[systemd-devel] sd_listen_fds: find file descriptor

[Lennart Poettering]

On Thu, 15.12.11 13:53, Michael D. Berger (m.d.berger at ieee.org) wrote:

> 
> [...]
> > 
> > Hmm? You should usually consume all fds you get passed. And 
> > if you cannot handle more than one, then exit quickly with an 
> > error message. And if any of the fds is invalid, then print 
> > an error message and leave, too.
> > 
> > This is what I'd usually use:
> > 
> > 
> > int fd, n;
> > 
> > n = sd_listen_fds();
> > if (n < 0)  {
> >         fprintf(stderr, "sd_listen_fds(): %s\n", strerror(-n));
> >         exit(1);
> > }
> > 
> > for (fd = SD_LISTEN_FDS_START; fd < SD_LISTEN_FDS_START + n; fd ++) {
> >         if (sd_is_socket_inet(fd, ...) <= 0) {
> >                 fprintf(stderr, "Invalid socket passed.\n");
> >                 exit(1);
> >         }
> > 
> >         /* do something with the fd... */ }
> > 
> > (This is for the generic case where the service can handle 
> > more than one fd. If you can handle ony one, then check 
> > whether n == 1 and if it isn't exit saying you can't deal 
> > with more than one fd.)
> > 
> > Lennart
> > 
> 
> What you say is clear, but I am perplexed by the fact that
> in a case where there will be one handle, that handle will
> always be 3.  How do we know that 3 is not being used by
> something else?  I thought that the system has to choose
> a handle that is available.

As Zbigniew already mentioned systemd as the process which spawns yours
has full control on which fds to pass you. It will guarantee that 0, 1,
2 are connected to a suitable stdin, stdout, stderr (/dev/null,
resp. syslog by default these days), and then starting with fd 3 all
sockets used for socket activation. No other fds will be passed to your
process.

In fact, one of the big advantages of systemd here is that it executes
all processes in a clearly defined, reset execution environment which
services can rely on, very much unlike classic SysV where the fds
passed and the process context properties set are dependent on the shell
or service that invoked another service, and where "leaked" fds and
settings are a frequent problem.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.