C++ to Java conversion question about extern "C"

Question

I have to convert some C/C++ code to Java. My C++ is extremely rusty.

In a .h file, I have the following:

#ifdef __cplusplus
extern "C" {
#endif

/* tons of declarations */

#ifdef __cplusplus
} /* extern C */
#endif

What is the use of extern "C"? What does it mean? Is it telling the compiler that corresponding code should be interpreted as pure C, rather than C++?

EDIT

Thanks for the answers so far. The history of the code I have to convert is that it seems like a part was written in C first, then the rest was written in C++. So my header file seems to correspond to 'old' C code.

I'll convert this code into a public final class with static method and attributes. No overriding.

Answer 1

It's telling the compiler that function signatures should be C compatible. Name mangling are different in C and C++ (i.e. C++ supports method overloading while C does not) so in order to make some function calls (i.e. some DLL dynamic calls) compatible with C signature, you use extern C in C++ code.

Take a look at this StackOverflow question for a great explanation on the topic.

Answer 2

Extern "C" is used to indicate a C-style linking instead of C++-style. The C++ compiler adds type information to the function symbol, but the C compiler does not.

If your library has to be used by an external application, extern "C" is mandatory because C++ compilers are mostly incompatible regarding type information. This is the case when you have to create a native implementation in Java, for instance.

Answer 3

Extern "C" is indicating a C linking style instead of a C++ style. It tells the compiler "please do not mangle my function names". It is used so that a C++ program can be linked with a C program. If you are converting something to Java, it is probably safe to ignore this. This line would only be important if you were trying to convert code to C or C++.