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8 Tips for interface design
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Writing a good library interface takes a lot of practice and thorough
understanding of the problem that the library is intended to solve.

   If you design a good interface, it won't have to change often, you
won't have to keep updating documentation, and users won't have to keep
relearning how to use the library.

   Here is a brief list of tips for library interface design that may
help you in your exploits:

Plan ahead
     Try to make every interface truly minimal, so that you won't need
     to delete entry points very often.

Avoid interface changes
     Some people love redesigning and changing entry points just for the
     heck of it (note: _renaming_ a function is considered changing an
     entry point).  Don't be one of those people.  If you must redesign
     an interface, then try to leave compatibility functions behind so
     that users don't need to rewrite their existing code.

Use opaque data types
     The fewer data type definitions a library user has access to, the
     better.  If possible, design your functions to accept a generic
     pointer (that you can cast to an internal data type), and provide
     access functions rather than allowing the library user to directly
     manipulate the data.  That way, you have the freedom to change the
     data structures without changing the interface.

     This is essentially the same thing as using abstract data types and
     inheritance in an object-oriented system.

Use header files
     If you are careful to document each of your library's global
     functions and variables in header files, and include them in your
     library source files, then the compiler will let you know if you
     make any interface changes by accident (*note C header files::).

Use the ‘static’ keyword (or equivalent) whenever possible
     The fewer global functions your library has, the more flexibility
     you'll have in changing them.  Static functions and variables may
     change forms as often as you like... your users cannot access them,
     so they aren't interface changes.

Be careful with array dimensions
     The number of elements in a global array is part of an interface,
     even if the header just declares ‘extern int foo[];’.  This is
     because on i386 and some other SVR4/ELF systems, when an
     application references data in a shared library the size of that
     data (whatever its type) is included in the application executable.
     If you might want to change the size of an array or string then
     provide a pointer not the actual array.

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