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the number of bytes in the send queue, and FIONSPACE reports the number of free bytes in the send queue. These ioctls permit applications to monitor file descriptor transmission dynamics. In examining prior art, FIONWRITE exists with the semantics given here. FIONSPACE is provided so that programs may easily determine how much space is left in the send queue; they do not need to know the send queue size. The fact that a write may block even if there is enough space in the send queue for it is noted in the documentation. FIONWRITE functionality may be used to implement TIOCOUTQ for Linux emulation - Linux extended this ioctl to sockets, even though they are not ttys.