31 Input/output library [input.output]

31.5 Iostreams base classes [iostreams.base]

31.5.2 Class ios_base [ios.base] Static members [ios.members.static]

static bool sync_with_stdio(bool sync = true);
Effects: If any input or output operation has occurred using the standard streams prior to the call, the effect is implementation-defined.
Otherwise, called with a false argument, it allows the standard streams to operate independently of the standard C streams.
Returns: true if the previous state of the standard iostream objects was synchronized and otherwise returns false.
The first time it is called, the function returns true.
Remarks: When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by fputc(f, c); is the same as the effect of str.rdbuf()->sputc(c); for any sequences of characters; the effect of extracting a character c by c = fgetc(f); is the same as the effect of c = str.rdbuf()->sbumpc(); for any sequences of characters; and the effect of pushing back a character c by ungetc(c, f); is the same as the effect of str.rdbuf()->sputbackc(c); for any sequence of characters.265
This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream.
In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer.