bdflush starts, flushes, or tunes the buffer-dirty-flush daemon. Only the super-user may call bdflush.
If func is negative or 0, and no daemon has been started, then bdflush enters the daemon code and never returns.
If func is 1, some dirty buffers are written to disk.
If func is 2 or more and is even (low bit is 0), then address is the address of a long word, and the tuning parameter numbered "(""func""-2)/2" is returned to the caller in that address.
If func is 3 or more and is odd (low bit is 1), then data is a long word, and the kernel sets tuning parameter numbered "(""func""-3)/2" to that value.
The set of parameters, their values, and their legal ranges are defined in the kernel source file fs/buffer.c.
If func is negative or 0 and the daemon successfully starts, bdflush never returns. Otherwise, the return value is 0 on success and -1 on failure, with errno set to indicate the error.