Re: [Qemu-devel] bug with io/channel-socket.c - variable-sized object may not be initialized

[Eric Blake]

On 01/13/2016 02:19 PM, Programmingkid wrote:
> This code causes an error to occur during compiling: 
> 
> char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)] = { 0 };
> 
> It is located at line 496 in io/channel-socket.c. 
> 
> Here is the full error message:
> io/channel-socket.c: In function 'qio_channel_socket_writev':
> io/channel-socket.c:496:18: error: variable-sized object may not be 
> initialized
>      char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)] = { 0 };
> 
> This is from gcc 4.9 running on Mac OS 10.6.8. 

Uggh. That sounds like a bug in the Mac OS headers, for making
CMSG_SPACE() not be a compile-time constant.  We do NOT want to be using
variable-sized objects here, so we need a compile-time constant for the
array size, even if we have to work around your platform's borked headers.

Can you capture what that line expands to after pre-processing, to see
if my guess is right?

> 
> A quick fix to this problem is to replace everything in the parentheses with 
> 1000. 

Yeah, but that's probably wrong, unless we know for sure that
sizeof(int)*SOCKET_MAX_FDS will never exceed 1000; in that case, it's
probably over-allocating.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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