How to call C function from lua?

Earlier today, I made some code that would basically run a lua script in C. It worked perfectly:

#include <lua5.1/lua.h>
#include <lua5.1/lualib.h>
#include <lua5.1/lauxlib.h>

lua_State *LuaInstance;

int main(int argc, const char *argv[])
{
    LuaInstance = luaL_newstate();
    luaL_openlibs(LuaInstance);
    luaL_dofile(LuaInstance, argv[1]);
}

Now, I know that it is possible to execute lua code in C. But, how can I call a function from C in lua?