std::ios_base::sync_with_stdio (3) - Linux Manuals
static bool sync_with_stdio( bool sync = true );
Sets whether the standard C++ streams are synchronized to the standard C streams after each input/output operation.
The standard C++ streams are the following: std::cin, std::cout, std::cerr, std::clog, std::wcin, std::wcout, std::wcerr and std::wclog
The standard C streams are the following: stdin, stdout and stderr
For a standard stream str, synchronized with the C stream f, the following pairs of functions have identical effect:
1) std::fputc(f, c) and str.rdbuf()->sputc(c)
2) std::fgetc(f) and str.rdbuf()->sbumpc()
3) std::ungetc(c, f) and str.rdbuf()->sputbackc(c)
In practice, this means that the synchronized C++ streams are unbuffered, and each I/O operation on a C++ stream is immediately applied to the corresponding C stream's buffer. This makes it possible to freely mix C++ and C I/O.
In addition, synchronized C++ streams are guaranteed to be thread-safe (individual characters output from multiple threads may interleave, but no data races occur)
If the synchronization is turned off, the C++ standard streams are allowed to buffer their I/O independently, which may be considerably faster in some cases.
By default, all eight standard C++ streams are synchronized with their respective C streams.
If this function is called after I/O has occurred on the standard stream, the behavior is implementation-defined: implementations range from no effect to destroying the read buffer.
sync - the new synchronization setting
synchronization state before the call to the function
// Run this code
cout (global object)
cerr (global object)
clog (global object)