Calling 'C' from PL/SQL

Does anyone out there know how to call a 'C' routine from PL/SQL?  

We are running Oracle 7.3.3 PS on an AIX 4.1.5 platform, and using Oracle procedure builder to build stored procedures. Some of our developers want to incorporate some 'C' routines that are already built. Typically, I've done the reverse (calling PL/SQL from 'C') but they want to keep things consistent with what they have already developed.  

Thanks in Advance,
Mike Rowland
MRowland1_at_compuserve.com

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