On Tuesday, July 20, 2010 12:37 PM, Robert McVicar wrote:
>I have some code which uses the sqlite3_blob_write() function, but I
> observe some odd behaviour.
> When I pass it a QVector (Qt vector class - similar to std::vector,
> contiguous memory) of a class C (for example) when the vector is of
> significant size (over 200 entries), towards the end of the blob, the
> data is just set to 0 - rather than the values in the vector members.
> Class C just consists of 6 floats and a short (as well as some functions
> in its definition). The precise point where this happens in the blob
> seems to vary between machines.
> I have checked the sqlite limits and the blob handle is of the desired
> size. I pass the function the value of constData() which returns a
> const pointer to the data in the vector.
>
> The call is as follows
> (body is a QVector,
> i_blobHandle is sqlite3_blob* where sqlite3_blob_bytes(i_blobHandle) ==
> ( length of vector * size of class C in bytes) )
>
> sqlite3_blob_write(i_blobHandle, (body.constData()), ( body.size() *
> SIZE_OF_C ), 0);
>
> If I immediately read the blob afterwards as follows
> QVector rmcv_d(body.size());
>
> sqlite3_blob_read(i_blobHandle, (rmcv_d.constData()), (body.size()*
> SIZE_OF_C), 0);
> for ( int rmcv_a(0); rmcv_a < rmcv_d.size(); ++rmcv_a )
> {
>if( d != body[rmcv_a] )
>{
> // GET HERE AROUND THE rmcv_a == 230 mark (though this varies)
>}
> }
>
> Has anybody seen this type of behaviour before? Is it a known
> limitation/bug with BLOB writing? If I write the items one at a time in
> a loop, there is no problem. But I thought to save time, passing in the
> address of the vector would be enough - yet it behaves in this odd way.
>
> Any feedback appreciated.
>
I use the sqlite3_blob_read/write commands a lot. I've found them to be
generally stable for blobs of several tens of megabytes. I see that you are
reserving enough space for your blob, but do you check for any errors on
your call to the write function?
I may be worth using the streaming operators to be sure of avoiding any
quirks in the Qt library (see
http://doc.trolltech.com/4.6/qvector.html#operator-lt-lt-75). You can then
do something along the lines of...
QBuffer b;
b.open(QBuffer::WriteOnly);
QDataStream s();
s << vector;
res = sqlite3_blob_write(blob, b.buffer().constData(), b.size(), 0);
... to write, and ...
QByteArray ba;
int data_size = sqlite3_blob_bytes(blob);
ba.reserve(data_size);
res = sqlite3_blob_read(blob, b.buffer().data(), data_size, 0);
QBuffer b();
b.open(QBuffer::ReadOnly);
QDataStream s();
QVector ret;
s >> ret;
... to read. I haven't actually tested this code, but it follows along the
general line of code that I use in my own software.
Hope this helps...
Andy
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