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POChecking for the existence of externally defined identifiers in C
 

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  1. POChecking for the existence of externally defined identifiers in C
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    <p>I ran into this problem while developing in Objective-C for iOS, but this should apply to any C/C++/Objective-C code using the Mac OS X/iOS linker. The solution is covered by <a href="https://stackoverflow.com/questions/3002833/weak-linking-on-iphone-refuses-to-work">another question</a>, but I'm interested in the <em>why</em>.</p> <p>Let's say I'm using a linking to a library which defines a constant. In a header file there is a declaration like this:</p> <pre><code>extern char * const BrandNewIdentifier; </code></pre> <p>I want to compile my application and run it on a system with an earlier version of the library, where that constant has no definition, so to be safe I don't assume it has been defined.</p> <p>Now, if there's a function that is only defined in the most recent version of the library, I can do this:</p> <pre><code>if (BrandNewFunc) { BrandNewFunc("foobar", 42); } else { printf("Your system does not support some thing."); } </code></pre> <p>Instead of containing the address of function code, <code>BrandNewFunc</code> evaluates to NULL. I would think that the constant would behave the same way, but if I try the same pattern, the app dies while performing a check (throws EXC_BAD_ACCESS on iOS). Specifically:</p> <pre><code>if (BrandNewIdentifier) ... // blows up here </code></pre> <p>What works instead is checking the address of the identifier:</p> <pre><code>if (&amp;BrandNewIdentifier) { printf("You got it!"); } </code></pre> <p>I can see the logic: <code>BrandNewIdentifier</code> has no value, so accessing it should fail. But then why does that syntax work in the case of <code>BrandNewFunc</code>? Shouldn't I be required to check its address also? Or is it actually consistent, and there is something I've overlooked?</p>
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    1. COI don't think this applies to C in general since it'd be dependent on the shared library implementation. For statically linked libraries, you'd simply have linker errors.
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    2. COFair enough, I've edited the question.
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