From cppreference.com
staticboolsync_with_stdio(boolsync=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:
,
,
,
,
,
,
and
.
The standard C streams are the following:
,
and
.
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>intmain(){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
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