end' does nothing, and you need not ever
     use it except for reasons of portability.


   Sometimes it is necessary to parse the list of parameters more than
once or one wants to remember a certain position in the parameter list.
To do this, one will have to make a copy of the current value of the
argument.  But `va_list' is an opaque type and one cannot necessarily
assign the value of one variable of type `va_list' to another variable
of the same type.

 - Macro: void __va_copy (va_list DEST, va_list SRC)
     The `__va_copy' macro allows copying of objects of type `va_list'
     even if this is not an integral type.  The argument pointer in
     DEST is initialized to point to the same argument as the pointer
     in SRC.

     This macro is a GNU extension but it will hopefully also be
     available in the next update of the ISO C standard.

   If you want to use `__va_copy' you should always be prepared for the
possibility that this macro will not be available.  On architectures
where a simple assignment is invalid, hopefully `__va_copy' _will_ be
available, so one should always write something like this:

     {
       va_list ap, save;
       ...
     #ifdef __va_copy
       __va_copy (save, ap);
     #else
       save = ap;
     #endif
       ...
     }