C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 32:
[32.5] How can I call a non-system C function f(int,char,float) from my C++ code?

If you have an individual C function that you want to call, and for some reason you don't have or don't want to #include a C header file in which that function is declared, you can declare the individual C function in your C++ code using the extern "C" syntax. Naturally you need to use the full function prototype:

extern "C" void f(int i, char c, float x);
A block of several C functions can be grouped via braces:
extern "C" {
  void   f(int i, char c, float x);
  int    g(char* s, char const* s2);
  double sqrtOfSumOfSquares(double a, double b);
After this you simply call the function just as if it were a C++ function:
int main()
  f(7, 'x', 3.14);   // Note: nothing unusual in the call