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fstat, fwstat, stat, wstat - get and put file status


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);


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 information 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 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 members, 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 irrelevant 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 discussed 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).

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 appropriate 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.


sys-intro(2), sys-dirread(2), sys-open(2)

SYS-STAT(2 ) Rev:  Thu Feb 15 14:43:26 GMT 2007