How do I call a C function from C++?/How do I call

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Just declare the C function ``extern "C"'' (in your C++ code) and call it (from your C or C++ code). For example:

	// C++ code

	extern "C" void f(int);	// one way

	extern "C" {	// another way
		int g(double);
		double h();
	};

	void code(int i, double d)
	{
		f(i);
		int ii = g(d);
		double dd = h();
		// ...
	}

The definitions of the functions may look like this:

	/* C code: */

	void f(int i)
	{
		/* ... */
	}

	int g(double d)
	{
		/* ... */
	}

	double h()
	{
		/* ... */
	}

Note that C++ type rules, not C rules, are used. So you can't call function declared ``extern "C"'' with the wrong number of argument. For example:

	// C++ code

	void more_code(int i, double d)
	{
		double dd = h(i,d);	// error: unexpected arguments
		// ...
	}

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Just declare the C++ function ``extern "C"'' (in your C++ code) and call it (from your C or C++ code). For example:

	// C++ code:

	extern "C" void f(int);

	void f(int i)
	{
		// ...
	}

Now f() can be used like this:

	/* C code: */

	void f(int);
	
	void cc(int i)
	{
		f(i);
		/* ... */
	}

Naturally, this works only for non-member functions. If you want to call member functions (incl. virtual functions) from C, you need to provide a simple wrapper. For example:

	// C++ code:

	class C {
		// ...
		virtual double f(int);
	};

	extern "C" double call_C_f(C* p, int i)	// wrapper function
	{
		return p->f(i);
	}

Now C::f() can be used like this:

	/* C code: */

	double call_C_f(struct C* p, int i);
	
	void ccc(struct C* p, int i)
	{
		double d = call_C_f(p,i);
		/* ... */
	}

If you want to call overloaded functions from C, you must provide wrappers with distinct names for the C code to use. For example:

	// C++ code:

	void f(int);
	void f(double);

	extern "C" void f_i(int i) { f(i); }
	extern "C" void f_d(double d) { f(d); }

Now the f() functions can be used like this:

	/* C code: */

	void f_i(int);
	void f_d(double);
	
	void cccc(int i,double d)
	{
		f_i(i);
		f_d(d);
		/* ... */
	}

Note that these techniques can be used to call a C++ library from C code even if you cannot (or do not want to) modify the C++ headers.

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