Question

我有下面编写的代码，当调用时从内部数据库中提取值。数据库3rd colum包含BLOB格式的值。因此，对此数据库的调用是在开始和结束时间之间提取所有列。我正在使用malloc

unsigned char *valueBlobData = (unsigned char*)malloc(valueBlobSize);

为值bcoz保留内存一次可以有多个值。现在，如果我使用：

free(valueBlobData)

列表historyRows不包含任何值。但是如果我没有freememory，那么我会在stl列表中收到值。但这会导致内存泄漏。

问题是，我是否在错误的位置释放记忆。如果是，我怎样才能使它发挥作用？

bool SQHISDB::getHisVArray(unsigned long startTime, unsigned long endTime,
    std::list<tHistoryRow> &historyRows)
{
    int rc;
    int counter = 0;
    tHistoryRow tempHistoryRow; // Struct having four variables
    unsigned int A1 =1, A2=2;
    const char *zSql = "SELECT * "
        "FROM node_values "
        "WHERE A1 = ? "
        "AND A2 = ? "
        "AND (sourceTS BETWEEN ? AND ? ) "
        ";";

    sqlite3_stmt *pStmt = NULL;

    rc = sqlite3_prepare_v2(this->db, zSql, std::strlen(zSql), &pStmt, NULL);

    if (rc != SQLITE_OK)
    {
        return rc;
    }

    sqlite3_bind_int(pStmt, 1, A1);
    sqlite3_bind_int(pStmt, 2, A2);
    sqlite3_bind_int64(pStmt, 3, startTime);
    sqlite3_bind_int64(pStmt, 4, endTime;

    while (sqlite3_step(pStmt) == SQLITE_ROW)
    {
        counter++;
        unsigned long sourceTimestamp = sqlite3_column_int64(pStmt, 4);
        unsigned int valueBlobSize = sqlite3_column_bytes(pStmt, 3);
        unsigned char *valueBlobData = (unsigned char*)malloc(valueBlobSize);

        std::memcpy(valueBlobData, sqlite3_column_blob(pStmt, SQLITE_HISTORYDB_INDEX_VALUE), valueBlobSize);

        tempHistoryRow.sourceTimestamp = sourceTS;
        tempHistoryRow.blobSize = valueBlobSize;
        tempHistoryRow.blob = valueBlobData;

        historyRows.push_back(tempHistoryRow);
        free(valueBlobData);
    }
    sqlite3_finalize(pStmt);

    return true;
}

Answer 1

实际上，你正试图将historyRows推向悬空指针。这绝对是错误的。

对我而言，您似乎想使用简单的向量来管理此缓冲区。

struct tHistoryRow {
    std::vector<unsigned char> blob;
    … … …
};

bool SQHISDB::getHisVArray(unsigned long startTime, unsigned long endTime,
    std::list<tHistoryRow> &historyRows)
{
    int rc;
    int counter = 0;
    unsigned int A1 =1, A2=2;
    static const std::string zSql{ "SELECT * "
        "FROM node_values "
        "WHERE A1 = ? "
        "AND A2 = ? "
        "AND (sourceTS BETWEEN ? AND ? ) "
        ";"  };

    sqlite3_stmt *pStmt = NULL;

    rc = sqlite3_prepare_v2(this->db, zSql.data(), zSql.length(), &pStmt, NULL);

    if (rc != SQLITE_OK)
    {
        return rc;
    }

    sqlite3_bind_int(pStmt, 1, A1);
    sqlite3_bind_int(pStmt, 2, A2);
    sqlite3_bind_int64(pStmt, 3, startTime);
    sqlite3_bind_int64(pStmt, 4, endTime;

    while (sqlite3_step(pStmt) == SQLITE_ROW)
    {
        counter++;
        unsigned long sourceTimestamp = sqlite3_column_int64(pStmt, 4);
        unsigned int valueBlobSize = sqlite3_column_bytes(pStmt, 3);


        tHistoryRow tempHistoryRow; // it would be nice to provide constructor
        tempHistoryRow.blob.resize(valueBlobSize);
        auto pBlob = sqlite3_column_blob(pStmt, SQLITE_HISTORYDB_INDEX_VALUE);
        std::copy(pBlob, pBlob + valueBlobSize, tempHistoryRow.blob.begin());

        tempHistoryRow.sourceTimestamp = sourceTS;

        historyRows.push_back(std::move(tempHistoryRow));
    }
    sqlite3_finalize(pStmt);

    return true;
}

Answer 2

释放内存并不意味着＆＃34; 清除＆＃34;内存，当你释放内存时，它只能用于其他分配，但是内存可能仍然包含与调用free之前相同的内容（或它的一些损坏版本，它取决于分配器的实现方式< / em>的）。

尝试检查（取消引用）已传递给free()的指针是未定义的行为。

无法释放由Malloc分配的记忆

2 个答案: