std::setbuf
<cstdio>
Sets the internal buffer to use for I/O operations performed on the C stream stream.
If buffer is not null, equivalent to std::setvbuf (stream, buffer, _IOFBF, BUFSIZ ).
If buffer is null, equivalent to std::setvbuf (stream, nullptr, _IONBF, 0), which turns off buffering.
Contents
[edit] Parameters
BUFSIZ characters
[edit] Return value
(none)
[edit] Notes
If BUFSIZ is not the appropriate buffer size, std::setvbuf can be used to change it.
std::setvbuf should also be used to detect errors, since std::setbuf does not indicate success or failure.
This function may only be used after stream has been associated with an open file, but before any other operation (other than a failed call to std::setbuf/std::setvbuf ).
A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:
int main() { char buf[BUFSIZ ]; std::setbuf(stdin, buf); } // lifetime of buf ends, undefined behavior
[edit] Example
std::setbuf may be used to disable buffering on streams that require immediate output.
#include <chrono> #include <cstdio> #include <thread> int main() { using namespace std::chrono_literals; std::setbuf(stdout, nullptr); // unbuffered stdout std::putchar ('a'); // appears immediately on unbuffered stream std::this_thread::sleep_for (1s); std::putchar ('b'); }
Output:
ab