INTRO(5) INTRO(5)
NAME
intro - introduction to the Plan 9 File Protocol, 9P
SYNOPSIS
#include <fcall.h>
DESCRIPTION
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 file
server. Another possibility for a server is to synthesize
files on demand, perhaps based on information on data struc-
tures 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 server: system calls oper-
ating on files are translated into requests and responses
transmitted on the connection to the appropriate service.
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
type.
Each message consists of a sequence of bytes. Two-, four-,
and eight-byte fields hold unsigned integers represented in
little-endian order (least significant byte first). Data
items of larger or variable lengths are represented by a
two-byte field specifying a count, n, followed by n bytes of
data. Text strings are represented this way, with the text
itself stored as a UTF-8 encoded sequence of Unicode charac-
ters (see utf(6)). Text strings in 9P messages are not NUL-
terminated: n counts the bytes of UTF-8 data, which include
no final zero byte. The NUL character is illegal in all
text strings in 9P, and is therefore excluded from file
names, user names, and so on.
Page 1 Plan 9 (printed 12/30/24)
INTRO(5) INTRO(5)
Each 9P message begins with a four-byte size field specify-
ing the length in bytes of the complete message including
the four bytes of the size field itself. The next byte is
the message type, one of the constants in the enumeration in
the include file <fcall.h>. The next two bytes are an iden-
tifying tag, described below. The remaining bytes are
parameters of different sizes. In the message descriptions,
the number of bytes in a field is given in brackets after
the field name. The notation parameter[n] where n is not a
constant represents a variable-length parameter: n[2] fol-
lowed by n bytes of data forming the parameter. The notation
string[s] (using a literal s character) is shorthand for
s[2] followed by s bytes of UTF-8 text. (Systems may choose
to reduce the set of legal characters to reduce syntactic
problems, for example to remove slashes from name compo-
nents, but the protocol has no such restriction. Plan 9
names may contain any printable character (that is, any
character outside hexadecimal 00-1F and 80-9F) except
slash.) Messages are transported in byte form to allow for
machine independence; fcall(2) describes routines that con-
vert to and from this form into a machine-dependent C struc-
ture.
MESSAGES
size[4] Tversion tag[2] msize[4] version[s]
size[4] Rversion tag[2] msize[4] version[s]
size[4] Tauth tag[2] afid[4] uname[s] aname[s]
size[4] Rauth tag[2] aqid[13]
size[4] Rerror tag[2] ename[s]
size[4] Tflush tag[2] oldtag[2]
size[4] Rflush tag[2]
size[4] Tattach tag[2] fid[4] afid[4] uname[s] aname[s]
size[4] Rattach tag[2] qid[13]
size[4] Twalk tag[2] fid[4] newfid[4] nwname[2]
nwname*(wname[s])
size[4] Rwalk tag[2] nwqid[2] nwqid*(wqid[13])
size[4] Topen tag[2] fid[4] mode[1]
size[4] Ropen tag[2] qid[13] iounit[4]
size[4] Tcreate tag[2] fid[4] name[s] perm[4] mode[1]
size[4] Rcreate tag[2] qid[13] iounit[4]
size[4] Tread tag[2] fid[4] offset[8] count[4]
size[4] Rread tag[2] count[4] data[count]
Page 2 Plan 9 (printed 12/30/24)
INTRO(5) INTRO(5)
size[4] Twrite tag[2] fid[4] offset[8] count[4]
data[count]
size[4] Rwrite tag[2] count[4]
size[4] Tclunk tag[2] fid[4]
size[4] Rclunk tag[2]
size[4] Tremove tag[2] fid[4]
size[4] Rremove tag[2]
size[4] Tstat tag[2] fid[4]
size[4] Rstat tag[2] stat[n]
size[4] Twstat tag[2] fid[4] stat[n]
size[4] Rwstat tag[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 NOTAG, defined as (ushort)~0
in <fcall.h>: the client can use it, when establishing a
connection, to override tag matching in version 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 version message identifies the version of the protocol
and indicates the maximum message size the system is pre-
pared to handle. It also initializes the connection and
aborts all outstanding I/O on the connection. The set of
messages between version requests is called a session.
Most T-messages contain a fid, a 32-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 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).
Page 3 Plan 9 (printed 12/30/24)
INTRO(5) INTRO(5)
Permission to attach to the service is proven by providing a
special fid, called afid, in the attach message. This afid
is established by exchanging auth messages and subsequently
manipulated using read and write messages to exchange
authentication information not defined explicitly by 9P.
Once the authentication protocol is complete, the afid is
presented in the attach to permit the user to access the
service.
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, or one of its subdirectories. Walk
returns a new fid that refers to the resulting file. Usu-
ally, a client maintains a fid for the root, and navigates
by walks from the root fid.
A client can send multiple T-messages without waiting for
the corresponding R-messages, but all outstanding T-messages
must specify different tags. The server may delay the
response to a request and respond to later ones; this is
sometimes necessary, for example when the client reads from
a file that the server synthesizes from external events such
as keyboard characters.
Replies (R-messages) to auth, 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 thirteen-byte
qid fields hold a one-byte type, specifying whether the file
is a directory, append-only file, etc., and two unsigned
integers: first the four-byte qid version, then the eight-
byte qid path. 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 directory, the old and new path
components of the qids should be different. The version is
a version 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 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
Page 4 Plan 9 (printed 12/30/24)
INTRO(5) INTRO(5)
public), access and modification times, and owner and group
identifications (see stat(2)). The owner and group identifi-
cations are textual names. The wstat transaction allows
some of a file's properties to be changed.
A request can be aborted with a flush 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 must wait
until it gets the Rflush (even if the reply to the original
message arrives in the interim), at which point oldtag may
be reused.
Because the message size is negotiable and some elements of
the protocol are variable length, it is possible (although
unlikely) to have a situation where a valid message is too
large to fit within the negotiated size. For example, a
very long file name may cause a Rstat of the file or Rread
of its directory entry to be too large to send. In most
such cases, the server should generate an error rather than
modify the data to fit, such as by truncating the file name.
The exception is that a long error string in an Rerror mes-
sage should be truncated if necessary, since the string is
only advisory and in some sense arbitrary.
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.
DIRECTORIES
Directories are created by create with DMDIR 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.
ACCESS PERMISSIONS
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
Page 5 Plan 9 (printed 12/30/24)
INTRO(5) INTRO(5)
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 (DMDIR) is set, the file is a
directory; if bit 30 (DMAPPEND) is set, the file is append-
only (offset is ignored in writes); if bit 29 (DMEXCL) is
set, the file is exclusive-use (only one client may have it
open at a time); if bit 27 (DMAUTH) is set, the file is an
authentication file established by auth messages; if bit 26
(DMTMP) is set, the contents of the file (or directory) are
not included in nightly archives. (Bit 28 is skipped for
historical reasons.) These bits are reproduced, from the
top bit down, in the type byte of the Qid: QTDIR, QTAPPEND,
QTEXCL, (skipping one bit) QTAUTH, and QTTMP. The name
QTFILE, defined to be zero, identifies the value of the type
for a plain file.
Page 6 Plan 9 (printed 12/30/24)