The function srv serves a 9P session by reading requests from
s–>infd, dispatching them to the function pointers kept in Srv,
and writing the responses to s–>outfd. (Typically, postmountsrv
or threadpostmountsrv initializes the infd and outfd structure
members. See the description below.)
Req and Fid structures are allocated one–to–one with uncompleted
requests and active fids, and are described in 9pfid(2).
The behavior of srv depends on whether there is a file tree (see
9pfile(2)) associated with the server, that is, whether the tree
element is nonzero. The differences are made explicit in the discussion
of the service loop below. The aux element is the client's, to
do with as it pleases.
Srv does not return until the 9P conversation is finished. Since
it is usually run in a separate process so that the caller can
exit, the service loop has little chance to return gracefully
on out of memory errors. It calls emalloc9p, erealloc9p, and estrdup9p
to obtain its memory. The default implementations of these
functions act as malloc, realloc, and strdup but abort the program
if they run out of memory. If alternate behavior is desired, clients
can link against alternate implementations of these functions.
Postmountsrv and threadpostmountsrv are wrappers that create a
separate process in which to run srv. They do the following:
If any error occurs during this process, the entire process is
terminated by calling sysfatal (see perror(2)).
If s–>nopipe is zero (the common case), initialize s–>infd and s–>outfd
to be one end of a freshly allocated pipe, with s–>srvfd initialized
as the other end.
If name is non–nil, call postfd(s–>srvfd, name) to post s–>srvfd as
Fork a child process via rfork (see fork(2)) or procrfork (see
thread(2)), using the RFFDG, RFNAMEG, and RFMEM flags. The child
process calls close(s–>srvfd) and then srv(s); it will exit once
If mtpt is non–nil, call amount(s–>srvfd, mtpt, flag, ""); otherwise,
The parent returns to the caller.
Listensrv and threadlistensrv create a separate process to announce
as addr. The process listens for incoming connections, creating
a new process to serve each. Using these functions results in
srv and the service functions being run in multiple processes
simultaneously. The library locks its own data structures as
necessary; the client may need to lock data it shares between
the multiple connections.
The functions in a Srv structure named after 9P transactions are
called to satisfy requests as they arrive. If a function is provided,
it must arrange for respond to be called when the request is satisfied.
The only parameter of each service function is a Req* parameter
(say r). The incoming request parameters are
stored in r–>ifcall; r–>fid and r–>newfid are pointers to Fid structures
corresponding to the numeric fids in r–>ifcall; similarly, r–>oldreq
is the Req structure corresponding to r–>ifcall.oldtag. The outgoing
response data should be stored in r–>ofcall. The one exception to
this rule is that stat should fill in
r–>d rather than r–>ofcall.stat: the library will convert the structure
into the machine–independent wire representation. Similarly, wstat
may consult r–>d rather than decoding r–>ifcall.stat itself. When
a request has been handled, respond should be called with r and
an error string. If the request was satisfied
successfully, the error string should be a nil pointer. Note that
it is permissible for a function to return without itself calling
respond, as long as it has arranged for respond to be called at
some point in the future by another proc sharing its address space,
but see the discussion of flush below. Once respond has been
called, the Req* as well as any pointers it once contained must
be considered freed and not referenced.
Responderror calls respond with the system error string (see errstr(2)).
If the service loop detects an error in a request (e.g., an attempt
to reuse an extant fid, an open of an already open fid, a read
from a fid opened for write, etc.) it will reply with an error
without consulting the service functions.
The service loop provided by srv (and indirectly by postmountsrv
and threadpostmountsrv) is single–threaded. If it is expected that
some requests might block, arranging for alternate processes to
handle them is suggested.
The constraints on the service functions are as follows. These
constraints are checked while the server executes. If a service
function fails to do something it ought to have, srv will call
endsrv and then abort.
AttachThe attach function should check the authentication state
of afid if desired, and set r–>fid–>qid and ofcall.qid to the qid
of the file system root. Attach may be nil only if file trees
are in use; in this case, the qid will be filled from the root
of the tree, and no authentication will be done.
Walk If file trees are in use, walk is handled internally, and
srv–>walk is never called.
Auth If authentication is desired, the auth function should record
that r–>afid is the new authentication fid and set r–>afid–>qid and
ofcall.qid. Auth may be nil, in which case it will be treated
as having responded with the error ``argv0: authentication not
required,'' where argv0 is the program name variable as set
If file trees are not in use, walk should consult r–>ifcall.wname
and r–>ifcall.nwname, filling in ofcall.qid and ofcall.nqid, and
also copying any necessary aux state from r–>fid to r–>newfid when
the two are different. As long as walk sets ofcall.nqid appropriately,
it can respond with a nil error string
even when 9P demands an error (e.g., in the case of a short walk);
the library detects error conditions and handles them appropriately.
Because implementing the full walk message is intricate and prone
to error, the helper routine walkandclone will handle the request
given pointers to two functions walk1 and (optionally) clone .
