HTML "Changes to the Programming Environment in the Fourth Release of Plan 9
FP lucidasans
TL
Changes to the Programming Environment
br
in the
br
Fourth Release of Plan 9
AU
Rob Pike
sp
[email protected]
SH
Introduction
PP
The fourth release of Plan 9 includes changes at many levels of the system,
with repercussions in the libraries and program interfaces.
This document summarizes the changes and describes how
existing programs must be modified to run in the new release.
It is not exhaustive, of course; for further detail about any of the
topics refer to the manual pages, as always.
PP
Programmers new to Plan 9 may find valuable tidbits here, but the
real audience for this paper is those with a need to update applications
and servers written in C for earlier releases of the Plan 9 operating system.
SH
9P, NAMELEN, and strings
PP
The underlying file service protocol for Plan 9, 9P, retains its basic form
but has had a number of adjustments to deal with longer file names and error strings,
new authentication mechanisms, and to make it more efficient at
evaluating file names.
The change to file names affects a number of system interfaces;
because file name elements are no longer of fixed size, they can
no longer be stored as arrays.
PP
9P used to be a fixed-format protocol with
CW NAMELEN -sized
byte arrays representing file name elements.
Now, it is a variable-format protocol, as described in
I intro (5),
in which strings are represented by a count followed by that many bytes.
Thus, the string
CW ken
would previously have occupied 28
CW NAMELEN ) (
bytes in the message; now it occupies 5: a two-byte count followed by the three bytes of
CW ken
and no terminal zero.
(And of course, a name could now be much longer.)
A similar format change has been made to
CW stat
buffers: they are no longer
CW DIRLEN
bytes long but instead have variable size prefixed by a two-byte count.
And in fact the entire 9P message syntax has changed: every message
now begins with a message length field that makes it trivial to break the
string into messages without parsing them, so
CW aux/fcall
is gone.
A new library entry point,
CW read9pmsg ,
makes it easy for user-level servers to break the client data stream into 9P messages.
All servers should switch from using
CW read
(or the now gone
CW getS)
to using
CW read9pmsg .
PP
This change to 9P affects the way strings are handled by the kernel and throughout
the system.
The consequences are primarily that fixed-size arrays have been replaced
by pointers and counts in a variety of system interfaces.
Most programs will need at least some adjustment to the new style.
In summary:
CW NAMELEN
is gone, except as a vestige in the authentication libraries, where it has been
rechristened
CW ANAMELEN .
CW DIRLEN
and
CW ERRLEN
are also gone.
All programs that mention
these constants
will need to be fixed.
PP
The simplest place to see this change is in the
CW errstr
system call, which no longer assumes a buffer of length
CW ERRLEN
but now requires a byte-count argument:
P1
char buf[...];

errstr(buf, sizeof buf);
P2
The buffer can be any size you like.
For convenience, the kernel stores error strings internally as 256-byte arrays,
so if you like \(em but it's not required \(em you can use the defined constant
CW ERRMAX= 256
as a good buffer size.
Unlike the old
CW ERRLEN
(which had value 64),
CW ERRMAX
is advisory, not mandatory, and is not part of the 9P specification.
PP
With names, stat buffers, and directories, there isn't even an echo of a fixed-size array any more.
SH
Directories and wait messages
PP
With strings now variable-length, a number of system calls needed to change:
CW errstr ,
CW stat ,
CW fstat ,
CW wstat ,
CW fwstat ,
and
CW wait
are all affected, as is
CW read
when applied to directories.
PP
As far as directories are concerned, most programs don't use the system calls
directly anyway, since they operate on the machine-independent form, but
instead call the machine-dependent
CW Dir
routines
CW dirstat ,
CW dirread ,
etc.
These used to fill user-provided fixed-size buffers; now they return objects allocated
by
CW malloc
(which must therefore be freed after use).
To `stat' a file:
P1
Dir *d;

d = dirstat(filename);
if(d == nil){
       fprint(2, "can't stat %s: %r\en", filename);
       exits("stat");
}
use(d);
free(d);
P2
A common new bug is to forget to free a
CW Dir
returned by
CW dirstat .
PP
CW Dirfstat
and
CW Dirfwstat
work pretty much as before, but changes to 9P make
it possible to exercise finer-grained control on what fields
of the
CW Dir
are to be changed; see
I stat (2)
and
I stat (5)
for details.
PP
Reading a directory works in a similar way to
CW dirstat ,
with
CW dirread
allocating and filling in an array of
CW Dir
structures.
The return value is the number of elements of the array.
The arguments to
CW dirread
now include a pointer to a
CW Dir*
to be filled in with the address of the allocated array:
P1
Dir *d;
int i, n;

