Question

#include <iostream>
#include <sstream>
#include <thread>

using namespace std;

int main()
{
    auto runner = []() {
        ostringstream oss;
        for (int i=0; i<100000; ++i)
            oss << i;
    };

    thread t1(runner), t2(runner);
    t1.join(); t2.join();
}

在g ++ 6.2.1中编译上面的代码，然后用valgrind --tool=helgrind ./a.out运行它。赫尔格林德会抱怨：

==5541== ----------------------------------------------------------------
==5541== 
==5541== Possible data race during read of size 1 at 0x51C30B9 by thread #3
==5541== Locks held: none
==5541==    at 0x4F500CB: widen (locale_facets.h:875)
==5541==    by 0x4F500CB: widen (basic_ios.h:450)
==5541==    by 0x4F500CB: fill (basic_ios.h:374)
==5541==    by 0x4F500CB: std::ostream& std::ostream::_M_insert<long>(long) (ostream.tcc:73)
==5541==    by 0x400CD0: main::{lambda()#1}::operator()() const (43.cpp:12)
==5541==    by 0x4011F7: void std::_Bind_simple<main::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) (functional:1391)
==5541==    by 0x401194: std::_Bind_simple<main::{lambda()#1} ()>::operator()() (functional:1380)
==5541==    by 0x401173: std::thread::_State_impl<std::_Bind_simple<main::{lambda()#1} ()> >::_M_run() (thread:197)
==5541==    by 0x4EF858E: execute_native_thread_routine (thread.cc:83)
==5541==    by 0x4C31A04: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5541==    by 0x56E7453: start_thread (in /usr/lib/libpthread-2.24.so)
==5541==    by 0x59E57DE: clone (in /usr/lib/libc-2.24.so)
==5541== 
==5541== This conflicts with a previous write of size 8 by thread #2
==5541== Locks held: none
==5541==    at 0x4EF3B1F: do_widen (locale_facets.h:1107)
==5541==    by 0x4EF3B1F: std::ctype<char>::_M_widen_init() const (ctype.cc:94)
==5541==    by 0x4F501B7: widen (locale_facets.h:876)
==5541==    by 0x4F501B7: widen (basic_ios.h:450)
==5541==    by 0x4F501B7: fill (basic_ios.h:374)
==5541==    by 0x4F501B7: std::ostream& std::ostream::_M_insert<long>(long) (ostream.tcc:73)
==5541==    by 0x400CD0: main::{lambda()#1}::operator()() const (43.cpp:12)
==5541==    by 0x4011F7: void std::_Bind_simple<main::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) (functional:1391)
==5541==    by 0x401194: std::_Bind_simple<main::{lambda()#1} ()>::operator()() (functional:1380)
==5541==    by 0x401173: std::thread::_State_impl<std::_Bind_simple<main::{lambda()#1} ()> >::_M_run() (thread:197)
==5541==    by 0x4EF858E: execute_native_thread_routine (thread.cc:83)
==5541==    by 0x4C31A04: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==5541==  Address 0x51c30b9 is 89 bytes inside data symbol "_ZN12_GLOBAL__N_17ctype_cE"

似乎两个线程都调用locale_facet.h:widen导致数据竞争，因为在此函数中似乎没有同步，即使在两个不同的operator <<对象上调用了ostringstream。所以我想知道这是真的是数据竞赛还是只是helgrind的误报。

Answer 1

更新：我承认在回答之前我没有完全阅读这个问题，所以我自己去研究这个问题。

TL; DR

这里有可变成员变量可能导致竞争条件。 ios每个语言环境都有静态变量，这些静态变量延迟加载（首次需要时初始化）查找表。如果要避免并发问题，只需确保在加入线程之前初始化语言环境，以便任何线程操作只读取到这些查找表。

详情

初始化流时，它会填充一个指针，该指针会加载语言环境的正确ctype（请参阅_M_ctype）： https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/basic_ios.h#L273

该错误指的是： https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/locale_facets.h#L875

如果两个线程同时初始化相同的区域设置，则可能是竞争条件。

这应该是线程安全的（虽然它可能仍然会出错）：

// Force ctype to be initialized in the base thread before forking
ostringstream dump;
dump << 1;

auto runner = []() {
    ostringstream oss;
    for (int i=0; i<100000; ++i)
        oss << i;
};

thread t1(runner), t2(runner);
t1.join(); t2.join();

Answer 2

对于2个不同的流，它是线程安全的：）

运算符＆lt;＆lt;（ostream＆amp;，obj）在两个不同的流线程上是否安全？

2 个答案: