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Re: [libvirt] [PATCH 1/3] util: add virendian.h macros



On 02/11/2013 05:56 PM, John Ferlan wrote:
> On 02/11/2013 07:07 PM, Eric Blake wrote:
>> We have several cases where we need to read endian-dependent
>> data regardless of host endianness; rather than open-coding
>> these call sites, it will be nicer to funnel things through
>> a macro.
>>
>> The virendian.h file can be expanded to add writer functions,
>> and/or 16-bit access patterns, if needed.  Also, if we need
>> to turn things into a function to avoid multiple evaluations
>> of buf, that can be done later.  But for now, a macro worked.
>>

>> +# define virReadBufInt32LE(buf)                          \
>> +    ((uint32_t)(uint8_t)((buf)[0]) |                     \
>> +     ((uint32_t)(uint8_t)((buf)[1]) << 8) |              \
>> +     ((uint32_t)(uint8_t)((buf)[2]) << 16) |             \
>> +     ((uint32_t)(uint8_t)((buf)[3]) << 24))
>> +
>> +#endif /* __VIR_ENDIAN_H__ */
> 
> Looking at byteswap.h.in I'm reminded of what I've seen in a previous job where
> we had lots of byteswapping to do. The macros will "isolate" each byte to move, eg:
> 
> #define bswap_32(x) ((((x) & 0x000000FF) << 24) | \
>                      (((x) & 0x0000FF00) << 8) | \
>                      (((x) & 0x00FF0000) >> 8) | \
>                      (((x) & 0xFF000000) >> 24))
> 
> Not sure if it's necessary in this case, but figured I'd ask.

In the byteswap case, you are going from an int to an int; so you do
have to mask out bytes on each part of the expression.  But this
function is going from a char[] to an int, so we are already starting
with bytes.  You will also notice that my code does a double cast: first
to uint8_t (to guarantee that the byte is not sign-extended) then to
uint{32,64}_t (to widen to the correct size of the resulting
expression); the first cast is the same as masking with 0xff.

> 
> ACK

Thanks for reviewing; and I have now pushed the series.

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

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