Hi guys, I am having a hard time understanding something right now:
struct FormatChunk
{
BYTE subchunk1ID[4];
DWORD subchunk1Size;
short audioFormat;
};
DWORD dwSize=sizeof(FormatChunk); // is 12 !!!!
dwSize=sizeof(short); // is 2 like it should be
What the hell ?
Ahh, ok guys, I thought I was going crazy.
I'm going to read up on it.
In general though, if I have to read 10 bytes into that struct I can't rely on sizeof() and should explicitly set it to read 10, is that right ?
Thanks.
If your structure is meant to be used to map in-memory file data you should enforce finer control over the alignment/padding of the structure with #pragma pack where available.
If not available, you should approach the problem in a different way period.
If your byte array was 3 bytes instead of 4 bytes in length, subchunk1Size would still be aligned to 4 bytes, with an extra byte added before it for padding.
Padding does not occur just at the end of structures, but also between members inside the structure, so if you can’t strictly control how it is padded, don’t even try to use that approach.
So no, don’t hard-code 10 anywhere. You should use sizeof(), and if that is not reliable then don’t use this method at all.
L. Spiro
Your safest option is to read and write each element individually.
sizeof(subchunk1ID) + sizeof(subchunk1Size) + sizeof(audioFormat) will always be 10 regardless of any structure padding.
The best option is typically to use #pragma pack (with push/pop). This allows you to eliminate the padding entirely (using a pack of 1), and at that point the representation in memory should be identical to that on disk (barring endian issues, which you are far less likely to run into since Mac's switched to Intel).
However, be aware that there is a performance penalty to read/write unaligned memory - you should only pack structures directly involved in I/O.
