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    <p>The C ABI and the C++ ABI are not guaranteed to be the same. So, an <code>extern "C"</code> function pointer is a different type from a C++ function pointer. You need something like this:</p> <pre><code>extern "C" { typedef int (*KlutzFuncType)(int); int Klutz (KlutzFuncType, int); } int Klutz (KlutzFuncType fptr, int x) { return (*fptr)(x); } </code></pre> <p>There is some discussion of this issue <a href="https://stackoverflow.com/q/11873939/315052">here</a>.</p> <hr> <p>I only have a copy of the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf" rel="nofollow noreferrer">draft</a>. From 7.5p1:</p> <blockquote> <p>Two function types with different language linkages are distinct types even if they are otherwise identical.</p> </blockquote> <p>My reading of this is that the first parameter of your first <code>Klutz</code> has a different type than the first parameter of your second <code>Klutz</code>, and so your second <code>Klutz</code> should have C++ linkage.</p> <hr> <p>There are C++ implementations that do not take language linkage into account for function types, despite what the standard says. In the following code snippet, <code>KlutzCxxFuncType</code> refers to a function with C++ linkage, while <code>KlutzCFuncType</code> refers to a function with C linkage.</p> <pre><code>typedef int (*KlutzCxxFuncType)(int); extern "C" { typedef int (*KlutzCFuncType)(int); int Klutz (KlutzCFuncType, int); } int Klutz (KlutzCxxFuncType fptr, int x) { return (*fptr)(x); } int Klutz (KlutzCFuncType fptr, int x) { return (*fptr)(x); } </code></pre> <p>A compiler that does not distinguish function types based on language linkage will generate a redefinition error on this code. For example, <code>g++ 4.7.2</code> will emit:</p> <pre><code>prog.cpp: In function ‘int Klutz(KlutzCFuncType, int)’: prog.cpp:9:5: error: redefinition of ‘int Klutz(KlutzCFuncType, int)’ prog.cpp:8:5: error: ‘int Klutz(KlutzCxxFuncType, int)’ previously defined here </code></pre>
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    1. POPossible ambiguity with extern "C", overloading, and function pointers
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    1. COI'm aware of this, but it does not answer my question. Is `Klutz` compiled from the code written in my question (a) defined to be C++ ABI, (b) defined to be C ABI, or (c) undefined/ambiguous according to the standard?
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    2. CO@Jed: I don't think it is ambiguous. I have updated my answer. Caveat, I am not a language lawyer. Neither do I play one on TV.
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    3. COInteresting, this would imply that GCC, Clang, ICC, PGI, and PathScale are all incorrect.
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