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<title>read(9P) - Plan 9 from User Space</title>
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<tr><td width=20><td><b>READ(9P)</b><td align=right><b>READ(9P)</b>
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<p><font size=+1><b>NAME </b></font><br>
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read, write – transfer data from and to a file<br>
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<p><font size=+1><b>SYNOPSIS </b></font><br>
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<i>size</i>[4] <tt><font size=+1>Tread</font></tt> <i>tag</i>[2] <i>fid</i>[4] <i>offset</i>[8] <i>count</i>[4]<br>
<i>size</i>[4] <tt><font size=+1>Rread</font></tt> <i>tag</i>[2] <i>count</i>[4] <i>data</i>[<i>count</i>]
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<i>size</i>[4] <tt><font size=+1>Twrite</font></tt> <i>tag</i>[2] <i>fid</i>[4] <i>offset</i>[8] <i>count</i>[4] <i>data</i>[<i>count</i>]<br>
<i>size</i>[4] <tt><font size=+1>Rwrite</font></tt> <i>tag</i>[2] <i>count</i>[4]<br>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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The <tt><font size=+1>read</font></tt> request asks for <i>count</i> bytes of data from the file identified
by <i>fid</i>, which must be opened for reading, starting <i>offset</i> bytes
after the beginning of the file. The bytes are returned with the
<tt><font size=+1>read</font></tt> reply message.
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The <i>count</i> field in the reply indicates the number of bytes returned.
This may be less than the requested amount. If the <i>offset</i> field
is greater than or equal to the number of bytes in the file, a
count of zero will be returned.
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For directories, <tt><font size=+1>read</font></tt> returns an integral number of directory
entries exactly as in <tt><font size=+1>stat</font></tt> (see <i>stat</i>(9P)), one for each member
of the directory. The <tt><font size=+1>read</font></tt> request message must have <tt><font size=+1>offset</font></tt> equal
to zero or the value of <tt><font size=+1>offset</font></tt> in the previous <tt><font size=+1>read</font></tt> on the directory,
plus the number of bytes returned in the previous
<tt><font size=+1>read</font></tt>. In other words, seeking other than to the beginning is illegal
in a directory.
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The <tt><font size=+1>write</font></tt> request asks that <i>count</i> bytes of data be recorded in
the file identified by <i>fid</i>, which must be opened for writing,
starting <i>offset</i> bytes after the beginning of the file. If the
file is append-only, the data will be placed at the end of the
file regardless of <i>offset</i>. Directories may not be written.
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The <tt><font size=+1>write</font></tt> reply records the number of bytes actually written.
It is usually an error if this is not the same as requested.
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Because 9P implementations may limit the size of individual messages,
more than one message may be produced by a single <i>read</i> or <i>write</i>
call. The <i>iounit</i> field returned by <i>open</i>(9P), if non-zero, reports
the maximum size that is guaranteed to be transferred atomically.<br>
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<p><font size=+1><b>ENTRY POINTS </b></font><br>
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<i>Fsread</i> and <i>fswrite</i> (see <a href="../man3/9pclient.html"><i>9pclient</i>(3)</a>) generate the corresponding
messages. Because they take an offset parameter, the <i>fspread</i> and
<i>fspwrite</i> calls correspond more directly to the 9P messages. Although
<i>fsseek</i> affects the offset, it does not generate a message.<br>
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