| va_arg | Function (Macro)

stdarg.h

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.