STAT(5)STAT(5)NAME
stat, wstat - inquire or change file attributes
SYNOPSIS
size[4] Tstat tag[2] fid[4]
size[4] Rstat tag[2] stat[n]
size[4] Twstat tag[2] fid[4] stat[n]
size[4] Rwstat tag[2]
DESCRIPTION
The stat transaction inquires about the file identified by fid. The
reply will contain a machine-independent directory entry, stat, laid
out as follows:
size[2]
total byte count of the following data
type[2]
for kernel use
dev[4] for kernel use
qid.type[1]
the type of the file (directory, etc.), represented as a bit
vector corresponding to the high 8 bits of the file's mode word.
qid.vers[4]
version number for given path
qid.path[8]
the file server's unique identification for the file
mode[4]
permissions and flags
atime[4]
last access time
mtime[4]
last modification time
length[8]
length of file in bytes
name[ s ]
file name; must be / if the file is the root directory of the
server
uid[ s ]
owner name
gid[ s ]
group name
muid[ s ]
name of the user who last modified the file
Integers in this encoding are in little-endian order (least significant
byte first). The unpackdir and packdir functions of styx(2) convert
between directory entries and the Limbo adt Sys->Dir. For C implemen‐
tations, the convM2D and convD2M routines (see styx(10.2)) convert
between directory entries and a C structure called Dir.
The mode contains permission bits as described in intro(5) and the fol‐
lowing: 16r80000000 (DMDIR, this file is a directory), 16r40000000
(DMAPPEND, append only), 16r20000000 (DMEXCL, exclusive use),
16r04000000 (DMTMP, temporary); these are echoed in Qid.type. Writes
to append-only files always place their data at the end of the file;
the offset in the write message is ignored, as is the OTRUNC bit in an
open. Exclusive use files may be open for I/O by only one fid at a
time across all clients of the server. If a second open is attempted,
it draws an error. Servers may implement a timeout on the lock on an
exclusive use file: if the fid holding the file open has been unused
for an extended period (of order at least minutes), it is reasonable to
break the lock and deny the initial fid further I/O. Temporary files
are not included in any automatic archives or dumps a server might cre‐
ate.
The two time fields are measured in seconds since the epoch (Jan 1
00:00 1970 GMT). The mtime field reflects the time of the last change
of content (except when later changed by wstat). For a plain file,
mtime is the time of the most recent create, open with truncation, or
write; for a directory it is the time of the most recent remove, cre‐
ate, or wstat of a file in the directory. Similarly, the atime field
records the last read of the contents; also it is set whenever mtime is
set. In addition, for a directory, it is set by an attach, walk, or
create, all whether successful or not.
The muid field names the user whose actions most recently changed the
mtime of the file.
The length records the number of bytes in the file. Directories and
most files representing devices have a conventional length of 0.
The stat request requires no special permissions.
The wstat request can change some of the file status information. The
name can be changed by anyone with write permission in the parent
directory; it is an error to change the name to that of an existing
file. The length can be changed (affecting the actual length of the
file) by anyone with write permission on the file. It is an error to
attempt to set the length of a directory to a non-zero value, and
servers may decide to reject length changes for other reasons. The
mode and mtime can be changed by the owner of the file or the group
leader of the file's current group. The directory bit cannot be
changed by a wstat; the other defined permission and mode bits can.
The gid can be changed: by the owner if also a member of the new group;
or by the group leader of the file's current group if also leader of
the new group (see intro(5) for more information about permissions and
users(6) for users and groups). None of the other data can be altered
by a wstat and attempts to change them will trigger an error. In par‐
ticular, it is illegal to attempt to change the owner of a file.
(These conditions may be relaxed when establishing the initial state of
a file server.)
Either all the changes in wstat request happen, or none of them does:
if the request succeeds, all changes were made; if it fails, none were.
A wstat request can avoid modifying some properties of the file by pro‐
viding explicit ``don't touch'' values in the stat data that is sent:
zero-length strings for text values and the maximum unsigned value of
appropriate size for integral values. As a special case, if all the
elements of the directory entry in a Twstat message are ``don't touch''
values, the server may interpret it as a request to guarantee that the
contents of the associated file are committed to stable storage before
the Rwstat message is returned. (Consider the message to mean, ``make
the state of the file exactly what it claims to be.'')
A read of a directory yields an integral number of directory entries in
the machine independent encoding given above (see read(5)).
Note that since the stat information is sent as a 9P variable-length
datum, it is limited to a maximum of 65535 bytes.
ENTRY POINTS
Stat messages are generated by fstat and stat.
Wstat messages are generated by fwstat and wstat.
SEE ALSOsys-stat(2), intro(5), read(5), intro(10), styx(10.2)
BUGS
To make the contents of a directory, such as returned by read(5), easy
to parse, each directory entry begins with a size field. For consis‐
tency, the entries in Twstat and Rstat messages also contain their
size, which means the size appears twice. For example, the Rstat mes‐
sage is formatted as ``(4+1+2+2+n)[4] Rstat tag[2] n[2] (n-2)[2]
type[2] dev[4]...,'' where n is the length of the array returned by
Styx->packdir.
STAT(5)
