C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 32:
[32.4] How can I modify my own C header files so it's easier to #include them in C++ code?

If you are including a C header file that isn't provided by the system, and if you are able to change the C header, you should strongly consider adding the extern "C" {...} logic inside the header to make it easier for C++ users to #include it into their C++ code. Since a C compiler won't understand the extern "C" construct, you must wrap the extern "C" { and } lines in an #ifdef so they won't be seen by normal C compilers.

Step #1: Put the following lines at the very top of your C header file (note: the symbol __cplusplus is #defined if/only-if the compiler is a C++ compiler):

#ifdef __cplusplus
extern "C" {
#endif
Step #2: Put the following lines at the very bottom of your C header file:
#ifdef __cplusplus
}
#endif
Now you can #include your C header without any extern "C" nonsense in your C++ code:
// This is C++ code

// Get declaration for f(int i, char c, float x)
#include "my-C-code.h"   // Note: nothing unusual in #include line

int main()
{
  f(7, 'x', 3.14);       // Note: nothing unusual in the call
  ...
}
Note: Somewhat different guidelines apply for C headers provided by the system (such as <cstdio>) and for C headers that you can't change.

Note: #define macros are evil in 4 different ways: evil#1, evil#2, evil#3, and evil#4. But they're still useful sometimes. Just wash your hands after using them.