c++11 - 使用 thread_local 时 gcc 4.8.1 中的内存泄漏？

Question

Valgrind 正在报告泄漏的 block ，显然每个线程一个，在以下代码中:

#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

注释:

• 如果更改创建的线程数，则泄漏 block 的数量与线程数匹配。
• 代码的结构方式可能允许资源重用(即泄漏的 block )(如果 GCC 是这样实现的话)。
• 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。

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

Answer 1

看来泄漏来自动态初始化。

这是一个带有 int 的示例:

thread_local int num=4; //static initialization

最后一个例子没有泄漏。我用 2 个线程尝试过，完全没有泄漏。

但现在:

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

这一次泄露了!对于 2 个线程，它提供总堆使用量:8 次分配、6 次释放、已分配 428 字节...

我建议您使用如下解决方法:

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

不要忘记在线程执行结束时要做的事情:

delete foo;

但最后一个例子是一个问题:如果线程在删除之前因错误退出怎么办？再次泄漏...

似乎没有什么太好的解决办法。也许我们应该向 g++ 开发人员报告这一点？