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Re: [libvirt] [PATCHv2] build: correctly check for SOICGIFVLAN GET_VLAN_VID_CMD command

On 02/10/2014 05:10 PM, Laine Stump wrote:

>> AC_CHECK_DECLS unconditionally defines the variable to either 0 or 1, so
>> checking if it is defined will still fail to compile on older headers
>> that lack the enum; 
> Well, it would have compiled successfully, but compiled the wrong code :-)
>> what you want is to check that it has a non-zero value.
> Now that I've seen this, it bugs me that all HAVE_* #defines aren't
> consistent about this - some are undefined if the capability is missing,
> and some are #defined, but to 0.

It's historical baggage of autoconf - the various checks were written at
different times, but autoconf can't change the older checks to always
define the macro because of back-compat.  The general rule of thumb is
HAVE_DECL_ is always defined, any other HAVE_ is likely undefined if the
feature is missing.

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

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