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I am familiar with the extern keyword, it is used to declare a variable present in some other file, but what does the following statement mean??

extern "C" const IMAGE_DOS_HEADER __ImageBase;
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  • possible duplicate of In C++ source, what is the effect of extern "C"? Commented May 22, 2012 at 18:26
  • Wnat is IMAGE_DOS_HEADER? Language linkage normally only applies to function types (included nested function types, such as pointer to function). Commented May 22, 2012 at 18:30
  • @James, global variable names are mangled too. Your point about linkage having further effect on functions still stands, though. Commented May 22, 2012 at 18:37
  • @FrédéricHamidi Mangling is up to the implementation; it may mangle nothing, everything or anything in between. In practice, most compilers, for most languages, have mangled everything; what has traditionally set C++ apart is the complexity of the mangling scheme (including information concerning the types of parameters) for functions. Traditionally, C and C++ have mangled variable names in the same manner. (A quick check with the compilers I have handy show that after extern "C", I can't even find the name in the object file. Which I definitely don't understand.) Commented May 23, 2012 at 7:55

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It means the __ImageBase global variable uses C linkage and that its name should be mangled using the rules for C instead of C++.

EDIT: It just so happens that Raymond Chen recently published an article that demonstrates my original answer was plain wrong: extern "C" does not disable name mangling, it only changes the rules used to perform it. C names can be mangled too.

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C linkage involves more than just mangling; it's possible for the arguments to be passed differently, for example. Also, the language linkage is part of the type; a function with extern "C" linkage cannot be assigned to a pointer to a C++ function.
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It means do not mangle the symbol name __ImageBase that follows the extern "C". In short it ensures you can use the variable in C++ code.

extern "C" specify's the linkage to be applied. In short a Linkage specification.
It tells the C++ compiler to apply linkage of the type of C to the symbol that follows.

Good Read:
Using extern to Specify Linkage
How to mix C and C++

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The now-deleted comment was right, as extern "C" is indeed a C++-only construct, but it still means C code that links to the C++ module defining this variable will be able to refer to it... So, it actually works both ways, C to C++ and C++ to C (section 32.6 of the FAQ seems to agree :).
@FrédéricHamidi: A C compiler won't understand the extern "C" construct, and so one must wrap the extern "C" { and` }` lines in an #ifdef so they won't be seen by the C compilers.
Yes, we agree that it cannot be used outside of C++ modules. But it both allows to use the decorated symbol if it is defined in a C module, or allows C modules to use the symbol if the C++ module itself defines it. That's what I meant by both ways (but I don't know if I was actually clearer this time around :)

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