[systemd-devel] Problem with sd_listen_fds(0)

[Mantas Mikulėnas]

On Thu, Jun 27, 2019 at 1:29 PM Donat Zenichev <donat.zenichev at gmail.com>
wrote:

> Hi systemd community.
> Recently I've come accross one interesting problem with API of systemd.
> The problem is that, I'm not able to read commands comming to the AF_UNIX
> socket, that was created by systemd and passed to my program using
> sd_listen_fds(0) function.
>
> What actually program requires, is AF_UNIX (AF_LOCAL) socket of type
> SOCK_STREAM.
> In case if I create this socket manually, using socket(AF_LOCAL,
> SOCK_STREAM, 0) function, it works out as expected.
>
> But, when I create a socket using systemd socket unit, reading fails with
> "-1" return value.
>
> What I actually do, is that I check if there are some sockets created for
> me using sd_listen_fds(0), and in case it returns 1 (so 1 socket created),
> program goes further and sets a file descriptor value to fd =
> SD_LISTEN_FDS_START + 0;
> Since the SD_LISTEN_FDS_START is pre-defined to value '3', my file
> descriptor will always be '3'.
>
> But when it comes to reading, with read() function, it fails with the
> errno EINVAL - Invalid argument.
> It's not a matter of a other parameters than a file descriptor, since with
> manual creation of socket it works out as I said.
>
> An initializing of the socket descriptor, when creating it by systemd
> passes as normal, so program starts.
> But any further command received on fd fails with '-1' return value.
>

As this is a stream socket, by default the program will receive the
*listener* socket. Systemd will start your service as soon as someone
connects, but your program still needs to accept() each connection first.


-- 
Mantas Mikulėnas
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