类型化一个函数指针

Question

I know this is a duplicate. I've searched already but none address the problem I'm having. NOTE in an attempt to single out my confusion I have tried to simplify the typecast from the original. Hopefully it isn't undefined behaviour

Declaring a pointer to a function

Int (*funcPtr)(void*,void*);

Strcmp declaration

int strcmp (char *, char *);

Typecasting a function pointer

FuncPtr = (int (*)(void*, void*))strcmp

This is where I get confused. I understand Typecasting a function pointer we have created. So type casting FuncPtr for example just changes what type of function funcPtr can point to. I have also read that the name of a function Is a function pointer to itself so Typecasting strcmp and using my understanding from above,

int (*)(void*, void*))strcmp 

Strcmp can now point to a function that takes two pointers to void and returns an int which Is clearly wrong as like arrays I think the function name can't be a pointer to another function.

Because of my understanding above I fail to see how you can assign the address of strcmp to FuncPtr with the following typecast

FuncPtr = (int (*)(void*, void*))strcmp

As changing what the function pointer strcmp points to doesn't change the arguments and return type of strcmp .

I would really appreciate some knowledge I'm just so confused.

Answer 1

There are functions and there are function pointers. strcmp is a function, linked to your project. When the identifier strcmp is used in an expression, you get a function pointer to that function. But you cannot change "where strcmp points to" because strcmp is a function and not a pointer.

C allows conversions between different function pointer types. However, should you try to call a function through a function pointer of incompatible type, you invoke undefined behavior.

int (*funcPtr)(void*,void*);

is not compatible with strcmp , which has the following format:

int strcmp(const char *s1, const char *s2);

Meaning:

funcPtr = (int(*)(void*, void*))strcmp; // this is ok but not very meaningful
funcPtr(); // this is NOT ok, invokes undefined behavior

In order to actually call the function strcmp through that function pointer, you would have to cast back to the correct type, which is int (*)(const char*, const char*) .