[libvirt] [PATCH] build: work around older systemtap header

[Eric Blake]

On 07/04/2011 01:10 AM, Matthias Bolte wrote:
>> +/* The double cast is necessary to silence gcc warnings; any pointer
>> + * can safely go to intptr_t and back to void *, which collapses
>> + * arrays into pointers; while any integer can be widened to intptr_t
>> + * then cast to void *.  */
>> +#  define VIR_ADD_CAST(a) ((void *)(intptr_t)(a))
> 
> Is this really true? What about a long long value (64bit) on a 32bit
> platform? intptr_t and void* are only 32bit on a 32bit platform,
> aren't they?

Hmm, interesting point.  On the other hand, can PROBE even operate on
'long long', or does it only operate on 'size_t'/'void *' operands?
That is, the whole point of system tap is to inject probe points that
make it easier to run system tap on a binary, where each use of the
PROBE() macro injects enough code to pass arbitrary bits through the
kernel so that a running systemtap can then reinterpret those bits with
the appropriate type according to the probe point that systemtap is tracing.

I also discovered that my cast is not always gcc silent - gcc insists on
warning for (intptr_t)(func()) for any func that returns a pointer
instead of an integer; but in those cases, the use of an intermediate
variable works around the warning.

It is really annoying that gcc does not provide any counterpart to
__typeof__ that would let you produce the decayed type of an input
argument - life would be much simpler if there were a way to decay an
expression of type 'char[100]' into an intermediate variable of type
'char *'.

At any rate, since none of our current uses of PROBE() pass either 'long
long' or a function call that returns 'char *', we aren't adding any
additional problems by accepting this patch, but we also might be adding
a latent bug that would be hard to diagnose in the future.  I'm not sure
whether we want to apply this patch, or just require systemtap 1.3 or
newer as a prerequisite for compiling libvirt.  Thoughts?  How prevalent
are systemtap 1.2 installations?

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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