C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 32:
[32.3] How can I include a non-system C header file in my C++ code?

If you are including a C header file that isn't provided by the system, you may need to wrap the #include line in an extern "C" { /*...*/ } construct. This tells the C++ compiler that the functions declared in the header file are C functions.

// This is C++ code

extern "C" {
  // Get declaration for f(int i, char c, float x)
  #include "my-C-code.h"
}

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 change.