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Re: [libvirt] [PATCH] Cygwin's GCC doesn't like this .sa_handler initialization for some reason



2010/4/26 Eric Blake <eblake redhat com>:
> On 04/25/2010 05:32 AM, Matthias Bolte wrote:
>> ---
>>  examples/domain-events/events-c/event-test.c |    7 ++++---
>>  1 files changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/examples/domain-events/events-c/event-test.c b/examples/domain-events/events-c/event-test.c
>> index 53a3195..74eabba 100644
>> --- a/examples/domain-events/events-c/event-test.c
>> +++ b/examples/domain-events/events-c/event-test.c
>> @@ -380,10 +380,11 @@ int main(int argc, char **argv)
>>      int callback5ret = -1;
>>      int callback6ret = -1;
>>      int callback7ret = -1;
>> +    struct sigaction action_stop;
>>
>> -    struct sigaction action_stop = {
>> -        .sa_handler = stop
>> -    };
>> +    memset(&action_stop, 0, sizeof action_stop);
>> +
>> +    action_stop.sa_handler = stop;
>
> ACK.  By the way, the "for some reason" boils down to how sa_handler is
> declared in <signal.h> on the two platforms.  On cygwin, sa_handler
> happens to be a member of an anonymous union (exploiting a gcc
> extension); whereas on Linux, it is a macro that accesses a member of a
> named union.  If I read POSIX correctly, both behaviors for the
> declaration of sa_handler are permitted.  But dotted assignment only
> works in the case of a named union.  Whether it is a gcc bug that you
> can't use dotted assignment to access a member of an anonymous union, or
> a cygwin bug for having a header that does not allow compilation like
> Linux (when cygwin's stated goal is to be Linux-compatible) is debatable.
>
> By the way, this use of memset() to initialize action_stop assumes that
> the null pointer is all 0 bits (which is probably okay for all platforms
> that libvirt will ever run on).  There is currently a discussion on
> gnulib on the fact that to be portable to C99, where a null pointer
> might not be all 0 bits, you would instead have to use the more
> drawn-out sequence:

You say null pointer, but where's the pointer in this example?

Do you really refer to pointers or to the initialization of memory?

I wonder what's the use case for this.

> static struct sigaction zero_sigaction = {0};
> struct sigaction action_stop = zero_sigaction;
> action_stop.sa_handler = stop;
>
> But don't go changing this commit just for that theoretical platform.
>

Thanks, pushed.

Matthias


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