`std::osyncstream` 如何管理输出流？

Question

我想知道 std::osyncstream 对象如何防止数据竞争条件？它是否锁定了一些互斥体？

我具体说的是下面这个程序：

#include <iostream>
#include <fstream>
#include <thread>
#include <syncstream>


void worker( const std::size_t startValue, const std::size_t stopValue, std::ostream& os )
{
    for ( auto idx { startValue }; idx < stopValue; ++idx )
    {
        std::osyncstream out { os };
        out << "thread: " << std::this_thread::get_id( ) << "; work: " << idx << '\n';
    }
}

void runThreads( std::ostream& os )
{
    std::jthread t1 { worker, 10000, 20000, std::ref( os ) };
    std::jthread t2 { worker, 20000, 30000, std::ref( os ) };
}

int main( )
{
    std::ofstream file { "out.txt" };
    runThreads( file );
}

以上代码源码可查看here。虽然我做了一些细微的修改，让它变得更好更安全。

这个简单的程序将 20,000 行打印到一个文件中，而不会产生混乱的输出。

可能的输出：

thread: 2; work: 10000
thread: 3; work: 20000
thread: 2; work: 10001
thread: 2; work: 10002
thread: 2; work: 10003
thread: 2; work: 10004
thread: 2; work: 10005
thread: 2; work: 10006
.
.
.

幕后发生了什么？这两个线程如何相互通信？他们有单独的 syncstream 对象副本吗？这个对象（即out）如何管理输出流os?

Answer 1

Does it lock some mutex?

是的，间接的。 std::basic_osyncstream class，其中 osyncstream 是 basic_osyncstream<char> 形式的特例，派生自 std::basic_ostream，通常只有一个 'extra' std::basic_syncbuf class 的成员。 From cppreference:

Typical implementation of std::basic_osyncstream holds only one member: the wrapped std::basic_syncbuf.

正是basic_syncbuf对象实现了输出同步，防止了数据竞争。同样，from cppreference（加粗我的）：

Typical implementation of std::basic_syncbuf holds a pointer to the wrapped std::basic_streambuf, a boolean flag indicating whether the buffer will transmit its contents to the wrapped buffer on sync (flush), a boolean flag indicating a pending flush when the policy is to not emit on sync, an internal buffer that uses Allocator (such as std::string), and a pointer to a mutex used to synchronize emit between multiple threads accessing the same wrapped stream buffer (these mutexes may be in a hash map with pointers to basic_streambuf objects used as keys).

`std::osyncstream` 如何管理输出流？

How does `std::osyncstream` manage the out stream?

c++

multithreading

mutex

c++20