Another advantage of typedefing the function is that you can let the compiler help ensure your function definition signatures are correct.

For example, if you typedef a callback type like so:

  typedef int callback_t(int foo);

And then declare a callback like so:

  callback_t my_callback;

And then later define the callback like so:

  int my_callback(int foo) {
    ...
  }

The compiler will produce an error if you screw up the function signature of my_callback() when defining it because it won't match the prototype you defined via the typedef. This only works in C. C++ allows multiple function signatures for the same function name, so you probably won't get a compiler error--though you'll likely wind up with a linker error.

Edit: The downside is that function declarations done this way will look a little odd--possibly mistaken for a variable definition. And, upon further thought, I'm not sure that this pattern really is a huge benefit since function pointer assignment will also produce an error if the signatures don't match. But interesting nonetheless, I guess.