srv – server registry

bind #s /srv

The srv device provides a one–level directory holding already–open channels to services. In effect, srv is a bulletin board on which processes may post open file descriptors to make them available to other processes within their srvgroup.

To install a channel, create a new file such as /srv/myserv and then write a text string (suitable for strtoul; see atof(2)) giving the file descriptor number of an open file. Any process in the same srvgroup may then open /srv/myserv to acquire another reference to the open file that was registered.

An entry in srv holds a reference to the associated file even if no process has the file open. Removing the file from /srv releases that reference.

It is an error to write more than one number into a server file, or to create a file with a name that is already being used.

When a fork(2) system call creates a new process, both the parent and the child continue to see exactly the same files in the srv device: changes made in either process can be noticed by the other. In contrast, an rfork system call with the RFCSRVG bit set (see fork(2)) causes a split: the new process sees a new srv device with no file descriptors posted, and any services added in one process group cannot be noticed by the other.

To drop one end of a pipe into /srv, that is, to create a named pipe:
int fd, p[2];
char buf[32];
fd = create("/srv/namedpipe", OWRITE, 0666);
fprint(fd, "%d", p[0]);
fprint(p[1], "hello");

At this point, any process may open and read /srv/namedpipe to receive the hello string. Data written to /srv/namedpipe can be received by executing
read(p[1], buf, sizeof buf);

in the above process.