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
...

void SetOutputExe();
void SetOutputMemory();
	
void Link(const char *fileName = 0);

#ifdef __unix__
#include "lib/Tcc.c"
#endif

#ifdef __unix__
#include "lib/tcc.c"
#endif

	unsigned long val;

	unsigned long val = 0;

why stdlib is included in a separate .a lib instead of being statically compiled with rest of tcc.

void NoStdlib(TCCState *s) {
	s->nostdlib = 1;
}

    if (!s1->nostdlib) {
        tcc_add_library(s1, "c");

        snprintf(buf, sizeof(buf), "%s/%s", tcc_lib_path, "libtcc1.a");
        tcc_add_file(s1, buf);
    }