std::ios_base::sync_with_stdio(3)

NAME
std::ios_base::sync_with_stdio - std::ios_base::sync_with_stdio

Synopsis
   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.

Parameters
   sync - the new synchronization setting

Return value
   Synchronization state before the call to the function.

Example
// Run this code
 #include <cstdio>
 #include <iostream>
 int main()
 {
     std::ios::sync_with_stdio(false);
     std::cout << "a\n";
     std::printf("b\n");
     std::cout << "c\n";
 }

Possible output:
 b
 a
 c
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                     it was unspecified (1) which state is actually
   LWG 49 C++98 returned and both specified
                     (2) what does 'synchronized' between standard C
                     and C++ streams mean

See also
   cout writes to the standard C output stream stdout
   wcout (global object)
   cerr writes to the standard C error stream stderr, unbuffered
   wcerr (global object)
   clog writes to the standard C error stream stderr
   wclog (global object)