plan9fox/sys/man/5/stat

.TH STAT 5 
.SH NAME
stat, wstat \- inquire or change file attributes
.SH SYNOPSIS
.ta \w'\fLTwstat 'u
.IR size [4]
.B Tstat
.IR tag [2]
.IR fid [4]
.br
.IR size [4]
.B Rstat
.IR tag [2]
.IR stat [ n ]
.PP
.IR size [4]
.B Twstat
.IR tag [2]
.IR fid [4]
.IR stat [ n ]
.br
.IR size [4]
.B Rwstat
.IR tag [2]
.SH DESCRIPTION
The
.B stat
transaction inquires about the file
identified by
.IR fid .
The reply will contain a
machine-independent
.I directory
.IR entry ,
.IR stat ,
laid out as follows:
.TP
.I size\f1[2]\fP
total byte count of the following data
.TP
.I type\f1[2]\fP
for kernel use
.TP
.I dev\f1[4]\fP
for kernel use
.TP
.I qid.type\f1[1]\fP
the type of the file (directory, etc.), represented as a bit vector
corresponding to the high 8 bits of the file's mode word.
.TP
.I qid.vers\f1[4]\fP
version number for given path
.TP
.I qid.path\f1[8]\fP
the file server's unique identification for the file
.TP
.I mode\f1[4]\fP
permissions and flags
.TP
.I atime\f1[4]\fP
last access time
.TP
.I mtime\f1[4]\fP
last modification time
.TP
.I length\f1[8]\fP
length of file in bytes
.TP
.I name\f1[ s ]\fP
file name; must be
.B /
if the file is the root directory of the server
.TP
.I uid\f1[ s ]\fP
owner name
.TP
.I gid\f1[ s ]\fP
group name
.TP
.I muid\f1[ s ]\fP
name of the user who last modified the file
.PD
.PP
Integers in this encoding are in little-endian order (least
significant byte first).
The
.I convM2D
and
.I convD2M
routines (see
.IR fcall (2))
convert between directory entries and a C structure called a
.BR Dir .
.PP
The
.I mode
contains permission bits as described in
.IR intro (5)
and the following:
.B 0x80000000
.RB ( DMDIR ,
this file is a directory),
.B 0x40000000
.RB ( DMAPPEND ,
append only),
.B 0x20000000
.RB ( DMEXCL ,
exclusive use),
.B 0x04000000
.RB ( DMTMP ,
temporary);
these are echoed in
.BR Qid.type .
Writes to append-only files always place their data at the
end of the file; the
.I offset
in the
.B write
message is ignored, as is the
.B 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 nightly archives.
.PP
The two time fields are measured in seconds since the epoch
(Jan 1 00:00 1970 GMT).
The
.I mtime
field reflects the time of the last change of content (except when later changed by
.BR wstat ).
For a plain file,
.I mtime
is the time of the most recent
.BR create ,
.B open
with truncation,
or
.BR write ;
for a directory it is the time of the most recent
.BR remove ,
.BR create ,
or
.B wstat
of a file in the directory.
Similarly, the
.I atime
field records the last
.B read
of the contents;
also it is set whenever
.I mtime
is set.
In addition, for a directory, it is set by
an
.BR attach ,
.BR walk ,
or
.BR create ,
all whether successful or not.
.PP
The
.I muid
field names the user whose actions most recently changed the
.I mtime
of the file.
.PP
The
.I length
records the number of bytes in the file.
Directories and most files representing devices have a conventional
length of 0.
.PP
The
.B stat
request requires no special permissions.
.PP
The
.B wstat
request can change some of the file status information.
The
.I 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
.I 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
.I mode
and
.I 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
.BR wstat ;
the other defined permission and mode bits can.
The
.I 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
.IR intro (5)
for more information about permissions and
.IR users (6)
for users and groups).
None of the other data can be altered by a
.B 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; see
.IR fsconfig (8).)
.PP
Either all the changes in
.B wstat
request happen, or none of them does: if the request succeeds,
all changes were made; if it fails, none were.
.PP
A
.B wstat
request can avoid modifying some properties of the file
by providing explicit ``don't touch'' values in the
.B 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
.I all
the elements of the directory entry in a
.B 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
.B Rwstat
message is returned.
(Consider the message to mean, ``make the state of the file
exactly what it claims to be.'')
.PP
A
.I read
of a directory yields an integral number of directory entries in
the machine independent encoding given above
(see
.IR read (5)).
.PP
Note that since the
.B stat
information is sent as a 9P variable-length datum, it is limited to a maximum of
65535 bytes.
.SH ENTRY POINTS
.B Stat
messages are generated by
.I fstat
and
.IR stat .
.PP
.B Wstat
messages are generated by
.I fwstat
and
.IR wstat .
.SH BUGS
To make the contents of a directory, such as returned by
.IR read (5),
easy to parse, each
directory entry begins with a size field.
For consistency, the entries in
.B Twstat
and
.B Rstat
messages also contain
their size, which means the size appears twice.
For example, the
.B Rstat
message is formatted as
.RI ``(4+1+2+2+ n )[4]
.B Rstat
.IR tag [2]
.IR n [2]
.RI ( n -2)[2]
.IR type [2]
.IR dev [4]...,''
where 
.I n
is the value returned by
.BR convD2M .