The Linux SCSI Generic (sg) HOWTO
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# 8.11. SG_SET_FORCE_PACK_ID

SG_SET_FORCE_PACK_ID 0x227b. Assumes 3rd argument is pointing to an int. 0 (default) instructs read() to return the oldest (written) packet if multiple packets are waiting to be read. 1 instructs read() to view the sg_io_hdr::pack_id (or sg_header::pack_id) as input and return the oldest packet matching that pack_id or wait until it arrives. If the file descriptor is in O_NONBLOCK state, rather than wait this ioctl() will yield EAGAIN. As a special case the pack_id of -1 given to read() in the mode will match the oldest packet. Only the current file descriptor is affected by this command.

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