while((n = dirread(fd, &d)) > 0){
       for(i=0; i<n; i++)
               use(&d[i]);
       free(d);
}
P2
A new library function,
CW dirreadall ,
has the same form as
CW dirread
but returns the entire directory in one call:
P1
n = dirreadall(fd, &d)
for(i=0; i<n; i++)
       use(&d[i]);
free(d);
P2
If your program insists on using the underlying
CW stat
system call or its relatives, or wants to operate directly on the
machine-independent format returned by
CW stat
or
CW read ,
it will need to be modified.
Such programs are rare enough that we'll not discuss them here beyond referring to
the man page
I stat (2)
for details.
Be aware, though, that it used to be possible to regard the buffer returned by
CW stat
as a byte array that began with the zero-terminated
name of the file; this is no longer true.
With very rare exceptions, programs that call
CW stat
would be better recast to use the
CW dir
routines or, if their goal is just to test the existence of a file,
CW access .
PP
Similar changes have affected the
CW wait
system call.  In fact,
CW wait
is no longer a system call but a library routine that calls the new
CW await
system call and returns a newly allocated machine-dependent
CW Waitmsg
structure:
P1
Waitmsg *w;

w = wait();
if(w == nil)
       error("wait: %r");
print("pid is %d; exit string %s\en", w->pid, w->msg);
free(w);
P2
The exit string
CW w->msg
may be empty but it will never be a nil pointer.
Again, don't forget to free the structure returned by
CW wait .
If all you need is the pid, you can call
CW waitpid ,
which reports just the pid and doesn't return an allocated structure:
P1
int pid;

pid = waitpid();
if(pid < 0)
       error("wait: %r");
print("pid is %d\en", pid);
P2
SH
Quoted strings and tokenize
PP
CW Wait
gives us a good opportunity to describe how the system copes with all this
free-format data.
Consider the text returned by the
CW await
system call, which includes a set of integers (pids and times) and a string (the exit status).
This information is formatted free-form; here is the statement in the kernel that
generates the message:
P1
n = snprint(a, n, "%d %lud %lud %lud %q",
       wq->w.pid,
       wq->w.time[TUser], wq->w.time[TSys], wq->w.time[TReal],
       wq->w.msg);
P2
Note the use of
CW %q
to produce a quoted-string representation of the exit status.
The
CW %q
format is like %s but will wrap
CW rc -style
single quotes around the string if it contains white space or is otherwise ambiguous.
The library routine
CW tokenize
can be used to parse data formatted this way: it splits white-space-separated
fields but understands the
CW %q
quoting conventions.
Here is how the
CW wait
library routine builds its
CW Waitmsg
from the data returned by
CW await :
P1
Waitmsg*
wait(void)
{
       int n, l;
       char buf[512], *fld[5];
       Waitmsg *w;

