double plus(double a, double b);	// External function has to be declared

...

	for (i = 0; i < 10000000; ++i)
		ret += 0.000000001*pow2(sqrt(plus(a, 10))); // plus() is called here

...

// Function to be called from the scripting program is defined here
double plus(double a, double b)
{
    return a + b;
} 

...
	    tcc.AddSymbol("plus", (void *)&plus); // And here we link the library function to the scripting program
...