In a C function declaration, what does "..." as the last parameter do?

it allows a variable number of arguments of unspecified type (like printf does).

you have to access them with va_start, va_arg and va_end

see http://publications.gbdirect.co.uk/c_book/chapter9/stdarg.html for more information

Variadic functions

Variadic functions are functions which may take a variable number of arguments and are declared with an ellipsis in place of the last parameter. An example of such a function is printf.

A typical declaration is

    int check(int a, double b, ...);

Variadic functions must have at least one named parameter, so, for instance,

    char *wrong(...);  

is not allowed in C.