Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"

On 01/20/2015 11:40 PM, Andy Lutomirski wrote:

On Tue, Jan 20, 2015 at 1:57 AM, Fam Zheng [off-list ref] wrote:

quoted

This adds a new system call, epoll_mod_wait. It's described as below:

[...]

quoted

       There is no guartantee that all the commands are executed in order. Only
       if all the commands are successfully executed (all the error fields are
       set to 0), events are polled.

If this doesn't happen, what error is returned?

If I read the code correctly: the error of the first epoll_ctl op that fails.

[...]

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RETURN VALUE

       When any error occurs, epoll_mod_wait() returns -1 and errno is set
       appropriately. All the "error" fields in cmds are unchanged before they
       are executed, and if any cmds are executed, the "error" fields are set
       to a return code accordingly. See also epoll_ctl for more details of the
       return code.

Does this mean that callers should initialize the error fields to an
impossible value first so they can tell which commands were executed?

Yes. (Ugly!)

[...]

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ERRORS

       These errors apply on either the return value of epoll_mod_wait or error
       status for each command, respectively.

Please clarify which errors are returned overall and which are per-command.

Yes, I think this would be valuable as well.

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/