intro - introduction to the Plan 9 File Protocol, 9P
A Plan 9 server is an agent that provides one or more hierarchical 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 pro-
totypical server is a separate machine that stores large numbers of user files on perma-
nent media; such a machine is called, somewhat confusingly, a file server. Another possi-
bility for a server is to synthesize 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 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-mes-
sages) to the client. The combined acts of transmitting (receiving) a request of a par-
ticular type, and receiving (transmitting) its reply is called a transaction of that type.
Each message consists of a sequence of bytes. The first byte is the message type, one of
the constants in the enumeration 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 significant 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. (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 restriction. 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.
Tsession tag chal
Rsession tag chal authid authdom
Rerror tag ename
Tflush tag oldtag
Tattach tag fid uid aname ticket auth
Rattach tag fid qid rauth
Tclone tag fid newfid
Rclone tag fid
Tclwalk tag fid newfid name
Rclwalk tag fid qid
Twalk tag fid name
Rwalk tag fid qid
Topen tag fid mode
Ropen tag fid qid
Tcreate tag fid name perm mode
Rcreate tag fid qid
Tread tag fid offset count
Rread tag fid count pad data[count]
Twrite tag fid offset count pad data[count]
Rwrite tag fid count
Tclunk tag fid
Rclunk tag fid
Tremove tag fid
Rremove tag fid
Tstat tag fid
Rstat tag fid stat
Twstat tag fid stat
Rwstat tag fid
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 out-
standing 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 con-
nection between a client and a server. It is never necessary. A session request initial-
izes 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
operating 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 spec-
ify 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 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 synthe-
sizes from external events such as keyboard characters.
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 directory, the old and new path components of the qids should be differ-
ent. Directories always have the CHDIR bit (0x80000000) set in their qid path. The ver-
sion is a version number for a file; typically, it is incremented every time the file is
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
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 identifications 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.
Directories are created by create with CHDIR set in the permissions 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 directory, although by conven-
tion 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 origi-
nal 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 permission, 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. Permissions are
kept in the low-order bits of the file mode: owner read/write/execute permission repre-
sented 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).