NAME

std::setbuf - std::setbuf

Synopsis

Defined in header <cstdio>
void setbuf( std::FILE* stream, char* buffer );

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.

Parameters

stream - the file stream to set the buffer to.
pointer to a buffer for the stream to use. If a null pointer is supplied,
buffer - the buffering is turned off. If not null, must be able to hold at least
BUFSIZ characters

Return value

(none)

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

Example

std::setbuf may be used to disable buffering on streams that require immediate
output

// Run this code

#include <cstdio>
#include <thread>
#include <chrono>

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