read, write - transfer data from and to a file
size Tread tag fid offset count
size Rread tag count data[count]
size Twrite tag fid offset count data[count]
size Rwrite tag count
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
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(5)), 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 (see seek(2)).
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(5), if non-zero, reports the maximum
size that is guaranteed to be transferred atomically.
Read and write messages are generated by the corresponding calls.
Because they include an offset, the pread and pwrite calls correspond
more directly to the 9P messages. Although seek(2) affects the offset,
it does not generate a message.