In C++, is it safe/portable to use static member function pointer for C API callbacks?

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It is not safe per the C++ standard. As stated in this SO posting:

A C callback function implemented in C++ must be extern “C”. It may seem to work as a static function in a class because class-static functions often use the same calling convention as a C function. However, doing that is a bug waiting to happen (see comments below), so please don’t – go through an extern “C” wrapper instead.

And according to comments made by Martin York in that answer there are real-world problems trying to do so on some platforms.

Make your C ABI callbacks extern "C".

Edit: Adding some supporting quotes from the standard (emphasis mine):

3.5 “Program and linkage”:

After all adjustments of types (during which typedefs (7.1.3) are replaced by their definitions), the types specified by all declarations referring to a given object or function shall be identical, except that declarations for an array object can specify array types that differ by the presence or absence of a major array bound (8.3.4). A violation of this rule on type identity does not require a diagnostic. [3.5/10]

[Note: linkage to non-C++ declarations can be achieved using a linkage-specification (7.5). ] [3.5/11]

And

7.5 “Linkage specifications”:

… Two function types with different language linkages are distinct types even if they are otherwise identical. [7.5/1]

So if the code making the callback is using C language bindings for the callback, then the callback target (in the C++ program) must as well.

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