unknown type - fd_set

["John Ray Thomas" <jt@xxxxxxxxxxx>]

I am porting OpenInventor to BeOS. Things are going fairly well as I have
made some good progress. Recently, I have run into an unknown data type
while building libInventor.so. The data type is fd_set. I grep'd the Be
posix headers for this type but could not find it. I get the feeling this is
some sort of file descriptor??? If anyone knows what this type is and can
tell me how I can emulate it, I would be very grateful.
Thanks,
//jt