ref: 0644aa11c120a581eec1d47c66208e08b946ec08
dir: /man/2/sys-stat/
.TH SYS-STAT 2
.SH NAME
fstat, fwstat, stat, wstat \- get and put file status
.SH SYNOPSIS
.EX
include "sys.m";
sys := load Sys Sys->PATH;
fstat: fn(fd: ref FD): (int, Dir);
fwstat: fn(fd: ref FD; d: Dir): int;
stat: fn(name: string): (int, Dir);
wstat: fn(name: string, d: Dir): int;
nulldir: con Dir(\fIspecial don't care values\fP);
zerodir: con Dir(\fIall elements set to zero\fP);
.EE
.SH DESCRIPTION
Given a file's
.IR name ,
or an open file descriptor
.IR fd ,
these routines retrieve or modify file status information.
.B Stat
and
.B fstat
retrieve information about
.I name
or
.I fd
into the
.B Dir
member of the return tuple.
The
.B int
member will be zero for success and \-1 for failure.
.B wstat
and
.B fwstat
write information back, thus changing file attributes according to
.IR d .
Both functions return zero for success and \-1 for failure.
.PP
File status is recorded as a
.B Dir
type:
.IP
.PP
.EX
Qid: adt
{
path: big; # unique id for file on server
vers: int; # write version number
qtype: int; # file type (see below)
};
Dir: adt
{
name: string; # last element of path
uid: string; # owner name
gid: string; # group name
muid: string; # last modifier name
qid: Qid; # unique id from server
mode: int; # permissions
atime: int; # last read time
mtime: int; # last write time
length: big; # file length
dtype: int; # server type
dev: int; # server subtype
};
.EE
.PP
If the file resides on permanent storage and is not a directory,
the
.B length
field returned in
.B Dir
by
.B stat
is the number of bytes in the file.
For directories, the length returned is zero.
Some devices, in particular files that are
streams such as pipes and network connections,
report a length that is the number of bytes that
may be read from the device without blocking.
.PP
Each file is the responsibility of some
.IR server :
it could be a file server, a kernel device, or a user process.
.B Dtype
identifies the server type, and
.B dev
says which of a group of servers of the same type is the one
responsible for this file.
.B Qid
is a type containing
.BR path ,
.B vers
and
.B qtype
members, each an integer:
.B path
is guaranteed to be
unique among all path names currently on the file server;
.B vers
changes each time the file is modified;
and
.B qtype
gives the file's characteristics (eg, directory or file).
The
.B path
is 64 bits
.RB ( big ),
and the
.B vers
is 32 bits
.RB ( int ).
Thus, if two files have the same
.BR dtype ,
.BR dev ,
and
.BR qid ,
they are the same file.
(Except when checking that the contents
are the same, as in a file cache, the version is often considered irrelevant in that comparison.)
The bits in
.B qtype
are defined by
.IP
.EX
16r80 # directory (Sys->QTDIR)
16r40 # append-only (Sys->QTAPPEND)
16r20 # exclusive-use (Sys->QTEXCL)
16r08 # authentication file (Sys->QTAUTH)
16r00 # any other file (Sys->QTFILE)
.EE
.PP
(They are the top 8 bits of
.B Dir.mode
for the file, as discussed below.)
.B Sys
defines constants for the bits:
.BR Sys->QTDIR ,
.BR Sys->QTAPPEND ,
and so on, as shown above.
The value
.B Sys->QTFILE
is not a particular bit; it is defined to be zero, to allow
a symbolic name to be used when creating
.B Qid
values for ordinary files.
.PP
The bits in
.B mode
are defined by
.IP
.EX
16r80000000 #directory (Sys->DMDIR)
16r40000000 #append-only (Sys->DMAPPEND)
16r20000000 #exclusive-use (Sys->DMEXCL)
16r08000000 #authentication file (Sys->DMAUTH)
8r400 #read permission by owner
8r200 #write permission by owner
8r100 #execute permission (search on directory) by owner
8r070 #read, write, execute (search) by group
8r007 #read, write, execute (search) by others
.EE
.PP
There are constants defined in
.B Sys
for the first four bits:
.BR Sys\->DMDIR ,
.B Sys\->DMAPPEND
and
.B Sys\->DMEXCL
for normal files, and
.B Sys\->DMAUTH
only for the special authentication file opened by
.IR sys-fauth (2).
.PP
The two time fields are measured in seconds since the epoch
(Jan 1 00:00 1970 GMT).
.B Mtime
is the time of the last change of content.
Similarly,
.B atime
is set whenever the contents are accessed;
also, it is set whenever
.B mtime
is set.
.PP
.B Uid
and
.B gid
are the names of the owner and group (of owners) of the file;
.B muid
is the name of the user that last modified the file (setting
.BR mtime ).
Groups are also users, but each server is free to associate
a list of users with any user name
.IR g ,
and that list is the
set of users in the group
.IR g .
When an initial attachment is made to a server,
the user string in the process group is communicated to the server.
Thus, the server knows, for any given file access, whether the accessing
process is the owner of, or in the group of, the file.
This selects which sets of three bits in
.B mode
is used to check permissions.
.PP
Only some of the fields may be changed by
.B wstat
calls.
The
.B name
can be changed by anyone with write permission in the parent directory.
The
.B mode
and
.B mtime
can be changed by the owner or the group leader of the file's current
group.
The
.B gid
can be changed by the owner if he or she is a member of the new group.
The
.B gid
can be changed by the group leader of the file's current group
if he or she is the leader of the new group.
The
.B length
can be changed by anyone with write permission, provided the operation
is implemented by the server.
(See
.IR intro (5)
and
.IR stat (5)
for more information about permissions, and
.IR users (6)
for how to configure users and groups
when using
.IR kfs (4)).
.PP
Special values in the fields of the
.B Dir
passed to
.I wstat
indicate that the field is not intended to be changed by the call.
The values are the maximum unsigned integer of appropriate size
for integral values (usually
.BR ~0 ,
but beware of conversions and size mismatches
when comparing values) and the empty or nil string for string values.
The constant
.B nulldir
in
.B Sys
has all its elements initialised to these ``don't care'' values.
Thus one may change the mode, for example, by assigning
.B sys->nulldir
to initialize a
.BR Dir ,
then setting the mode, and then doing
.IR wstat ;
it is not necessary to use
.I stat
to retrieve the initial values first.
.PP
The constant
.B zerodir
has all its elements initialised to zero.
It can be used to initialise a
.B Dir
structure, for use with
.IR styx (2)
or
.IR styxservers-nametree (2),
for instance.
.SH SEE ALSO
.IR sys-intro (2),
.IR sys-dirread (2),
.IR sys-open (2)