Guy Harris wrote:
> Davide Schiera wrote:
>
>> The structures are used to wrap the header of a 802.11 packet.
>> The base format of these structures are
>> UCHAR fc[2];
>> UCHAR dur[2];
>> UCHAR addr1[AIRPDCAP_MAC_LEN]; // addr1[6]
>> UCHAR addr2[AIRPDCAP_MAC_LEN];
>> UCHAR addr3[AIRPDCAP_MAC_LEN];
>> UCHAR seq[2];
>> If any compiler align each field at the desired position (fc:0, dur:2,
>> addr1:4, ...) the #pragma pack is definitely unneeded. If not, there is
>> another way to do the same thing?
>
> If the alignment were at the desired position, it would be because the
> compiler added extra padding between the fields. Alignment is
> implementation-defined in C89, but I suspect the chances that a compiler
> would add padding between arrays of 1-byte quantities are less than the
> chances that a compiler would support "#pragma pack" in that particular
> fashion, so leaving the "#pragma pack" out is probably safer.
I've consistently seen both VC and gcc putting padding between values in
an array, especially in a case like this one: there's good chance that
dur will be at byte 4 instead of 2. To avoid the problem, I use #pragma
pack with VC, and __attribute__((__packed__)) with GCC
typedef struct _foo
{
char first;
char second;
}__attribute__((__packed__))
foo;
With the right #ifdefs, you can easily make the thing portable.
Loris
> (UCHAR
> expands to guchar, which is ultimately typedeffed to "unsigned char";
> the chances that a compiler for a platform supporting Wirehark doesn't
> implement "unsigned char" as an 8-bit byte is probably also less than
> the chances that a compiler doesn't support "#pragma pack" in that
> particular fashion.)
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