Valgrind检测内存泄漏

Detecting a memory leak with Valgrind

本文关键字：泄漏 内存 检测 Valgrind      更新时间：2023-10-16

我正在用C++编写一个守护进程，它的任务很简单，就是将一些事件插入到mysql数据库中。

当我运行top命令时，我看到进程的内存需要增加，我认为我有内存泄漏，我开始使用Valgrind

我经营valgrind是这样的：

valgrind--tool=memcheck--leak check=yes--show reachable=yes--num个呼叫者=20——跟踪fds=是/my_app

我收到以下报告：

==17045== 128 bytes in 1 blocks are definitely lost in loss record 6 of 11
==17045==    at 0x402A629: calloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==17045==    by 0x40AAB63: my_thread_init (in /usr/lib/i386-linux-gnu/libmysqlclient.so.18.0.0)
==17045==    by 0x40AAE43: my_thread_global_init (in /usr/lib/i386-linux-gnu/libmysqlclient.so.18.0.0)
==17045==    by 0x40A92D7: my_init (in /usr/lib/i386-linux-gnu/libmysqlclient.so.18.0.0)
==17045==    by 0x40863FA: mysql_server_init (in /usr/lib/i386-linux-gnu/libmysqlclient.so.18.0.0)
==17045==    by 0x4087B28: mysql_init (in /usr/lib/i386-linux-gnu/libmysqlclient.so.18.0.0)
==17045==    by 0x8049890: write_db(std::string, std::string, std::string) (Listener.cpp:76)
==17045==    by 0x804A692: SocketListener(void*) (Listener.cpp:182)
==17045==    by 0x4052D4B: start_thread (pthread_create.c:308)
==17045==    by 0x4582D3D: clone (clone.S:130)

函数write_db是这样的：

void write_db(std::string userid,std::string zona,std::string eveniment)
{
    try
    {
    MYSQL * connect;
    connect = mysql_init(NULL);
    connect = mysql_real_connect(connect,"127.0.0.1","myusr","mypwd","mytbl",0,NULL,0);
    std::string stmt = "INSERT INTO t_evenimente(placaid,codev,zona,cand) VALUES("" + userid + ""," + eveniment + ","" + zona + "",NOW())";
    std::cout << stmt << std::endl;
    mysql_query(connect,stmt.c_str());
    mysql_close(connect);
    std::cout << "Inserat eveniment obiectiv " << userid << std::endl;
    }
    catch (...)
    {
        std::cout <<"Exceptie MYSQL" << std::endl;
    }
}

内存泄漏在哪里？我正在使用mysql_init，并按照文档中所说的关闭。。。可能是假阳性吗？

由于您只在函数内部使用connect，因此不动态分配它更容易，并有内存泄漏的风险（很可能就是您看到的）。此外，MySQL API是一个C API，不会抛出任何异常供您捕获，这将简化您现在拥有的东西；

void write_db(std::string userid,std::string zona,std::string eveniment)
{
    MYSQL connect;
    mysql_init(&connect);
    mysql_real_connect(&connect,"127.0.0.1","myusr","mypwd","mytbl",0,NULL,0);
    std::string stmt = "INSERT INTO t_evenimente(placaid,codev,zona,cand) VALUES("" + userid + ""," + eveniment + ","" + zona + "",NOW())";
    std::cout << stmt << std::endl;
    mysql_query(&connect,stmt.c_str());
    mysql_close(&connect);
    std::cout << "Inserat eveniment obiectiv " << userid << std::endl;
}

当然，这仍然忽略了错误处理，您需要将其添加回来。

mysql_init返回一个堆分配的对象，该对象需要由mysql_close释放。当mysql_init和mysql_close之间的某个东西抛出异常时，您永远不会调用close。快速修复-将mysql_close添加到catch块中。更好的修复-了解/使用RAII。

正如Angew所指出的，当您调用mysql_real_connect时，实际上很可能会泄漏连接（除非您在日志中看到异常文本）。你也可以使用RAII来避免这种情况。

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