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
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STAT(5) STAT(5)
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 func-
tions of styx(2) convert between directory entries and the
Limbo adt Sys->Dir. For C implementations, 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 following: 16r80000000 (DMDIR, this file is a direc-
tory), 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 create.
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 direc-
tory it is the time of the most recent remove, create, 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. Direc-
tories and most files representing devices have a conven-
tional length of 0.
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The stat request requires no special permissions.
The wstat request can change some of the file status infor-
mation. The name can be changed by anyone with write per-
mission 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 particular, 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 providing 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 inte-
gral values. As a special case, if all the elements of the
directory entry in a Twstat message are ``don't touch'' val-
ues, 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. (Con-
sider 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.
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STAT(5) STAT(5)
SEE ALSO
sys-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 consistency, the entries in Twstat and
Rstat messages also contain their size, which means the size
appears twice. For example, the Rstat message 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.
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