> Atomic operations on these objects, such as extractions and insertions, are protected against simultaneous access from different threads. (中略) Thus, you can safely share iostream objects across threads without worrying about loss of integrity.
ということなので、別々のスレッドから同時にアクセスしても保護されるということですよね。
> If you want changes to the file to be reflected after each atomic write, you must change the definition of the macro _IOSTREAM_OP_LOCKS from 0 to 1 (後略)
private:
_Mutex(const _Mutex&); // not defined
_Mutex& operator=(const _Mutex&); // not defined
void *_Mtx;
#endif /* !_MULTI_THREAD || !_IOSTREAM_OP_LOCKS */