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.TH READ 5
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.SH NAME
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read, write \- transfer data from and to a file
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.SH SYNOPSIS
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.ta \w'\fLTwrite 'u
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.IR size [4]
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.B Tread
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.IR tag [2]
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.IR fid [4]
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.IR offset [8]
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.IR count [4]
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.br
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.IR size [4]
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.B Rread
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.IR tag [2]
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.IR count [4]
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.IR data [ count ]
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.PP
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.IR size [4]
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.B Twrite
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.IR tag [2]
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.IR fid [4]
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.IR offset [8]
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.IR count [4]
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.IR data [ count ]
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.br
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.IR size [4]
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.B Rwrite
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.IR tag [2]
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.IR count [4]
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.SH DESCRIPTION
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The
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.B read
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request
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asks for
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.I count
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bytes of data
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from the file identified by
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.IR fid ,
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which must be opened for reading,
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starting
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.I offset
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bytes after the beginning of the file.
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The bytes are returned with the
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.B read
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reply message.
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.PP
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The
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.I count
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field in the reply indicates the number of bytes returned.
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This may be less than the requested amount.
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If the
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.I offset
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field is greater than or equal to the number of bytes in the file,
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a count of zero will be returned.
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.PP
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For directories,
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.B read
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returns an integral number of
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directory entries exactly as in
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.B stat
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(see
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.IR stat (5)),
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one for each member of the directory.
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The
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.B read
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request message must have
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.B offset
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equal to zero or the value of
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.B offset
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in the previous
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.B read
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on the directory, plus the number of bytes
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returned in the previous
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.BR read .
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In other words, seeking other than to the beginning
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is illegal in a directory (see
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.IR seek (2)).
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.PP
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The
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.B write
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request asks that
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.I count
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bytes of data be recorded in the file identified by
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.IR fid ,
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which must be opened for writing, starting
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.I offset
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bytes after the beginning of the file.
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If the file is append-only,
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the data will be placed at the end of the file regardless of
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.IR offset .
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Directories may not be written.
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.PP
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The
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.B write
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reply records the number of bytes actually written.
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It is usually an error
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if this is not the same as requested.
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.PP
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Because 9P implementations may limit the size of individual
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messages,
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more than one message may be produced by a single
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.I read
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or
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.I write
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call.
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The
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.I iounit
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field returned by
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.IR open (5),
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if non-zero, reports the maximum size that is guaranteed
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to be transferred atomically.
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.SH ENTRY POINTS
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.B Read
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and
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.B write
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messages are generated by the corresponding calls.
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Because they include an offset, the
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.I pread
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and
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.I pwrite
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calls correspond more directly to the 9P messages.
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Although
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.IR seek (2)
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affects the offset, it does not generate a message.