Clone, if non–nil, is called to signal the creation of newfid from
oldfid. Typically a clone routine will copy or increment
a reference count in oldfid's aux element. Walk1 should walk fid
to name, initializing fid–>qid to the new path's qid. Both should
return nil on success or an error message on error. Walkandclone
will call respond after handling the request.
Open If file trees are in use, the file metadata will be consulted
on open, create, remove, and wstat to see if the requester has
the appropriate permissions. If not, an error will be sent back
without consulting a service function.
If the client provides functions srv–>walk1 and (optionally) srv–>clone,
the 9P service loop will call walkandclone with these functions
to handle the request. Unlike the walk1 above, srv–>walk1 must fill
in both fid–>qid and *qid with the new qid on a successful walk.
CreateThe create function must fill in both r–>fid–>qid and r–>ofcall.qid
on success. When using file trees, create should allocate a new
File with createfile; note that createfile may return nil (because,
say, the file already exists). If the create function is nil,
srv behaves as though it were a function that always
If not using file trees or the user has the appropriate permissions,
open is called with r–>ofcall.qid already initialized to the one
stored in the Fid structure (that is, the one returned in the
previous walk). If the qid changes, both should be updated.
responded with the error ``create prohibited''.
Read The read function must be provided; it fills r–>ofcall.data
with at most r–>ifcall.count bytes of data from offset r–>ifcall.offset
of the file. It also sets r–>ofcall.count to the number of bytes
being returned. If using file trees, srv will handle reads of
directories internally, only calling read for requests on
should mark the file as removed, whether by calling removefile
when using file trees, or by updating an internal data structure.
In general it is not a good idea to clean up the aux information
associated with the corresponding File at this time, to avoid
memory errors if other fids have references to that
file. Instead, it is suggested that remove simply mark the file
as removed (so that further operations on it know to fail) and
wait until the file tree's destroy function is called to reclaim
the aux pointer. If not using file trees, it is prudent to take
the analogous measures. If remove is not provided, all remove
requests will draw ``remove prohibited'' errors.
WriteThe write function is similar but need not be provided. If
it is not, all writes will draw ``write prohibited'' errors. Otherwise,
write should attempt to write the r–>ifcall.count bytes of r–>ifcall.data
to offset r–>ifcall.offset of the file, setting r–>ofcall.count to
the number of bytes actually written. Most
files. Readstr and readbuf are useful for satisfying read requests
on a string or buffer. Consulting the request in r–>ifcall, they
fill r–>ofcall.data and set r–>ofcall.count; they do not call respond.
Similarly, dirread9p can be used to handle directory reads in
servers not using file trees. The passed gen
function will be called as necessary to fill dir with information
for the nth entry in the directory. The string pointers placed
in dir should be fresh copies made with estrdup9p; they will be
freed by dirread9p after each successful call to gen. Gen should
return zero if it successfully filled dir, minus one on end of
Stat Stat should fill r–>d with the stat information for r–>fid. If
using file trees, r–>d will have been initialized with the stat
info from the tree, and stat itself may be nil.
programs consider it an error to write less than the requested
WstatThe wstat consults r–>d in changing the metadata for r–>fid as
described in stat(5). When using file trees, srv will take care
to check that the request satisfies the permissions outlined in
stat(5). Otherwise wstat should take care to enforce permissions
Flush Servers that always call respond before returning from the
service functions need not provide a flush implementation: flush
is only necessary in programs that arrange for respond to be called
asynchronously. Flush should cause the request r–>oldreq to be cancelled
or hurried along. If oldreq is cancelled, this
Destroyfid, destroyreq, and end are auxiliary functions, not called
in direct response to 9P requests.
should be signalled by calling respond on oldreq with error string
`interrupted'. Flush must respond to r with a nil error string.
Flush may respond to r before forcing a response to r–>oldreq. In
this case, the library will delay sending the Rflush message until
the response to r–>oldreq has been sent.
When a Fid's reference count drops to zero (i.e., it has been
clunked and there are no outstanding requests referring to it),
destroyfid is called to allow the program to dispose of the fid–>aux
End Once the 9P service loop has finished (end of file been reached
on the service pipe or a bad message has been read), end is called
(if provided) to allow any final cleanup. For example, it was
used by the Palm Pilot synchronization file system (never finished)
to gracefully terminate the serial conversation once the
Similarly, when a Req's reference count drops to zero (i.e., it
has been handled via respond and other outstanding pointers to
it have been closed), destroyreq is called to allow the program
to dispose of the r–>aux pointer.
If the chatty9p flag is at least one, a transcript of the 9P session
is printed on standard error. If the chatty9p flag is greater
than one, additional unspecified debugging output is generated.
By convention, servers written using this library accept the –D
option to increment chatty9p.
file system had been unmounted. After calling end, the service
loop (which runs in a separate process from its caller) terminates
using _exits (see exits(2)).