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Static Member Function Pointer in C for C API Callbacks: Is It a Safe and Portable Practice?

Despite the apparent similarity in calling convention, using a static member function pointer as a callback for C APIs is not considered safe or portable according to the C standard.

C Language Bindings and Linkage

Section 3.5 of the C standard states that objects or functions must have identical types across all declarations referencing them. This includes the linkage, which determines the conventions for cross-language interoperability.

A linkage specification is used to achieve linkage to non-C declarations, but when dealing with C API callbacks, the callback target in the C program must use the same language linkage as the code making the callback.

Extern "C" for C API Callbacks

Section 7.5 of the C standard clarifies that function types with different language linkages are considered distinct types. Therefore, if the callback code uses C language bindings, the callback target in C must also be declared as extern "C".

Potential Issues

Comments on a related Stack Overflow question highlight potential problems with using static member function pointers for C API callbacks on certain platforms.

Recommendation

To ensure safety and portability, it is advised to use extern "C" wrapped functions as C ABI callbacks rather than using static member function pointers directly.

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