Object oriented programming in C [duplicate]
You can implement polymorphism with regular functions and virtual tables (vtables). Here's a pretty neat system that I invented (based on C++) for a programming exercise: 
The constructors allocate memory and then call the class's init function where the memory is initialized. Each init function should also contain a static vtable struct that contains the virtual function pointers (NULL for pure virtual). Derived class init functions call the superclass init function before doing anything else.
A very nice API can be created by implementing the virtual function wrappers (not to be confused with the functions pointed to by the vtables) as follows (add static inline in front of it, if you do this in the header):
int playerGuess(Player* this) { return this->vtable->guess(this); }Single inheritance can be done by abusing the binary layout of a struct: 
Notice that multiple inheritance is messier as then you often need to adjust the pointer value when casting between types of the hierarchy.
Other type-specific data can be added to the virtual tables as well. Examples include runtime type info (e.g. type name as a string), linking to superclass vtable and the destructor chain. You probably want virtual destructors where derived class destructor demotes the object to its super class and then recursively calls the destructor of that and so on, until the base class destructor is reached and that finally frees the struct.
Encapsulation was done by defining the structs in player_protected.h and implementing the functions (pointed to by the vtable) in player_protected.c, and similarly for derived classes, but this is quite clumsy and it degrades performance (as virtual wrappers cannot be put to headers), so I would recommend against it.
How would you emulate encapsulation and inheritance though?
Actually, encapsulation is the easiest part. Encapsulation is a design philosophy, it has nothing at all to do with the language and everything to to with how you think about problems.
For example, the Windows FILE api is completely encapsulated. When you open a file, you get back an opaque object that contains all of the state information for the file 'object'. You hand this handle back to each of the file io apis. The encapsulation is actually much better than C++ because there is no public header file that people can look at and see the names of your private variables.
Inheritance is harder, but it isn't at all necessary in order for your code to be object oriented. In some ways aggregation is better than inheritance anyway, and aggregation is just as easy in C as in C++. see this for instance.
In response to Neil see Wikipedia for an explanation of why inheritance isn't necessary for polymorphism.
Us old-timers wrote object oriented code years before C++ compilers were available, it's a mind-set not a tool-set.