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

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 sys-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 Styx 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.