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    <p>The least confusing thing (at least for me) to do is to make the declarations in the header match the function definitions. As other answers have mentioned, it's the declaration/prototype that matters most - the <code>extern "C"</code> linkage specification can be omitted from the function definition as long as the prototype has been 'seen' by the compiler already when it gets to the definition. However, personally I find it preferable and less potentially confusing to have the declarations and the definitions match, but that's a preference that other programmers might not share.</p> <p>Note that you don't have to <code>extern "C"</code> everything in a header or implementation file - the <code>extern "C"</code> can be applied to a single name or to a group of names (by applying it to a block).</p> <p>So, in your header you can have something like:</p> <pre><code>// entrypoints.h #ifdef __cplusplus // this opens an extern "C" block, but only if being included for a C++ compile // if this is being included in a C compile, the extern "C" bits won't be seen extern "C" { #endif int foo(void); int bar( void*); #ifdef __cplusplus // close the extern "C" block, but only if we started it in the first place } #endif </code></pre> <p>And your implementation of them:</p> <pre><code>// entrypoints.cpp // Note: since this is a .cpp file, it's compiled as C++ so we // don't need the #ifdefs around the extern "C" #include "entrypoints.h" extern "C" int foo(void) { return 42; } extern "C" int bar( void* p) { return -42; } </code></pre>
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    1. POHow does C's "extern" work?
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    1. USMichael Burr
    1. USMichael Burr
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    1. COThe problem is that if I wrap the implementations in extern "C", It won't link to the mangled function names of the C++ functions in other files... This iswhy I only extern "C" the .h file, which is what felt weird...
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      1. USBrian Postow
    2. COWhat makes you say that? Are you saying that you think extern "c" will cause the linker to search for dependencies by their unmangled names? If so, please be aware that extern "C" only applies to _declarations_ at the file scope, not to method/function calls inside the function implementations.
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      1. USDavid Gladfelter
    3. COThe functions that get called by the `extern "C"` function implementations are not impacted at all - the implementation is still C++ and the declarations/prototypes that `foo()` and `bar()` see when `entrypoints.cpp` is compiled do not need to have `extern "C"` applied to them (unless the called functions actually are `extern "C"`) - the functions implemented in `entrypoints.cpp` can call any C++ code and can use all C++ facilities that a non-`extern "C"` function can use..
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      1. USMichael Burr
 

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