Casts between pointer-to-function and pointer-to-object in C and C++

Question

Am i wrong about the following?

C++ standards says that conversion between pointer-to-function and pointer-to-object (and back) is conditionnaly-supported with implementation-defined semantics, while all C standards says that this is illegal in all cases, right?

void foo() {}

int main(void)
{
    void (*fp)() = foo;
    void* ptr = (void*)fp;
    return 0;
}

ISO/IEC 14882:2011

5.2.10 Reinterpret cast [expr.reinterpret.cast]

8 Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning of such a conversion is implementation-defined, except that if an implementation supports conversions in both directions, converting a prvalue of one type to the other type and back, possibly with different cvqualification, shall yield the original pointer value.

I can't find anything about it in C standard right now...

Answer 1

• In C++03, such conversions were illegal (not UB). The compiler was supposed to issue a diagnostic. A lot of compilers on Unix systems didn't issue a diagnostic. This was essentially a clash between standards, POSIX vs C++.
• In C++11, such conversions are "conditionally supported". No diagnostic is required if the system does supports such conversions; there's nothing to diagnose.
• In C, such conversions officially are undefined behavior, so no diagnostic is required. If the system happens to do the "right" thing, well that's one way to implement UB.
• In C99, this is once again UB. However, the standard also lists such conversions as one of the "common extensions" to the language:

  J.5.7 Function pointer casts
  A pointer to an object or to void may be cast to a pointer to a function, allowing data to be invoked as a function (6.5.4).
  A pointer to a function may be cast to a pointer to an object or to void, allowing a function to be inspected or modified (for example, by a debugger) (6.5.4).

Answer 2

You're right, the C(99) standard says nothing about conversion from pointer-to-function to pointer-to-object, therefore it's undefined behaviour.^*

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_{*Note, however, that it does define behaviour between pointer-to-function types.}