#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();
}
用g ++ 6.2.1编译上面的代码,然后用valgrind --tool=helgrind ./a.out
运行它。 Helgrind会抱怨:运算符<<(ostream&,obj)在两个不同的线程上是否安全?
==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"
看来,这两个线程调用locale_facet.h:widen
造成数据竞争,因为有出现在该函数没有同步,即使operator <<
被称为在两个不同的ostringstream
对象。所以我想知道这是真的是数据竞赛还是只是一个误报helgrind
。
不管是什么标准说,这应该* *是线程安全的。 –
标准状态*多线程并发访问流对象(27.8,27.9),流缓冲对象(27.6)或C库流(27.9.2) 可能会导致数据竞争(1.10),除非另有说明( 27.4)。 [注意:数据竞赛 导致未定义的行为(1.10)。 - 结束注释] *所以我的猜测是流在后端使用了一些不同步的全局状态。或者这是一个误报。我的直觉说这应该是安全的。 – NathanOliver
我认为这应该是安全的根据§17.6.5.9/ 2:*“C++标准库函数不应直接或间接访问除当前线程之外的线程可访问的对象(1.10),除非通过直接或间接访问对象函数的参数,包括'this'。“*所以我会说这是一个不符合的实现或者是一个误报。 –