SYS-STAT(2) SYS-STAT(2)
NAME
fstat, fwstat, stat, wstat - get and put file status
SYNOPSIS
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(special don't care values);
zerodir: con Dir(all elements set to zero);
DESCRIPTION
Given a file's name, or an open file descriptor fd, these
routines retrieve or modify file status information. Stat
and fstat retrieve information about name or fd into the Dir
member of the return tuple. The int member will be zero for
success and -1 for failure. wstat and fwstat write informa-
tion back, thus changing file attributes according to d.
Both functions return zero for success and -1 for failure.
File status is recorded as a Dir type:
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
};
If the file resides on permanent storage and is not a
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SYS-STAT(2) SYS-STAT(2)
directory, the length field returned in Dir by 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.
Each file is the responsibility of some server: it could be
a file server, a kernel device, or a user process. Dtype
identifies the server type, and dev says which of a group of
servers of the same type is the one responsible for this
file. Qid is a type containing path, vers and qtype mem-
bers, each an integer: path is guaranteed to be unique among
all path names currently on the file server; vers changes
each time the file is modified; and qtype gives the file's
characteristics (eg, directory or file). The path is 64
bits (big), and the vers is 32 bits (int). Thus, if two
files have the same dtype, dev, and 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 irrele-
vant in that comparison.) The bits in qtype are defined by
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)
(They are the top 8 bits of Dir.mode for the file, as dis-
cussed below.) Sys defines constants for the bits:
Sys->QTDIR, Sys->QTAPPEND, and so on, as shown above. The
value Sys->QTFILE is not a particular bit; it is defined to
be zero, to allow a symbolic name to be used when creating
Qid values for ordinary files.
The bits in mode are defined by
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
There are constants defined in Sys for the first four bits:
Sys->DMDIR, Sys->DMAPPEND and Sys->DMEXCL for normal files,
and Sys->DMAUTH only for the special authentication file
opened by sys-fauth(2).
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SYS-STAT(2) SYS-STAT(2)
The two time fields are measured in seconds since the epoch
(Jan 1 00:00 1970 GMT). Mtime is the time of the last
change of content. Similarly, atime is set whenever the
contents are accessed; also, it is set whenever mtime is
set.
Uid and gid are the names of the owner and group (of owners)
of the file; muid is the name of the user that last modified
the file (setting mtime). Groups are also users, but each
server is free to associate a list of users with any user
name g, and that list is the set of users in the group 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 mode is
used to check permissions.
Only some of the fields may be changed by wstat calls. The
name can be changed by anyone with write permission in the
parent directory. The mode and mtime can be changed by the
owner or the group leader of the file's current group. The
gid can be changed by the owner if he or she is a member of
the new group. The 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 length can be changed by anyone with
write permission, provided the operation is implemented by
the server. (See intro(5) and stat(5) for more information
about permissions, and users(6) for how to configure users
and groups when using kfs(4)).
Special values in the fields of the Dir passed to wstat
indicate that the field is not intended to be changed by the
call. The values are the maximum unsigned integer of appro-
priate size for integral values (usually ~0, but beware of
conversions and size mismatches when comparing values) and
the empty or nil string for string values. The constant
nulldir in Sys has all its elements initialised to these
``don't care'' values. Thus one may change the mode, for
example, by assigning sys->nulldir to initialize a Dir, then
setting the mode, and then doing wstat; it is not necessary
to use stat to retrieve the initial values first.
The constant zerodir has all its elements initialised to
zero. It can be used to initialise a Dir structure, for use
with styx(2) or styxservers-nametree(2), for instance.
SEE ALSO
sys-intro(2), sys-dirread(2), sys-open(2)
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