The usual form of function pointer definitions is:
int function(int, int);
int (*ptr)(int, int);
int (*close) __P((struct __db *));
__P() macro is usually used to support C implementations from the days of K&R C, when there were no prototypes (which were introduced to C with C89). Basically the logic is
#if SOME_LOGIC_TO_TEST_WHETHER_IMPLEMENTATION_SUPPORTS_PROTOTYPES # define __P(argument_list) argument_list #else # define __P(argument_list) () #endif
Can you see how this works when applied to your example? Note that for this to work and not cause a syntax error, the argument list must include the parentheses of the function call, not just the parentheses of the function-like macro. Hence the double parentheses when the macro is used. That's probably the reason why it looks unusual.