       n = await(buf, sizeof buf-1);
       if(n < 0)
               return nil;
       buf[n] = '\0';
       if(tokenize(buf, fld, nelem(fld)) != nelem(fld)){
               werrstr("couldn't parse wait message");
               return nil;
       }
       l = strlen(fld[4])+1;
       w = malloc(sizeof(Waitmsg)+l);
       if(w == nil)
               return nil;
       w->pid = atoi(fld[0]);
       w->time[0] = atoi(fld[1]);
       w->time[1] = atoi(fld[2]);
       w->time[2] = atoi(fld[3]);
       w->msg = (char*)&w[1];
       memmove(w->msg, fld[4], l);
       return w;
}
P2
PP
This style of quoted-string and
CW tokenize
is used all through the system now.
In particular, devices now
CW tokenize
the messages written to their
CW ctl
files, which means that you can send messages that contain white space, by quoting them,
and that you no longer need to worry about whether or not the device accepts a newline.
In other words, you can say
P1
echo message > /dev/xx/ctl
P2
instead of
CW echo
CW -n
because
CW tokenize
treats the newline character as white space and discards it.
PP
While we're on the subject of quotes and strings, note that the implementation of
CW await
used
CW snprint
rather than
CW sprint .
We now deprecate
CW sprint
because it has no protection against buffer overflow.
We prefer
CW snprint
or
CW seprint ,
to constrain the output.
The
CW %q
format is cleverer than most in this regard:
if the string is too long to be represented in full,
CW %q
is smart enough to produce a truncated but correctly quoted
string within the available space.
SH
Mount
PP
Although strings in 9P are now variable-length and not zero-terminated,
this has little direct effect in most of the system interfaces.
File and user names are still zero-terminated strings as always;
the kernel does the work of translating them as necessary for
transport.
And of course, they are now free to be as long as you might want;
the only hard limit is that their length must be represented in 16 bits.
PP
One example where this matters is that the file system specification in the
CW mount
system call can now be much longer.
Programs like
CW rio
that used the specification string in creative ways were limited by the
CW NAMELEN
restriction; now they can use the string more freely.
CW Rio
now accepts a simple but less cryptic specification language for the window
to be created by the
CW mount
call, e.g.:
P1
% mount $wsys /mnt/wsys 'new -dx 250 -dy 250 -pid 1234'
P2
In the old system, this sort of control was impossible through the
CW mount
interface.
PP
While we're on the subject of
CW mount ,
note that with the new security architecture
(see
I factotum (4)),
9P has moved its authentication outside the protocol proper.
(For a full description of this change to 9P, see
I fauth (2),
I attach (5),
and the paper
I "Security in Plan 9\f1.)
The most explicit effect of this change is that
CW mount
now takes another argument,
CW afd ,
a file descriptor for the
authentication file through which the authentication will be made.
For most user-level file servers, which do not require authentication, it is
sufficient to provide
CW -1
as the value of
CW afd:
P1
if(mount(fd, -1, "/mnt/wsys", MREPL,
  "new -dx 250 -dy 250 -pid 1234") < 0)
       error("mount failed: %r");
P2
To connect to servers that require authentication, use the new
CW fauth
system call or the reimplemented
CW amount
(authenticated mount) library call.
In fact, since
CW amount
handles both authenticating and non-authenticating servers, it is often
easiest just to replace calls to
CW mount
by calls to
CW amount ;
see
I auth (2)
for details.
SH
Print
PP
The C library has been heavily reworked in places.
Besides the changes mentioned above, it
now has a much more complete set of routines for handling
CW Rune
strings (that is, zero-terminated arrays of 16-bit character values).
The most sweeping changes, however, are in the way formatted I/O is performed.
PP
The
CW print
routine and all its relatives have been reimplemented to offer a number
of improvements:
IP (1)
Better buffer management, including the provision of an internal flush
routine, makes it unnecessary to provide large buffers.
For example,
CW print
uses a much smaller buffer now (reducing stack load) while simultaneously
removing the need to truncate the output string if it doesn't fit in the buffer.
IP (2)
Global variables have been eliminated so no locking is necessary.
IP (3)
The combination of (1) and (2) means that the standard implementation of
CW print
now works fine in threaded programs, and
CW threadprint
is gone.
IP (4)
The new routine
CW smprint
prints into, and returns, storage allocated on demand by
CW malloc .
IP (5)
It is now possible to print into a
CW Rune
string; for instance,
CW runesmprint
is the
CW Rune
analog of
CW smprint .
IP (6)
There is improved support for custom
print verbs and custom output routines such as error handlers.
The routine
CW doprint
is gone, but
CW vseprint
can always be used instead.
However, the new routines
CW fmtfdinit ,
CW fmtstrinit ,
CW fmtprint ,
and friends
are often a better replacement.
The details are too long for exposition here;
I fmtinstall (2)
explains the new interface and provides examples.
IP (7)
Two new format flags, space and comma, close somewhat the gap between
Plan 9 and ANSI C.
PP
Despite these changes, most programs will be unaffected;
CW print
is still
CW print .
Don't forget, though, that
you should eliminate calls to
CW sprint
and use the
CW %q
format when appropriate.
SH
Binary compatibility
PP
The discussion so far has been about changes at the source level.
Existing binaries will probably run without change in the new
environment, since the kernel provides backward-compatible
system calls for
CW errstr ,
CW stat ,
CW wait ,
etc.
The only exceptions are programs that do either a
CW mount
system call, because of the security changes and because
the file descriptor in
CW mount
must point to a new 9P connection; or a
CW read
system call on a directory, since the returned data will
be in the new format.
A moment's reflection will discover that this means old
user-level file servers will need to be fixed to run on the new system.
SH
File servers
PP
A full description of what user-level servers must do to provide service with
the new 9P is beyond the scope of this paper.
Your best source of information is section 5 of the manual,
combined with study of a few examples.
CW /sys/src/cmd/ramfs.c
is a simple example; it has a counterpart
CW /sys/src/lib9p/ramfs.c
that implements the same service using the new
I 9p (2)
library.
PP
That said, it's worth summarizing what to watch for when converting a file server.
The
CW session
message is gone, and there is a now a
CW version
message that is exchanged at the start of a connection to establish
the version of the protocol to use (there's only one at the moment, identified by
the string
CW 9P2000 )
and what the maximum message size will be.
This negotiation makes it easier to handle 9P encapsulation, such as with
CW exportfs ,
and also permits larger message sizes when appropriate.
PP
If your server wants to authenticate, it will need to implement an authentication file
and implement the
CW auth
message; otherwise it should return a helpful error string to the
CW Tauth
request to signal that authentication is not required.
PP
The handling of
CW stat
and directory reads will require some changes but they should not be fundamental.
Be aware that seeking on directories is forbidden, so it is fine if you disregard the
file offset when implementing directory reads; this makes it a little easier to handle
the variable-length entries.
You should still never return a partial directory entry; if the I/O count is too small
to return even one entry, you should return two bytes containing the byte count
required to represent the next entry in the directory.
User code can use this value to formulate a retry if it desires.
See the
DIAGNOSTICS section of
I stat (2)
for a description of this process.
PP
The trickiest part of updating a file server is that the
CW clone
and
CW walk
messages have been merged into a single message, a sort of `clone-multiwalk'.
The new message, still called
CW walk ,
proposes a sequence of file name elements to be evaluated using a possibly
cloned fid.
The return message contains the qids of the files reached by
walking to the sequential elements.
If all the elements can be walked, the fid will be cloned if requested.
If a non-zero number of elements are requested, but none
can be walked, an error should be returned.
If only some can be walked, the fid is not cloned, the original fid is left
where it was, and the returned
CW Rwalk
message should contain the partial list of successfully reached qids.
See
I walk (5)
for a full description.

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