// The 'rb_define_global_function' can be found in class.c

/*!
 * Defines a global function
 * \param name    name of the function
 * \param func    the method body
 * \param argc    the number of parameters, or -1 or -2. see \ref defmethod.
 */
void
rb_define_global_function(const char *name, VALUE (*func)(ANYARGS), int argc)
{
    rb_define_module_function(rb_mKernel, name, func, argc);
}

// As you can see, this just defines a method on the "Kernel"-Module (which is defined in object.c)

/*!
 * Defines a module function for \a module.
 * \param module  an module or a class.
 * \param name    name of the function
 * \param func    the method body
 * \param argc    the number of parameters, or -1 or -2. see \ref defmethod.
 */
void
rb_define_module_function(VALUE module, const char *name, VALUE (*func)(ANYARGS), int argc)
{
    rb_define_private_method(module, name, func, argc);
    rb_define_singleton_method(module, name, func, argc);
}