(libc.info.gz) How Many Arguments

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 A.2.2.3 How Many Arguments Were Supplied
 ........................................
 
 There is no general way for a function to determine the number and type
 of the optional arguments it was called with.  So whoever designs the
 function typically designs a convention for the caller to specify the
 number and type of arguments.  It is up to you to define an appropriate
 calling convention for each variadic function, and write all calls
 accordingly.
 
    One kind of calling convention is to pass the number of optional
 arguments as one of the fixed arguments.  This convention works provided
 all of the optional arguments are of the same type.
 
    A similar alternative is to have one of the required arguments be a
 bit mask, with a bit for each possible purpose for which an optional
 argument might be supplied.  You would test the bits in a predefined
 sequence; if the bit is set, fetch the value of the next argument,
 otherwise use a default value.
 
    A required argument can be used as a pattern to specify both the
 number and types of the optional arguments.  The format string argument
 to 'printf' is one example of this ( Formatted Output Functions).
 
    Another possibility is to pass an "end marker" value as the last
 optional argument.  For example, for a function that manipulates an
 arbitrary number of pointer arguments, a null pointer might indicate the
 end of the argument list.  (This assumes that a null pointer isn't
 otherwise meaningful to the function.)  The 'execl' function works in
 just this way; see  Executing a File.
 

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