SQLite3 INSERT 查询的随机语法错误

Question

我在 Qt 上使用 SQLite3 和 Spatialite 进行插入查询，有时它只是无法返回随机语法错误。如果我运行在 SpatialiteGUI 上查询它永远不会失败。

我正在使用 SQLite3 版本 3.27.2。

构建和运行查询的方法：

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; 
}

一些查询示例：

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));

输出：

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

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

Answer 1

strQuery.toLatin1() 是临时值，.data() 获取该值内的指针。这实际上是一个悬挂指针。

添加一个中间保持变量：（并使用 UTF8 而不是 Latin1）

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

Answer 2

要解开这个谜，我们所要做的就是通过阅读 Qt 的文档仔细检查发生了什么。

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

根据 Qt 的文档，QString 的 toLatin1() method returns a QByteArray 对象。

接下来，我们确定 QByteArray 的 data() 方法 returns 一个 "pointer [that] remains valid as long as the byte array isn't reallocated or destroyed".

好了，到此，问题就清楚了。 toLatin1() returns 一个临时 QByteArray 对象，根据 C++ 管理临时对象的通常规则，它在表达式末尾被销毁。在此之前 data() returns 指向其内部的指针，保存在 query.

但是该指针立即变成指向已释放内存的指针，任何进一步使用该指针都会导致未定义的行为。

此处最简单的解决方法是获取 toLatin1() 的对象，并将其保存为局部变量，只有在不再需要其 data() 后才会销毁。类似于：

QByteArray strarray = strQuery.toLatin1();

char *query=strarray.data();

SQLite3 INSERT 查询的随机语法错误

Random syntax error on SQLite3 INSERT query

c++

sqlite

qt

syntax-error

spatialite