INTRO(5) INTRO(5)
NAME
intro - introduction to the Inferno File Protocol, Styx
SYNOPSIS
#include <lib9.h>
#include <styx.h>
DESCRIPTION
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 device
prog(3) 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) interface.
More complex Limbo file service applications can use
styxlib(2) to handle the protocol messages directly.
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 or mount calls; see sys-bind(2) and intro(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 Inferno File Protocol, Styx, 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. The first
byte is the message type, one of the constants in the enu-
meration in the C include file <styx.h> (see styx(10.2)).
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-
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INTRO(5) INTRO(5)
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 pro-
tocol has no such restriction. Inferno names may contain
any printable character (that is, any character outside hex-
adecimal 00-1F and 80-9F) except slash and blank. Messages
are transported in byte form to allow for machine indepen-
dence; styx(10.2) describes C routines that convert to and
from this form into a machine-dependent C structure.
MESSAGES
Tnop tag[2]
Rnop tag[2]
Rerror tag[2] ename[64]
Tflush tag[2] oldtag[2]
Rflush tag[2]
Tattach tag[2] fid[2] uid[28] aname[28]
Rattach tag[2] fid[2] qid[8]
Tclone tag[2] fid[2] newfid[2]
Rclone tag[2] fid[2]
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 NOTAG, value 16rFFFF, meaning
`no tag': the client can use it, when establishing a connec-
tion, to override tag matching in messages.
The type of an R-message will either be one greater than the
type of the corresponding T-message or Rerror indicating
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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.
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 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 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, which must be
a directory, to be a named file in that directory. 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
synthesizes from external events such as keyboard charac-
ters.
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
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INTRO(5) INTRO(5)
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 (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 sys-stat(2)). The owner and group iden-
tifications 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 Styx protocol directly; instead
calls to library routines that access files are translated
by the mount driver, mnt(3), into Styx messages.
DIRECTORIES
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.
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
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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 permis-
sions 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 read-
ing 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 an additional attribute besides the
permissions: if bit 31 is set, the file is a directory.
SEE ALSO
intro(2), styxlib(2), sys-bind(2), sys-stat(2), mnt(3),
prog(3), read(5), stat(5), styx(10.2)
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