The Linux SCSI Generic (sg) HOWTO
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# 6.12. pack_id

This value is not normally acted upon by the sg driver. It is provided so the user can identify the request. This is useful when command queuing is being used. The "abnormal" case is when SG_SET_FORCE_PACK_ID is set and a 'pack_id' other than -1 is given to read(). In this case the read() will wait to fetch a request that matches this 'pack_id'. If this mode is used be careful to set 'dxfer_direction' to a valid value (actually any of the SG_DXFER_* values will do) on input to the read(), together with the wanted pack_id. The type of pack_id is int.

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