QT 5.8 image from database error

[Kutyus]

Hi!

I stored an image (ca.: 1.6 Mb) on MSSQL2012 Express database, when i query it, i get an error:

QSqlQuery query(db);
	query.prepare("select picture from ads");
		
	if (query.exec())
	{
		while (query.next())
		{
			pixdata = query.value(0).toByteArray();
			qDebug() << "image:" << pixdata.toHex();
		}
	} else qDebug() << query.lastError().text();
	pixmap.loadFromData(pixdata);

The pixdata is only 8 byte long, the qDebug output: image: "89504e470d0a1a0a".
Query the pixmap from the sqsh or management studio, I get all of the data: 0x89504E470D0A1A0A0000000D494844520000078000000438080200000067B1... etc.
Interestingly, the first missing byte is NULL.

Thanks, in advance!

[SGaist]

Hi,

How did you store this picture in the first place ?
What type of fields are you using ?

[Kutyus]

Hi SGaist!

I uploaded the file to a varbinary(max) column.

[VRonin]

I load images with that method from the same version of server successfully.

The problem you are experiencing is just an unfortunate coincidence: toHex() returns QByteArray, qDebug() << tries to display it as it was an ascii string so stops when it reaches '\0'.

qDebug() << "image size:" << pixdata.size(); should return the correct amount of bytes.

pixmap.loadFromData(pixdata); is maybe in the wrong place? there it will load the image of the last row of the table

[Kutyus]

@VRonin pixdata.size() is 8.
I have only one record in the database.

[VRonin]

@Kutyus said in QT 5.8 image from database error:

pixdata

What type is pixdata ?

[Kutyus]

@VRonin pixdata is QByteArray, pixmap is QPixmap.

[SGaist]

Might be a silly question but did you check that the data was correctly saved in the database ?

[Kutyus]

@SGaist I modified the query to:

query.prepare("select DATALENGTH(picture), picture from ads");

and

qDebug() << "image size:" << query.value(0).toInt();

The image size is 1715323, it is the uploaded file size in bytes, and the sqsh and management studio restore more than 8 bytes.

[VRonin]

Ok, after some digging, the problem is with varbinary(MAX) change it to varbinary(8000) and everything works. of course this is a problem for you as picture > 8000 bytes.

---

EDIT.
I was trying to replicate a minimal example to submit a bug report but couldn't reproduce (Qt 5.5 and 5.9).
What version of Qt are you using?

CREATE TABLE [dbo].[TestBinary](
	[TestMax] [varbinary](max) NULL,
	[TestNum] [varbinary](8000) NULL
) 

INSERT INTO TestBinary
           (TestMax
           ,TestNum)
     VALUES
           (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           ,0x89504E470D0A1A0A0000000D494844520000078000000438080200000067B1
		   )

#include <QApplication>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QDebug>
bool openDB(QSqlDatabase& db)
{
    if (!db.isValid()) {
        db = QSqlDatabase::addDatabase("QODBC", "MyDatabase");
        db.setDatabaseName(<INSERT HERE YOUR CONNECTION STRING>);
    }
    if (db.isOpen())
        return true;
    if (db.open())
        return true;
    return false;
}
int main(int argc, char *argv[])
{
    QApplication app(argc,argv);
    QSqlDatabase db;
    if(!openDB(db))
        return -1;
    QSqlQuery testQuery(db);
    testQuery.prepare("SELECT TestMax,TestNum FROM TestBinary");
    if(!testQuery.exec())
         return -1;
    if(!testQuery.next())
         return -1;
    const QByteArray TestMax = testQuery.value(0).toByteArray();
    const QByteArray TestNum = testQuery.value(1).toByteArray();
    qDebug() << "varbinary(MAX) size: " << TestMax.size();
    qDebug() << "varbinary(8000) size: " << TestNum.size();
    return 0;
}

correctly outputs:

varbinary(MAX) size:  8122
varbinary(8000) size:  31

[Kutyus]

@VRonin I used a tds plugin earlier, I have rebuild Qt with odbc driver, but I can not connect to the database, which driver you use?

[VRonin]

@Kutyus ODBC from online installer of Qt 5.5 and 5.9 on Windows for MSVC2013

[Kutyus]

@VRonin Ahh, I use a Banana Pi with linux.
I connected with odbc over freetds, but the pixdata size is 4096 :(
I try to upgrade freetds now. I think it is a driver bug.

[Kutyus]

@VRonin Not working, i compiled the freetds 1.00.47, and i recompiled the tds plugin, but the pixdata is only 8 byte :(

[VRonin]

Ok, now that we know it's a plugin problem we can see if we can do something about it.

The first thing that comes to mind is storing your binarydata in a varchar (no need for unicode) using base64.

This is easy if you use QByteArray for both reading and writing, just use toBase64()

[Kutyus]

@VRonin Thank you for tip, the trick with base64 encoded file (to varchar(max) field) is working.