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

     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).

     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)