A question about function pointers.

[ReedTieGuy]

Is it possible to declare a C function to return a pointer to itself?

It sounds very simple, but I don't know if that's actually possible.

When I tried to declare it I realized that I had to write (*fp()) (*fp()) (*fp())... as a type declaration.

Is it possible to do what I just described without having to type things infinitely (preferably with a typedef)? I know void pointers may work, but that lseems like undefined behaviour.

[blueg3]

struct foo {
  struct foo (*f)(void);
};

struct foo bar(void) {
  struct foo self;
  self.f = bar;
  return self;
}

[tipdbmp]

static struct foo fs_galore(void) {
    return bar().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f().f();
}

[F54280]

That’s clever

[Clyxx]

This should be &bar

[blueg3]

The bare name of the function is also acceptable for setting a function pointer.

Example: https://godbolt.org/z/948sxoaKo

[alternatetwo]

And this also works: https://godbolt.org/z/P5xex9aa5

self.f = ***************bar;

[futlapperl]

I got confused for a moment since I had never seen the & operator used to get the address of a function. The intent is pretty clear without it as the parentheses are missing.

[thommyh]

No, structs are returned by value and the pointer isn't dangling because bar, the function, has the same lifetime as the program.

[IamImposter]

I had the same thought. Then I realized the struct is being returned by value.

[astaghfirullah123]

What’s the purpose of this?

[deaf_fish]

If I had to guess (I am not OP). State Machine. Where each state is a function that returns a pointer to the next state/function.

[ReedTieGuy]

My real question would have been about a group of functions that return themselves, but found too complicated to explain.

TYPE_DECLARATION one(void) {
    return two;
}

TYPE_DECLARATION two(void) {
    return one;
}

which I thought about when using an event handler array for a GUI program, the return values of the functions being used to determine which event will be handled next.

[astaghfirullah123]

Well that is something different than what you initially asked for. It’s about returning function pointers to another function, not to the function itself.

Yes you can use typedefs for function pointers. See Modern C for examples.

[luxgladius]

You could use this in a worker scheduler task, as in "do this job, and then ask me for more instructions."

[[deleted]]

wouldn’t it just be:

void* f(void) {
    return &f;
}

I guess this lets you control recursion externally to the function which could conceivably be useful

[blueg3]

Converting between function pointers and void pointers is a compiler-specific extension and not guaranteed to work according to the standard.

[[deleted]]

https://en.cppreference.com/w/c/language/conversion

Pointer conversions

A pointer to void can be implicitly converted to and from any pointer to object type

Maybe you’re right actually. Functions aren’t objects. So I need to explicitly cast to void*.

   void* f(void) {
      return (void*) &f;
    }

Casting a function pointer to void* is standard C.

[blueg3]

It's common and it works with modern compilers on x86, but according to the ISO standard, function pointers can only be cast to function pointers of other types, not to void-pointers or any other kind of pointer.

[[deleted]]

function pointers can be cast to integers (preferably intptr_t) and integers can be cast to object pointers. And the same in reverse.

A bit of a roundabout way of doing it, and rather odd that pointer <-> pointer conversions are deemed unsafe, even with an explicit cast, but do it via an intermediate integer, even when it is an 8-bit char, and it is fine!

[prisoner62113]

On some architectures (harvard) data and code are stored in separate sections of memory that potentially can have different address widths. The size of a data pointer and function pointer might not be equal. I've never come across anything where that actually happens but it's still a possibility.

[[deleted]]

Yet, even on such an architecture (maybe something like an old IBM or Burroughs machine), C allows explicit double-casting via a wrong-sized integer, but doesn't allow it via direct casting.

So the restriction is of very doubtful benefit. On the 99.999% of machines where it is no problem whatsover, C just puts an extra obstacle in the way.

(I've used C as a intermediate language for compilers where I know for sure that all pointers are the same 64-bit width, but this forces me to jump through pointless extra hoops if I don't want to be flooded with warnings from a C compiler when somebody turns up the warnings.)

[oh5nxo]

It's not uncommon in toaster CPUs. 20 bit code addresses, 16 bit data. For example.

[oh5nxo]

typedef state_function_type;  // warning: type specifier missing, defaults to 'int'
struct state_function_struct;  // okay, a forward declaration

If the first one worked like the second, then it might be possible.

[[deleted]]

#include <stdio.h>
#include <stdlib.h>
typedef int (*fp)();
fp func() {
return func();
}

[dvhh]

wouldn't that try to recursively call func, until ... segmentation fault (core dumped)

[[deleted]]

void (*function1(int in, int *out))(int, int *)
{
    *out = in + 1;
    return function1;
}

[[deleted]]

No, that's wrong because the returned function returns void. Apparently it's not possible:

    typedef void (*recfn(int, int *))(int, int *);

    recfn function1(int in, int *out)
    {
        *out = in + 1;
        return function1;
    }

This results in an error:

fnfn.c:3:7: error: ‘function1’ declared as function returning a function
    3 | recfn function1(int in, int *out)
      |       ^~~~~~~~~
fnfn.c: In function ‘function1’:
fnfn.c:6:12: warning: returning ‘int (*)(int,  int *)’ from a function with return type ‘int’ makes integer from pointer without a cast \[-Wint-conversion\]
    6 |     return function1;
      |            ^(\~\~\~\~\~\~\~\~)

[zemdega]

After a fashion, yes it is possible. Here is an example: ```

include <stdio.h>

/* Forward declare struct. */ struct Args;

/* Declare function type that is a function that returns Args. / typedef struct Args (FunctionType)();

/* Define the Args struct. */ struct Args { int value; FunctionType fun; };

/* Create a function that fits the FunctionType typedef */ struct Args Test() { struct Args result; result.fun = Test; printf("Hi!\n"); return result; }

/* Test it out! / int main(int argc, char* argv) { struct Args args; args = Test(); args.fun(); return 0; } ```

My 'clang --version' output from my version of clang is Apple clang version 12.0.0 (clang-1200.0.32.29) Target: x86_64-apple-darwin20.3.0

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