14

Valgrind 在以下代码中报告泄漏的块，显然每个线程一个：

#include <iostream>
#include <thread>
#include <mutex>
#include <list>
#include <chrono>

std::mutex cout_mutex;

struct Foo
{
    Foo() 
    { 
        std::lock_guard<std::mutex> lock( cout_mutex );
        std::cout << __PRETTY_FUNCTION__ << '\n'; 
    }

    ~Foo() 
    { 
        std::lock_guard<std::mutex> lock( cout_mutex );
        std::cout << __PRETTY_FUNCTION__ << '\n'; 
    }

    void 
    hello_world() 
    { 
        std::lock_guard<std::mutex> lock( cout_mutex );
        std::cout << __PRETTY_FUNCTION__ << '\n'; 
    }
};

void
hello_world_thread()
{
    thread_local Foo foo;

    // must access, or the thread local variable may not be instantiated
    foo.hello_world();

    // keep the thread around momentarily
    std::this_thread::sleep_for( std::chrono::milliseconds( 100 ) );
}

int main()
{
    for ( int i = 0; i < 100; ++i )
    {
        std::list<std::thread> threads;

        for ( int j = 0; j < 10; ++j )
        {
            std::thread thread( hello_world_thread );
            threads.push_back( std::move( thread ) );
        }

        while ( ! threads.empty() )
        {
            threads.front().join();
            threads.pop_front();
        }
    }
}

编译器版本：

$ g++ --version
g++ (GCC) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

GCC 构建选项：

--enable-shared
--enable-threads=posix
--enable-__cxa_atexit
--enable-clocale=gnu
--enable-cxx-flags='-fno-omit-frame-pointer -g3'
--enable-languages=c,c++
--enable-libstdcxx-time=rt
--enable-checking=release
--enable-build-with-cxx
--disable-werror
--disable-multilib
--disable-bootstrap
--with-system-zlib

程序编译选项：

g++ -std=gnu++11 -Og -g3 -Wall -Wextra -fno-omit-frame-pointer thread_local.cc

valgrind 版本：

$ valgrind --version
valgrind-3.8.1

Valgrind 选项：

valgrind --leak-check=full --verbose ./a.out > /dev/null

valgrind 输出的尾部：

==1786== HEAP SUMMARY:
==1786==     in use at exit: 24,000 bytes in 1,000 blocks
==1786==   total heap usage: 3,604 allocs, 2,604 frees, 287,616 bytes allocated
==1786== 
==1786== Searching for pointers to 1,000 not-freed blocks
==1786== Checked 215,720 bytes
==1786== 
==1786== 24,000 bytes in 1,000 blocks are definitely lost in loss record 1 of 1
==1786==    at 0x4C29969: operator new(unsigned long, std::nothrow_t const&) (vg_replace_malloc.c:329)
==1786==    by 0x4E8E53E: __cxa_thread_atexit (atexit_thread.cc:119)
==1786==    by 0x401036: hello_world_thread() (thread_local.cc:34)
==1786==    by 0x401416: std::thread::_Impl<std::_Bind_simple<void (*())()> >::_M_run() (functional:1732)
==1786==    by 0x4EE4830: execute_native_thread_routine (thread.cc:84)
==1786==    by 0x5A10E99: start_thread (pthread_create.c:308)
==1786==    by 0x573DCCC: clone (clone.S:112)
==1786== 
==1786== LEAK SUMMARY:
==1786==    definitely lost: 24,000 bytes in 1,000 blocks
==1786==    indirectly lost: 0 bytes in 0 blocks
==1786==      possibly lost: 0 bytes in 0 blocks
==1786==    still reachable: 0 bytes in 0 blocks
==1786==         suppressed: 0 bytes in 0 blocks
==1786== 
==1786== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)
--1786-- 
--1786-- used_suppression:      2 dl-hack3-cond-1
==1786== 
==1786== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)

构造函数和析构函数为每个线程运行一次：

$ ./a.out | grep 'Foo::Foo' | wc -l
1000

$ ./a.out | grep hello_world | wc -l
1000

$ ./a.out | grep 'Foo::~Foo' | wc -l
1000

笔记：

  • 如果更改创建的线程数，则泄漏块数与线程数匹配。
  • 如果 GCC 是这样实现的，那么代码的结构可能允许资源重用（即泄露的块）。
  • 从 valgrind 堆栈跟踪中，thread_local.cc:34 是以下行：thread_local Foo foo;
  • 由于 sleep_for() 调用，程序运行大约需要 10 秒左右。

知道这个内存泄漏是在 GCC 中，是我的配置选项的结果，还是我的程序中的一些错误？

4

2 回答 2

3

似乎泄漏来自动态初始化。

这是一个带有 的示例int：

thread_local int num=4; //static initialization

最后一个例子没有泄漏。我用 2 个线程试了一下，完全没有泄漏。

但现在 ：

int func()
{
    return 4;
}
thread_local int num2=func(); //dynamic initialization

这一漏！有 2 个线程，它提供total heap uage: 8 allocs, 6 frees, 428 bytes allocated...

我建议您使用以下解决方法：

thread_local Foo *foo = new Foo; //dynamic initialization

不要忘记在线程执行结束时做：

delete foo;

但是最后一个例子是一个问题：如果线程在您删除之前退出并出现错误怎么办？又漏...

似乎没有很好的解决方案。也许我们应该向g++开发人员报告这一点？

于 2013-07-16T10:37:55.490 回答
0

尝试删除 thread_local 并使用以下代码

    void
    hello_world_thread()
    {
        Foo foo;

        // must access, or the thread local variable may not be instantiated
        foo.hello_world();

        // keep the thread around momentarily
        std::this_thread::sleep_for( std::chrono::milliseconds( 100 ) );
    }

hello_world_thread 中的 foo 应该在每个线程的本地堆栈中。所以每个线程都会维护自己的 foo 副本。无需将其显式标记为 thread_local。当您有静态或命名空间级别变量但您希望每个变量为每个线程维护自己的副本时，应该在上下文中使用 thread_local。

问候卡哈尔

于 2013-07-16T09:00:03.093 回答