OBJC_EXTERN: what's the purpose?

Question

Hi was reviewing some Objective-C code and found out the following statement:

OBJC_EXTERN void CLSLog(NSString *format, ...) NS_FORMAT_FUNCTION(1,2);

What does this mean? Also, what is supposed to be the syntax of this statement?

Thanks in advance.

Answer 1

OBJC_EXTERN is defined in <objc/objc-api.h> as

#if !defined(OBJC_EXTERN)
#   if defined(__cplusplus)
#       define OBJC_EXTERN extern "C" 
#   else
#       define OBJC_EXTERN extern
#   endif
#endif

and therefore prevents "C++ name mangling" even if the above declaration is included from a C++ source file, as for example explained here:

• In C++ source, what is the effect of extern "C"?

For pure C code, you can just remove the OBJC_EXTERN, because the extern keyword is not needed in a function declaration.

---

NS_FORMAT_FUNCTION is defined as

#define NS_FORMAT_FUNCTION(F,A) __attribute__((format(__NSString__, F, A)))

and __attribute__((format(...))) is a GCC specific extension, also understood by Clang:

• http://clang.llvm.org/docs/LanguageExtensions.html#format-string-checking
• http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

It allows the compiler to check the number and types of the variable argument list against the format string. For example

CLSLog(@"%s", 123);

would cause a compiler warning, because %s is the placeholder for a string, but 123 is an integer.