243 lines
4.5 KiB
HTML
243 lines
4.5 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>write()</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
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"><LINK
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REL="HOME"
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TITLE="The Linux SCSI Generic (sg) HOWTO"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="System calls"
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HREF="syscalls.html"><LINK
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REL="PREVIOUS"
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TITLE="open()"
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HREF="open.html"><LINK
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REL="NEXT"
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TITLE="read()"
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HREF="read.html"></HEAD
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><BODY
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CLASS="SECT1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>The Linux SCSI Generic (sg) HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="open.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 7. System calls</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="read.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="WRITE">7.2. write()</H1
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><DIV
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CLASS="FORMALPARA"
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><P
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><B
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>write(int sg_fd, const void * buffer, size_t count). </B
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>The action of write() with a control block based on struct sg_header is
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discussed in the earlier document:
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<A
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HREF="http://www.torque.net/sg/p/scsi-generic.txt"
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TARGET="_top"
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><TT
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CLASS="LITERAL"
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>www.torque.net/sg/p/scsi-generic.txt</TT
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></A
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>
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(i.e the sg version 2 documentation). This section describes the action of
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write() when it is given a control block based on struct sg_io_hdr.</P
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></DIV
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><P
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>The 'buffer' should point to an object of type sg_io_hdr_t and 'count'
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should be sizeof(sg_io_hdr_t) [it can be larger but the excess is ignored].
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If the write() call succeeds then the 'count' is returned as the result.</P
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><P
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>Up to SG_MAX_QUEUE (16) write()s can be queued up before any finished
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requests are completed by read(). An attempt to queue more than that will
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result in an EDOM error.
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<A
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NAME="AEN425"
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HREF="#FTN.AEN425"
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>[1]</A
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>
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The write() command should return more or
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less immediately.
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<A
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NAME="AEN427"
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HREF="#FTN.AEN427"
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>[2]</A
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></P
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><P
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>The version 2 sg driver defaulted the maximum queue length to 1 (and
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made available the SG_SET_COMMAND_Q ioctl() to switch it to SG_MAX_QUEUE).
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So for backward compatibility a file descriptor that only receives
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sg_header structures in its write() will have a default "max" queue length
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of 1. As soon as a sg_io_hdr_t structure is seen by a write() then the
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maximum queue length is switched to SG_MAX_QUEUE on that file descriptor.</P
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><P
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>The "const" on the 'buffer' pointer is respected by the sg driver. Data
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is read in from the sg_io_hdr object that is pointed to. Significantly
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this is when the 'sbp' and the 'dxferp' are recorded internally (i.e.
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not from the sg_io_hdr object given to the corresponding read() ).</P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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BORDER="0"
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN425"
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HREF="write.html#AEN425"
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>[1]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>The command queuing capabilities of the SCSI device and the adapter driver
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should also be taken into account. To this end the sg_scsi_id::h_cmd_per_lun
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and sg_scsi_id::d_queue_depth values returned bu ioctl(SG_GET_SCSI_ID) may
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be useful. Also some devices that indicate in their INQUIRY response that
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they can accept command queuing react badly when queuing is actually
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attempted.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN427"
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HREF="write.html#AEN427"
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>[2]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>There is a small probability it will spend some time waiting for a
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command block to become available. In this case the wait is interruptible.
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If O_NONBLOCK is active then this scenario will cause a EAGAIN.</P
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></TD
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></TR
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></TABLE
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="open.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
|
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
|
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ALIGN="right"
|
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VALIGN="top"
|
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><A
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HREF="read.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
|
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><TR
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><TD
|
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WIDTH="33%"
|
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ALIGN="left"
|
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VALIGN="top"
|
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>open()</TD
|
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><TD
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WIDTH="34%"
|
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ALIGN="center"
|
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VALIGN="top"
|
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><A
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HREF="syscalls.html"
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ACCESSKEY="U"
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>Up</A
|
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></TD
|
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><TD
|
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WIDTH="33%"
|
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ALIGN="right"
|
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VALIGN="top"
|
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>read()</TD
|
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></TR
|
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></TABLE
|
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></DIV
|
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></BODY
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></HTML
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> |