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]
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 requested amount is not all currently available. However, servers should try to avoid returning less data than requested if possible. In particular, merely crossing a storage block boundary is not sufficient reason to return a short count; exec(2) in particular relies upon this property. 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.|