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copied!<p><code>OBJC_EXTERN</code> is defined in <code><objc/objc-api.h></code> as</p> <pre><code>#if !defined(OBJC_EXTERN) # if defined(__cplusplus) # define OBJC_EXTERN extern "C" # else # define OBJC_EXTERN extern # endif #endif </code></pre> <p>and therefore prevents "C++ name mangling" even if the above declaration is included from a C++ source file, as for example explained here:</p> <ul> <li><a href="https://stackoverflow.com/questions/1041866/in-c-source-what-is-the-effect-of-extern-c">In C++ source, what is the effect of extern "C"?</a></li> </ul> <p>For pure C code, you can just remove the <code>OBJC_EXTERN</code>, because the <code>extern</code> keyword is not needed in a function declaration.</p> <hr> <p><code>NS_FORMAT_FUNCTION</code> is defined as </p> <pre><code>#define NS_FORMAT_FUNCTION(F,A) __attribute__((format(__NSString__, F, A))) </code></pre> <p>and <code>__attribute__((format(...)))</code> is a GCC specific extension, also understood by Clang:</p> <ul> <li><a href="http://clang.llvm.org/docs/LanguageExtensions.html#format-string-checking" rel="nofollow noreferrer">http://clang.llvm.org/docs/LanguageExtensions.html#format-string-checking</a></li> <li><a href="http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html" rel="nofollow noreferrer">http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html</a></li> </ul> <p>It allows the compiler to check the number and types of the variable argument list against the format string. For example</p> <pre><code>CLSLog(@"%s", 123); </code></pre> <p>would cause a compiler warning, because <code>%s</code> is the placeholder for a string, but <code>123</code> is an integer.</p>

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