read, write – transfer data from and to a file

size[4] Tread tag[2] fid[4] offset[8] count[4]
size[4] Rread tag[2] count[4] data[count]
size[4] Twrite tag[2] fid[4] offset[8] count[4] data[count]
size[4] Rwrite tag[2] count[4]

The read request asks for count bytes of data from the file identified by fid, which must be opened for reading, starting offset bytes after the beginning of the file. The bytes are returned with the read reply message.
The count field in the reply indicates the number of bytes returned. This may be less than the requested amount. If the offset field is greater than or equal to the number of bytes in the file, a count of zero will be returned.
For directories, read returns an integral number of directory entries exactly as in stat (see stat(9P)), one for each member of the directory. The read request message must have offset equal to zero or the value of offset in the previous read on the directory, plus the number of bytes returned in the previous read. In other words, seeking other than to the beginning is illegal in a directory.
The write request asks that count bytes of data be recorded in the file identified by fid, which must be opened for writing, starting offset 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 offset. Directories may not be written.
The write reply records the number of bytes actually written. It is usually an error if this is not the same as requested.
Because 9P implementations may limit the size of individual messages, more than one message may be produced by a single read or write call. The iounit field returned by open(9P), if non-zero, reports the maximum size that is guaranteed to be transferred atomically.

Fsread and fswrite (see 9pclient(3)) generate the corresponding messages. Because they take an offset parameter, the fspread and fspwrite calls correspond more directly to the 9P messages. Although fsseek affects the offset, it does not generate a message.

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