Wireshark-dev: Re: [Wireshark-dev] R: Problems encountered on Windows building from a "source t
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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