Is stdout.flush() unsafe?

[tsbockman via Digitalmars-d-learn]

Trying to compile this:

void main() @safe
{
     import std.stdio;
     stdout.flush();
}

Fails with this message:
> Error: safe function 'main' cannot access __gshared data 
> 'stdout'

Is this necessary? If so, what are the synchronization 
requirements for access to `stdout`?