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)