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Re: [libvirt] [PATCH v4 2/9] New functions for virBitmap



On 09/18/2012 08:29 AM, Eric Blake wrote:
> On 09/18/2012 01:27 AM, Osier Yang wrote:
>> On 2012年09月14日 15:46, Hu Tao wrote:
>>> In many places we store bitmap info in a chunk of data
>>> (pointed to by a char *), and have redundant codes to
>>> set/unset bits. This patch extends virBitmap, and convert
>>> those codes to use virBitmap in subsequent patches.
>>> ---
> 
>>> +
>>> +    memcpy(bitmap->map, data, len);
>>> +    for (i = 0; i<  bitmap->map_len; i++)
>>> +        bitmap->map[i] = le64toh(bitmap->map[i]);
>>
>>
>> le64toh is not portable. Such as on mingw platform.
> 
> Agreed - gnulib does not yet provide it, so we need to open-code this
> ourselves instead of relying on a non-standard function.

Also, this is broken on 32-bit platforms.

https://www.redhat.com/archives/libvir-list/2012-September/msg01326.html

> 
>>> +    l = (unsigned long *)*data;
>>> +    for (i = 0; i<  bitmap->map_len; i++, l++)
>>> +        *l = htole64(*l);
>>
>> Likewise.
> 
> Same here.

Even worse, this use of htole64 can trigger unaligned data access, and
trigger SIGBUS on platforms not as forgiving as x86_64.

>>> +
>>> +    return ffsl(bits) - 1 + nl * VIR_BITMAP_BITS_PER_UNIT;
>>
>> And ffsl.
> 
> But here, gnulib guarantees ffsl.  We just need to modify bootstrap.conf
> to pull it in.

As well as include the right header (the fact that glibc leaks ffs, and
therefore ffsl, through <string.h>, does not mean that it happens like
that on other platforms).

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

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