180 lines
3.1 KiB
HTML
180 lines
3.1 KiB
HTML
<HTML
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>iovec_count</TITLE
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>The Linux SCSI Generic (sg) HOWTO</TH
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>Chapter 6. The sg_io_hdr_t structure in detail</TD
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NAME="AEN192">6.5. iovec_count</H1
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><P
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> This is the number of scatter gather elements in an array pointed
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to by 'dxferp'. If the value is zero then scatter gather (in the
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user space) is _not_ being used and 'dxferp' points to the data
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transfer buffer. If the value is greater than zero then each
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element of the array is assumed to be of the form:
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<TABLE
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><PRE
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CLASS="PROGRAMLISTING"
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> typedef struct sg_iovec
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{
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void * iov_base; /* starting address */
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size_t iov_len; /* length in bytes */
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} sg_iovec_t;</PRE
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></FONT
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>
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Note that this structure has been named and defined in such a way to
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parallel "struct iovec" used by the readv() and writev() system calls
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in Linux. See "man 2 readv".</P
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><P
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>Note that the scatter gather capability offered by 'iovec_count' is unrelated
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to the scatter gather capability (often associated with DMA) offered by
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most modern SCSI adapters. Furthermore iovec_count's variety of scatter
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gather (into the user space) is only available when normal (or "indirect")
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IO is being used. Hence when the SG_FLAG_DIRECT_IO or SG_FLAG_MMAP_IO are
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set in 'flags' then 'iovec_count' should be zero.</P
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><P
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>The type of iovec_count is unsigned short.</P
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