[zeromq-dev] How to handle EINTR especially when using High-Level APIs as CZMQ

Franz Hollerer f.hollerer at gmx.net
Sun Mar 22 13:53:06 CET 2020


I wonder how to correctly handle EINTR especially when using a
high-level API as CZMQ.

The ZeroMQ guide states in section "The CZMQ High-Level API":

> One thing CZMQ provides is clean interrupt handling. This means that
> Ctrl-C will cause any blocking
> ZeroMQ call to exit with a return code -1 and errno set to EINTR . The
> high-level recv methods will
> return NULL in such cases. So, you can cleanly exit a loop like this:

> while (true) {
>         zstr_send (client, "Hello");
>         char *reply = zstr_recv (client);
>         if (!reply)
>                 break;  // Interrupted
>         printf ("Client: %s\n", reply);
>         free (reply);
>         sleep (1);
> }

The example above assumes that the call is interrupted because someone
has pressed Ctrl-C. But on an Unix-like operating system there is not
only the SIGTERM and SIGINT (Ctrl+C) signal which may interrupt the
system call.

Also alarm() and interval timers might be implemented using signals.
 From the alarm() manpage on Debian 10:

> alarm()  arranges  for  a SIGALRM signal to be delivered to the calling
> process in seconds seconds.

To my understanding handling EINTR needs to be done close to system call
where it occurs. In most cases it is appropriate to restart the system call.

I wonder how to deal with EINTR at a high-level API. As zstr_recv() maps
it to NULL there is no way to distinguish EINTR from other errors. And
even if it can be distinguished somehow, it might be a bad idea to
restart the high-level API function.

Leaving the loop as shown above works for SIGTERM and SIGINT. But what
if some other part of the program uses interval timers and the call was
interrupted by a SIGALARM?

Any recommendations?


Franz Hollerer

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