运算符<<(ostream&,obj)在两个不同的流线程上是否安全？

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,然后运行它valgrind --tool=helgrind ./a.out.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operator <<在两个不同的ostringstream对象上调用.所以我想知道这是真的数据竞赛还是只是误报helgrind.

Answer 1

对于 2 个不同的流，它是线程安全的:)