Hi,
I'm having an issue with parsing a file that contains structures that are
defined with type 'double'. These are 64-bit doubles, in little endian format.
In my code, I have typdefs of the various structures and sub-structures within
the file that are defined using fixed size types - SInt32, UInt32, Float64 and
so on.
I'm using the relevant swapping routines to read the file into my local format.
Everything is fine when I compile in 32-bit, but fails to work in 64 bit. It's
acting as if the fields in the structures are misaligned.
Could this be caused by the 64 bit compiler padding out my structures in a
different way for 64-bit builds? If so is there a way to tell it not to for
these types?
Here's some example code:
typedef struct
{
SInt32 fileCode; // 9994, big
endian
SInt32 unused[5];
UInt32 fileLength; // big endian,
length expressed as # of 16-bit words, including header
SInt32 version; //
little endian, = 1000
UInt32 shapeType; // all little
endian from here on
Float64 xMin; // bbox
Float64 yMin;
Float64 xMax;
Float64 yMax;
Float64 zMin;
Float64 zMax;
Float64 mMin;
Float64 mMax;
}
ESRIFileHeader, *ESRIFileHeaderPtr;
typedef struct
{
SInt32 recordNumber; // big endian
UInt32 contentlength; // big endian, length
expressed as # of 16-bit words
SInt32 shapeType; // little endian
}
ESRIRecordHeader, *ESRIRecordHeaderPtr;
// this fragment of code is where I do the pointer arithmetic:
p = (unsigned char*)[_data bytes];
eof = p + [_data length];
p += sizeof( ESRIFileHeader );
while( !done && !mShouldAbort )
{
// parse the record header
recP = (ESRIRecordHeaderPtr) p;
recNum =
(NSInteger)CFSwapInt32BigToHost(recP->recordNumber);
recLength =
(NSUInteger)CFSwapInt32BigToHost(recP->contentlength) * 2;
shapeType =
(NSInteger)CFSwapInt32LittleToHost(recP->shapeType);
NSLog( @"record #%ld, length=%lu, type=%ld", recNum,
recLength, shapeType ); // displays garbage values in 64-bit build
In the file, the header is followed by a series of records, each of which has
the header structure shown, and a variable length data portion. I use pointer
arithmetic to skip past the header, using the sizeof(ESRIFileHeader) to compute
the size. Even from the very first record, the following header is misaligned,
indicating that sizeof() gives a different result for 32 versus 64 bit builds.
Indeed, I get 100 in 32-bits, and 104 in 64-bit builds for the header structure.
That's bad enough, but the values in the Float64 fields of the header appear
garbage in the debugger in the 64-bit build, so even though I believe I'm
swapping them correctly, the values returned in the 64-bit version are
nonsense. I'm less clear about why that should be, but again it could be
padding - if the padding is not at the end of the structure but somewhere
before the float values.
Basically, I need to know if there's a way to get the 64-bit compiler to treat
these structures the same way as it does in a 32-bit build.
--Graham
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