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     INTRO(5)                                                 INTRO(5)

          intro - introduction to the Plan 9 File Protocol, 9P

          #include <fcall.h>

          A Plan 9 server is an agent that provides one or more hier-
          archical file systems - file trees - that may be accessed by
          Plan 9 processes.  A server responds to requests by clients
          to navigate the hierarchy, and to create, remove, read, and
          write files.  The prototypical server is a separate machine
          that stores large numbers of user files on permanent media;
          such a machine is called, somewhat confusingly, a
          fileserver.  Another possibility for a server is to synthe-
          size files on demand, perhaps based on information on data
          structures inside the kernel; the proc(3) kernel device is a
          part of the Plan 9 kernel that does this.  User programs can
          also act as servers.

          A connection to a server is a bidirectional communication
          path from the client to the server.  There may be a single
          client or multiple clients sharing the same connection.  A
          server's file tree is attached to a process group's name
          space by bind(2) and mount calls; see intro(2). Processes in
          the group are then clients of the servers: system calls
          operating on files are translated into requests and
          responses transmitted on the connection to the appropriate

          The Plan 9 File Protocol, 9P, is used for messages between
          clients and servers. A client transmits requests (T-
          messages) to a server, which subsequently returns replies
          (R-messages) to the client.  The combined acts of transmit-
          ting (receiving) a request of a particular type, and receiv-
          ing (transmitting) its reply is called a transaction of that

          Each message consists of a sequence of bytes.  The first
          byte is the message type, one of the constants in the enu-
          meration in the include file <fcall.h>.  The remaining bytes
          are parameters.  Each parameter consists of a fixed number
          of bytes (except the data fields of write requests or read
          replies); in the message descriptions below, the number of
          bytes in a field is given in brackets after the field name.
          The two-, four-, and eight-byte fields may hold unsigned
          integers represented in little-endian order (least signifi-
          cant byte first).  Fields that contain names are 28-byte
          strings (including a terminal NUL (zero) byte).  Other than
          the NUL terminator, all characters are legal in file names.

     INTRO(5)                                                 INTRO(5)

          (Systems may choose to reduce the set of legal characters to
          reduce syntactic problems, for example to remove slashes
          from name components, but the protocol has no such restric-
          tion.  Plan 9 names may contain any printable character
          (that is, any character outside hexadecimal 00-1F and 80-9F)
          except slash and blank.)  Messages are transported in byte
          form to allow for machine independence; fcall(2) describes
          routines that convert to and from this form into a machine-
          dependent C structure.

               Tnop      tag[2]
               Rnop      tag[2]

               Tsession  tag[2] chal[8]
               Rsession  tag[2] chal[8] authid[28] authdom[48]

               Rerror    tag[2] ename[64]

               Tflush    tag[2] oldtag[2]
               Rflush    tag[2]

               Tattach   tag[2] fid[2] uid[28] aname[28] ticket[72]
               Rattach   tag[2] fid[2] qid[8] rauth[13]

               Tclone    tag[2] fid[2] newfid[2]
               Rclone    tag[2] fid[2]

               Tclwalk   tag[2] fid[2] newfid[2] name[28]
               Rclwalk   tag[2] fid[2] qid[8]

               Twalk     tag[2] fid[2] name[28]
               Rwalk     tag[2] fid[2] qid[8]

               Topen     tag[2] fid[2] mode[1]
               Ropen     tag[2] fid[2] qid[8]

               Tcreate   tag[2] fid[2] name[28] perm[4] mode[1]
               Rcreate   tag[2] fid[2] qid[8]

               Tread     tag[2] fid[2] offset[8] count[2]
               Rread     tag[2] fid[2] count[2] pad[1] data[count]

               Twrite    tag[2] fid[2] offset[8] count[2] pad[1]
               Rwrite    tag[2] fid[2] count[2]

               Tclunk    tag[2] fid[2]
               Rclunk    tag[2] fid[2]

               Tremove   tag[2] fid[2]

     INTRO(5)                                                 INTRO(5)

               Rremove   tag[2] fid[2]

               Tstat     tag[2] fid[2]
               Rstat     tag[2] fid[2] stat[116]

               Twstat    tag[2] fid[2] stat[116]
               Rwstat    tag[2] fid[2]

          Each T-message has a tag field, chosen and used by the
          client to identify the message.  The reply to the message
          will have the same tag.  Clients must arrange that no two
          outstanding messages on the same connection have the same
          tag.  An exception is the tag 0xFFFF, meaning `no tag': the
          client can use it, when establishing a connection, to over-
          ride tag matching in nop and session messages.

          The type of an R-message will either be one greater than the
          type of the corresponding T-message or Rerror, indicating
          that the request failed.  In the latter case, the ename
          field contains a string describing the reason for failure.

          The nop message request has no obvious effect.  Its main
          purpose is in debugging the connection between a client and
          a server.  It is never necessary.  A session request ini-
          tializes a connection and aborts all outstanding I/O on the
          connection.  The set of messages between session requests is
          called a session.

