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The Linux SCSI Generic (sg) HOWTO
PrevChapter 6. The sg_io_hdr_t structure in detailNext

6.5. iovec_count

This is the number of scatter gather elements in an array pointed to by 'dxferp'. If the value is zero then scatter gather (in the user space) is _not_ being used and 'dxferp' points to the data transfer buffer. If the value is greater than zero then each element of the array is assumed to be of the form: 
            typedef struct sg_iovec
            {
                void * iov_base; /* starting address */
                size_t iov_len;  /* length in bytes */
            } sg_iovec_t;
Note that this structure has been named and defined in such a way to parallel "struct iovec" used by the readv() and writev() system calls in Linux. See "man 2 readv".

Note that the scatter gather capability offered by 'iovec_count' is unrelated to the scatter gather capability (often associated with DMA) offered by most modern SCSI adapters. Furthermore iovec_count's variety of scatter gather (into the user space) is only available when normal (or "indirect") IO is being used. Hence when the SG_FLAG_DIRECT_IO or SG_FLAG_MMAP_IO are set in 'flags' then 'iovec_count' should be zero.

The type of iovec_count is unsigned short.

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