The Linux SCSI Generic (sg) HOWTO
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# 6.7. dxferp

If 'iovec_count' is zero then this value is a pointer to user memory of at least 'dxfer_len' bytes in length. If there is a data transfer associated with the command then the data will be transferred to or from this user memory. If 'iovec_count' is greater than zero then this value points to a scatter-gather array in user memory. Each element of this array should be an object of type sg_iovec_t. Note that data is sometimes written to user memory (e.g. from a failed SCSI READ) even when an error has occurred.

If mmap-ed IO is selected then the value in 'dxferp' is ignored and any data transfers will be to and from the address returned by the prior mmap() call.

The type of dxferp is void * .

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