INTRO(5) INTRO(5)
NAME
intro - introduction to the Plan 9 File Protocol 9P in
Inferno
SYNOPSIS
#include <lib9.h>
#include <styx.h>
DESCRIPTION
Inferno uses the Plan 9 protocol 9P to make resources visi-
ble as file hierarchies. There have been two versions of
9P, the most recent being 9P2000. Inferno originally used
its own protocol called Styx, which was a simplified version
of the first 9P protocol. When 9P was subsequently revised
as 9P2000, Inferno adopted that, but still using the name
Styx. To reduce confusion and to emphasise that Inferno and
Plan 9 use the same protocol, the name Styx is being
replaced by 9P. References to `Styx' remain in the programs
and documentation, but will eventually be eliminated.
An Inferno server is an agent that provides one or more
hierarchical file systems - file trees - that may be
accessed by Inferno processes. A server responds to
requests by clients to navigate the hierarchy, and to cre-
ate, remove, read, and write files. The prototypical server
is a separate machine that stores large numbers of user
files on permanent media. Another possibility for a server
is to synthesize files on demand, perhaps based on informa-
tion on data structures inside the kernel; the prog(3)
kernel device is a part of the Inferno kernel that does
this. User programs can also act as servers. One easy way
is to serve a set of files using the sys-file2chan(2) inter-
face. More complex Limbo file service applications can use
styx(2) to handle the protocol messages directly, or use
styxservers(2) to provide a higher-level framework for file
serving.
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 and mount calls; see intro(2) and sys-bind(2).
Processes in the group are then clients of the server: sys-
tem calls operating on files are translated into requests
and responses transmitted on the connection to the appropri-
ate 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
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INTRO(5) INTRO(5)
(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.
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 module
styx(2), and in the C include file <styx.h> (see
styx(10.2)). The next two bytes are an identifying tag,
described below. The remaining bytes are parameters of dif-
ferent 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 repre-
sents a variable-length parameter: n[2] followed 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 exam-
ple to remove slashes from name components, but the protocol
has no such restriction. Inferno names may contain any
printable character (that is, any character outside hexadec-
imal 00-1F and 80-9F) except slash.) Messages are trans-
ported in byte form to allow for machine independence;
styx(10.2) describes routines that convert to and from this
form into a machine-dependent C structure.
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]
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INTRO(5) INTRO(5)
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]
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 16rFFFF in
styx(2) and styx(10.2): the client can use it, when estab-
lishing 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.
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INTRO(5) INTRO(5)
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 sys-
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 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).
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-
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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
public), access and modification times, and owner and group
identifications (see sys-stat(2)). The owner and group iden-
tifications 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
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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)). 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 (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.
SEE ALSO
intro(2), styx(2), styxservers(2), sys-bind(2), sys-stat(2),
mnt(3), prog(3), read(5), stat(5), styx(10.2)
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