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POIs extern "C" only required on the function declaration?
 

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  1. POIs extern "C" only required on the function declaration?
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    <p>I wrote a C++ function that I need to call from a C program. To make it callable from C, I specified <code>extern "C"</code> on the function <strong>declaration</strong>. I then compiled the C++ code, but the compiler (Dignus Systems/C++) generated a <a href="http://en.wikipedia.org/wiki/Name_mangling" rel="noreferrer">mangled name</a> for the function. So, it apparently did not honor the <code>extern "C"</code>. </p> <p>To resolve this, I added <code>extern "C"</code> to the function <strong>definition</strong>. After this, the compiler generated a function name that is callable from C.</p> <p>Technically, the <code>extern "C"</code> only needs to be specified on the function declaration. Is this right? (The <a href="http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.6" rel="noreferrer">C++ FAQ Lite</a> has a good example of this.) Should you also specify it on the function definition? </p> <p>Here's an example to demonstrate this:</p> <pre><code>/* ---------- */ /* "foo.h" */ /* ---------- */ #ifdef __cplusplus extern "C" { #endif /* Function declaration */ void foo(int); #ifdef __cplusplus } #endif /* ---------- */ /* "foo.cpp" */ /* ---------- */ #include "foo.h" /* Function definition */ extern "C" // &lt;---- Is this needed? void foo(int i) { // do something... } </code></pre> <p><em>My issue may be the result of incorrectly coding something, or I may have found a compiler bug. In any case, I wanted to consult stackoverflow to make sure I know which is technically the "right" way.</em></p>
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      1. USBrian Liang
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    1. COAre you certain that you actually get the mangling of foo if you leave off the extern "C" in foo.c in the example code you show? Or was this just something that occurred in other more complicated code? I've quite often seen this problem as a symptom of forgetting to include foo.h in foo.c.
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      1. USBrooks Moses
    2. CO@Brooks Moses: That's a great point. In my actual code, which is slightly more complex than this "foo" example, I am including the header in the "cpp" source file. What made me think that the compiler was mangling the name was this: If I do not include the 'extern "C"' on the function definition, then the compiler listing shows the external symbol, "foo_FPFPCc_v". When the 'extern "C"' is included, the listing shows the external symbol, "foo".
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      1. USbporter
    3. CO@bporter - it might be interesting to experiment with what the compiler does with your simplified example. If it shows the same behavior, then you might want to send a note to the vendor. If it doesn't show the same, then you should track down what's going on in your real build, because that would indicate that the wrong header is being brought in (or something else is making the function declaration to be missed).
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      1. USMichael Burr
 

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