variable types in external C modules

["D. Mattes" <dmattes(at)u.washington.edu>]

hello IDL gurus:

i am using call_external to run computations in external C modules.  i
want to send arrays of different types to the C module and have them be
correctly declared in the C module.  for example, if the IDL procedure
sends floats, then the C module needs to declare the incoming arguments as
floats, and be able to create some scratch memory of type float.  
however, if the IDL procedure sends doubles, then the C module needs to
declare the variables as doubles.  how can i determine the variable types
of the input arguments from within the C module?

thanks!
david