The function foreign-symbol-address is used to find out whether a foreign symbol is defined, by looking for it in the foreign-symbol table. If its associated object code has been loaded into the image, its address is returned. Otherwise nil is returned, unless errorp is nil.
The errorp keyword defines the behavior of the function when a symbol has not been defined. If it is non-nil (which is the default value), then an error will be signaled. If it is nil, no error will be reported, and the function will return nil.
The functionp keyword is used to specify the kind of symbol sought. If it is t, foreign-symbol-address will assume that name is the name of a function. If it is nil it will assume that name is the name of a variable. The default value is t.
LispWorks User Guide and Reference Manual - 13 Feb 2015