# std::setbuf

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< cpp‎ | io‎ | c

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.
buffer | - | pointer to a buffer for the stream to use. If a null pointer is supplied, 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

### See also

setvbuf | sets the buffer and its size for a file stream (function)

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