INTRO(5) File Formats Manual 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 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 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 structures inside the ker-
nel; 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 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 transmitting (receiving) a request of a particular
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.
MESSAGES
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] auth[13]
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] data[count]
Rwrite tag[2] fid[2] count[2]
Tclunk tag[2] fid[2]
Rclunk tag[2] fid[2]
Tremove tag[2] fid[2]
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 override 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 initializes a connection and aborts all outstanding I/O on the connection. The set of messages between ses-
sion 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 some-
what 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 identi-
fies 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 mes-
sage 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. Usu-
ally, 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 differ-
ent tags. The server may delay the response to a request on one fid and respond to later requests on other fids; this is sometimes neces-
sary, for example when the client reads from a file that the server synthesizes 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 different. Directories always have the CHDIR bit (0x80000000) set in their qid path. The version is a version number for a file; typi-
cally, 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 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
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 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 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 read-
ing is allowed, whether writing is allowed, and whether executing is allowed. A walk in a directory is regarded as executing the direc-
tory, not reading it. Permissions 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).
INTRO(5)