Random syntax error on SQLite3 INSERT query

Question

I'm doing an insertion query using SQLite3 with Spatialite on Qt and sometimes it just fails returning random syntax errors. If I run the queries on SpatialiteGUI it never fails.

I'm using SQLite3 version 3.27.2.

The method that builds and runs the query:

bool DatabaseManager::insertPolygons(QList<QList<QGeoCoordinate>> polygons, int workId) 
{
    sqlite3_stmt *dbStatement = nullptr;

    QString strQuery = "INSERT INTO \"WORK_" + QString::number(workId) + "\" (Geometry) VALUES ";
    bool inserted = false;

    for(int i = 0; i < polygons.size(); i++) {
        strQuery += "(GeomFromText('POLYGON((";

        foreach (QGeoCoordinate coordinate, polygons[i]) {
            strQuery += QString::number(coordinate.longitude(), 'f', 10) + " " +
                        QString::number(coordinate.latitude(), 'f', 10)  + ",";
        }

        strQuery = strQuery.left(strQuery.size() - 1) + "))', 4326)),";
    }

    strQuery = strQuery.left(strQuery.size() - 1) + ";";

    char *query = strQuery.toLatin1().data();

    int status = sqlite3_prepare_v2(database, query, strQuery.toLatin1().size(), &dbStatement, 0);

    if(status == SQLITE_OK) {
        int status = 0;

        status = sqlite3_step(dbStatement);

        if(status == SQLITE_DONE)
            inserted = true;
    }else
        qDebug().noquote() << "status:" << status << "error:" << sqlite3_errmsg(database);

    sqlite3_finalize(dbStatement);

    return inserted; 
}

Some queries examples:

INSERT INTO "WORK_264" (Geometry) VALUES 
(GeomFromText('POLYGON((-52.3855298461 -28.2283621371,-52.3855220463 -28.2283563298,-52.3855103297 -28.2283685464,-52.3855181295 -28.2283743537,-52.3855298461 -28.2283621371))', 4326)),
(GeomFromText('POLYGON((-52.3855454459 -28.2283737516,-52.3855376460 -28.2283679443,-52.3855259294 -28.2283801609,-52.3855337292 -28.2283859682,-52.3855454459 -28.2283737516))', 4326)),
(GeomFromText('POLYGON((-52.3855610456 -28.2283853661,-52.3855532457 -28.2283795588,-52.3855415291 -28.2283917755,-52.3855493289 -28.2283975828,-52.3855610456 -28.2283853661))', 4326)),
(GeomFromText('POLYGON((-52.3855766453 -28.2283969805,-52.3855688455 -28.2283911733,-52.3855571288 -28.2284033900,-52.3855649286 -28.2284091973,-52.3855766453 -28.2283969805))', 4326));

INSERT INTO "WORK_264" (Geometry) VALUES 
(GeomFromText('POLYGON((-52.3868293314 -28.2269741900,-52.3868371280 -28.2269800006,-52.3868488522 -28.2269677737,-52.3868410531 -28.2269619658,-52.3868293314 -28.2269741900))', 4326)),
(GeomFromText('POLYGON((-52.3868137382 -28.2269625689,-52.3868215348 -28.2269683795,-52.3868332540 -28.2269561579,-52.3868254549 -28.2269503500,-52.3868137382 -28.2269625689))', 4326)),
(GeomFromText('POLYGON((-52.3867981450 -28.2269509478,-52.3868059416 -28.2269567584,-52.3868176557 -28.2269445420,-52.3868098566 -28.2269387341,-52.3867981450 -28.2269509478))', 4326)),
(GeomFromText('POLYGON((-52.3867825518 -28.2269393267,-52.3867903484 -28.2269451373,-52.3868020575 -28.2269329262,-52.3867942584 -28.2269271183,-52.3867825518 -28.2269393267))', 4326));

Output:

status: 1 error: near "database": syntax error

status: 1 error: near "�": syntax error

Botje · Accepted Answer · 2021-11-26 15:11:38Z

1

strQuery.toLatin1() is a temporary value, and .data() grabs a pointer within that value. This is effectively a dangling pointer.

Add an intermediate holding variable: (and use UTF8 instead of Latin1 while you're at it)

auto queryBA = strQuery.toUtf8();
int status = sqlite3_prepare_v2(database, queryBA.data(), queryBA.size(), &dbStatement, 0);

answered Nov 26, 2021 at 15:11

Botje

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Sam Varshavchik · Accepted Answer · 2021-11-26 15:12:41Z

1

To solve this mystery all we have to do is double-check what's going on by reading Qt's documentation.

char *query = strQuery.toLatin1().data();

According to Qt's documentation, QString's toLatin1() method returns a QByteArray object.

Next, we determine that QByteArray's data() method returns a "pointer [that] remains valid as long as the byte array isn't reallocated or destroyed".

Well, now, the problem is clear. toLatin1() returns a temporary QByteArray object that, as per C++ usual rules governing temporary objects, gets destroyed at the end of the expression. Before that happens data() returns a pointer to its innards, which gets saved in query.

But that pointer immediately becomes a pointer to deallocated memory, and any further use of this pointer results in undefined behavior.

The simplest fix here is to take toLatin1()'s object, and save it as a local variable, that only gets destroyed after its data() is no longer needed. Something like:

QByteArray strarray = strQuery.toLatin1();

char *query=strarray.data();

answered Nov 26, 2021 at 15:12

Sam Varshavchik

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1 Comment

JonatasCV Over a year ago

Giving the answer to Botje because he replied first, but thanks for the explanation. Have a good one!!

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