Its a hazardous business:

• Data-type compatibility, e.g., number of bytes for an integer (machine-dependent, compiler-dependent...);
• case-sensitivity;
• underscorse in routine names;
• argument passing by reference or value?
• array element ordering: Fortran versus C;
• IO: channels versus file descriptors.

Resources

• The Fortran Programming Guide at docs.sun.com (follow link to "C-Fortran Interface" --- or: local copy of this section).
  
  
• Mixed language programming in the Salford FTN95 compiler Users Guide.

Example

• Output Library Example