          Most T-messages contain a fid, a 16-bit unsigned integer
          that the client uses to identify a ``current file'' on the
          server.  Fids are somewhat like file descriptors in a user
          process, but they are not restricted to files open for I/O:
          directories being examined, files being accessed by stat(2)
          calls, and so on - all files being manipulated by the oper-
          ating system - are identified by fids.  Fids are chosen by
          the client.  All requests on a connection share the same fid
          space; when several clients share a connection, the agent
          managing the sharing must arrange that no two clients choose
          the same fid.

          The first fid supplied (in an attach message) will be taken
          by the server to refer to the root of the served file tree.
          The attach identifies the user to the server and may specify
          a particular file tree served by the server (for those that
          supply more than one).  A walk message causes the server to
          change the current file associated with a fid to be a file
          in the directory that is the old current file.  Usually, a
          client maintains a fid for the root, and navigates by walks
          on a fid cloned from the root fid.

          A client can send multiple T-messages without waiting for
          the corresponding R-messages, but all outstanding T-messages

     INTRO(5)                                                 INTRO(5)

          must specify different tags.  The server may delay the
          response to a request on one fid and respond to later
          requests on other fids; this is sometimes necessary, for
          example when the client reads from a file that the server
          synthesizes from external events such as keyboard charac-

          Replies (R-messages) to attach, walk, open, and create
          requests convey a qid field back to the client.  The qid
          represents the server's unique identification for the file
          being accessed: two files on the same server hierarchy are
          the same if and only if their qids are the same.  (The
          client may have multiple fids pointing to a single file on a
          server and hence having a single qid.)  The eight-byte qid
          fields represent two four-byte unsigned integers: first the
          qid path, then the qid version. The path is an integer
          unique among all files in the hierarchy.  If a file is
          deleted and recreated with the same name in the same direc-
          tory, the old and new path components of the qids should be
          different.  Directories always have the CHDIR bit
          (0x80000000) set in their qid path.  The version is a ver-
          sion number for a file; typically, it is incremented every
          time the file is modified.

          An existing file can be opened, or a new file may be created
          in the current (directory) file.  I/O of a given number of
          bytes (limited to 8192) at a given offset on an open file is
          done by read and write.

          A client should clunk any fid that is no longer needed.  The
          remove transaction deletes files.

          The stat transaction retrieves information about the file.
          The stat field in the reply includes the file's name, access
          permissions (read, write and execute for owner, group and
          public), access and modification times, and owner and group
          identifications (see stat(2)). The owner and group identifi-
          cations are 28-byte names.  The wstat transaction allows
          some of a file's properties to be changed.

          A request can be aborted with a Tflush request.  When a
          server receives a Tflush, it should not reply to the message
          with tag oldtag (unless it has already replied), and it
          should immediately send an Rflush.  The client should ignore
          replies with tag oldtag until it gets the Rflush, at which
          point oldtag may be reused.

          Most programs do not see the 9P protocol directly; instead
          calls to library routines that access files are translated
          by the mount driver, mnt(3), into 9P messages.


     INTRO(5)                                                 INTRO(5)

          Directories are created by create with CHDIR set in the per-
          missions argument (see stat(5)). The members of a directory
          can be found with read(5). All directories must support
          walks to the directory .. (dot-dot) meaning parent direc-
          tory, although by convention directories contain no explicit
          entry for .. or . (dot).  The parent of the root directory
          of a server's tree is itself.

          Each file server maintains a set of user and group names.
          Each user can be a member of any number of groups.  Each
          group has a group leader who has special privileges (see
          stat(5) and users(6)). Every file request has an implicit
          user id (copied from the original attach) and an implicit
          set of groups (every group of which the user is a member).

          Each file has an associated owner and group id and three
          sets of permissions: those of the owner, those of the group,
          and those of ``other'' users.  When the owner attempts to do
          something to a file, the owner, group, and other permissions
          are consulted, and if any of them grant the requested per-
          mission, the operation is allowed.  For someone who is not
          the owner, but is a member of the file's group, the group
          and other permissions are consulted.  For everyone else, the
          other permissions are used.  Each set of permissions says
          whether reading is allowed, whether writing is allowed, and
          whether executing is allowed.  A walk in a directory is
          regarded as executing the directory, not reading it.  Per-
          missions are kept in the low-order bits of the file mode:
          owner read/write/execute permission represented as 1 in bits
          8, 7, and 6 respectively (using 0 to number the low order).
          The group permissions are in bits 5, 4, and 3, and the other
          permissions are in bits 2, 1, and 0.

          The file mode contains some additional attributes besides
          the permissions.  If bit 31 is set, the file is a directory;
          if bit 30 is set, the file is append-only (offset is ignored
          in writes); if bit 29 is set, the file is exclusive-use
          (only one client may have it open at a time).