ref: 6bb619c8db2867ddd9cd19c0aec05065f5ee0cae
dir: /man/2/sys-file2chan/
.TH SYS-FILE2CHAN 2
.SH NAME
file2chan \- create file connected to Limbo channel
.SH SYNOPSIS
.EX
include "sys.m";
sys := load Sys Sys->PATH;
Rread: type chan of (array of byte, string);
Rwrite: type chan of (int, string);
FileIO: adt
{
read: chan of (big, int, int, Rread);
write: chan of (big, array of byte, int, Rwrite);
};
file2chan: fn(dir, file: string): ref FileIO;
.EE
.fi
.SH DESCRIPTION
.B File2chan
presents an interface for binding Limbo channels to files in the file name space.
A
.I server
program calls
.B file2chan
to create a
.I file
in a directory
.IR dir ,
which must be a directory on which device
.RB ` #s '
has been bound
(see
.IR srv (3)).
A
.I client
program can open the file for reading or writing (see
.IR sys-open (2)
and
.IR sys-read (2))
to communicate with the server.
.PP
.B File2chan
returns a
.B FileIO
type holding two channels used to deliver tuples representing
the contents of the
.B Tread
and
.B Twrite
9P messages received by the system on the server's behalf; see
.IR intro (5).
.PP
When the client invokes the
.B read
system call on the file, the server
receives a tuple, say
.BI ( offset\fP,\fP\ count\fP,\fP\ fid\fP,\fP\ rc )\f1,
on the
.B read
channel.
The request asks for
.I count
bytes of data, starting at
.I offset
bytes from the beginning of the file
associated with
.IR fid .
The server sends its reply to the client by transmitting a tuple, say
.BI ( data\fP,\fP\ error )\f1,
that contains the
.I data
for the
.BR read ,
on the channel
.I rc
received as part of the
.B read
tuple.
If the request was successful, the
.I error
string should be
.BR nil .
If an error occurred,
.I error
should be a diagnostic string,
and the
.I data
array should be
.BR nil .
The client blocks in the
.B read
system call until the server sends its reply.
The client receives only
.I count
bytes even if
.I data
is larger.
.PP
When the client does a
.B write
system call on the file,
the server receives a tuple,
.BI ( offset\fP,\fP\ data\fP,\fP\ fid\fP,\fP\ wc )\f1,
on the
.B write
channel.
A
.BI ( count\fP,\fP\ error )
response is sent back on the
.I wc
channel received in the
.B write
tuple:
.I count
gives the number of bytes written (zero if an error occurs), and
.I error
is an empty or
.B nil
string or an explanation of the problem.
.PP
The
.I fid
received by the server can be used to manage the multiplexing of multiple
active clients sharing a single file; see
.IR intro (5)
for details.
Each distinct
open of the file will yield a distinct
.IR fid ,
unique to that open, which will appear in the read and write
messages corresponding to reads and writes on that open instance of the file.
.PP
When the client closes the file,
the server will be sent a
.B read
message with a nil
.IR rc ,
then a
.B write
message with a nil
.IR wc .
A server typically ignores the former and uses the latter as a signal to stop processing for the given client
(more precisely, the given
.IR fid ).
Note that the
.I fid
can later be re-used by another client.
.SH SOURCE
.B /emu/port/devsrv.c
.br
.B /os/port/devsrv.c
.SH SEE ALSO
.IR sh-file2chan (1),
.IR sys-intro (2),
.IR sys-open (2),
.IR sys-read (2),
.IR sys-bind (2),
.IR intro (5)
.SH BUGS
.B Read
and
.B write
system calls for the file will not return until the
server sends its reply on the appropriate channel,
so the process doing the
.B read
or
.B write
cannot be the one serving.