ANSI routines for creating functions with variable numbers of arguments
|type va_arg (va_list &ap, type);|
Returns the current argument in the list.
This function (also implemented as a macro) expands to an expression that
has the same type and value as the next argument being passed (one of the
variable arguments). The variable ap to va_arg should be the same
ap that va_start initialized. Because of
default promotions, you can't use char or unsigned char types with va_arg.
The first time va_arg is used, it returns the first argument in the list. Each successive time va_arg is used, it returns the next argument in the list. It does this by first dereferencing ap, and then incrementing ap to point to the following item.
va_arg uses the parameter type (which must be an expected type name) to both perform the dereference and to locate the following item. Each successive time va_arg is invoked, it modifies ap to point to the next argument in the list.
|void va_end (va_list &ap);|
va_end helps the called function perform a normal return.
va_end might modify ap in such a way that it cannot be used unless
va_start is recalled. va_end should be called
after va_arg has read all the arguments.
Note: va_end is introduced here only to increase compatibility with ANSI C. In this implementation, va_end in fact does nothing.
|void va_start (va_list &ap, &lastfix);|
Initializes a pointer to a variable argument list.
Some C functions, such as sprintf have a variable number of arguments. This
function, together with va_arg and va_end,
provides a portable way to access
these argument lists. They are used for stepping through a list of arguments
when the called function does not know the number and types of the arguments
being passed. Function va_start (implemented as a macro) sets ap to point to the
first of the variable arguments being passed to the function. It must be
used before the first call to va_arg or va_end.
va_start takes two parameters: ap and lastfix. ap is explained above, and lastfix is the name of the last fixed parameter being passed to the called function.
Note: I used notation "&ap" in the prototype description, although passing by reference does not exist in ordinary C (only in C++). However, this macro is implemented in such a way that it simulates "passing by reference".
|typedef void *va_list;|
A void pointer which can be interpreted as an argument list.
va_list is the type of the void pointer passed to one of the functions that accepts a pointer to a list of arguments. This array holds information needed by va_arg and va_end.