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

Return value

synchronization state before the call to the function.


#include <iostream>
#include <cstdio>
int main()
    std::cout << "a\n";
    std::cout << "c\n";

Possible output:


Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 49 C++98 it was unspecified (1) which state is actually returned and
(2) what does 'synchronized' between standard C and C++ streams mean
both specified

See also

writes to the standard C output stream stdout
(global object)
writes to the standard C error stream stderr, unbuffered
(global object)
writes to the standard C error stream stderr
(global object)

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