What means :: on the front of a function call

Question

I have this code:

1. bool MyClass::open() {
2.   int fd = ::open("file.txt",flags);
3. }

Does the "::" from the line 2 before calling open means something?

Answer 1

It means "open from the global namespace". It is a way to disambiguate with MyClass::open, which is a name that would be picked up if you said open without the leading ::.

Answer 2

It's scope resolution operator and it says, that the function (open in this case) is in the global namespace.