我的问题
当使用两个线程通过 asio::ip::tcp::iostream
发送和接收数据时,如何避免数据竞争?
设计
我正在编写一个使用 asio::ip::tcp::iostream
进行输入和输出的程序。该程序通过端口 5555 接受来自(远程)用户的命令,并通过相同的 TCP 连接向用户发送消息。因为这些事件(从用户收到的命令或发送给用户的消息)异步发生,所以我有单独的传输和接收线程。
在这个玩具版本中,命令是“一”、“二”和“退出”。当然“退出”退出程序。其他命令什么都不做,任何无法识别的命令都会导致服务器关闭 TCP 连接。
传输的消息是简单的序列号消息,每秒发送一次。
在这个玩具版本和我尝试编写的真实代码中,传输和接收进程都使用阻塞 IO,因此似乎没有使用 std::的好方法mutex
或其他同步机制。 (在我的尝试中,一个进程会获取互斥锁然后阻塞,这不会为此工作。)
构建和测试
为了构建和测试它,我在 64 位 Linux 机器上使用 gcc 7.2.1 版和 valgrind 3.13。构建:
g++ -DASIO_STANDALONE -Wall -Wextra -pedantic -std=c++14 concurrent.cpp -o concurrent -lpthread
为了测试,我使用以下命令运行服务器:
valgrind --tool=helgrind --log-file=helgrind.txt ./concurrent
然后我在另一个窗口中使用 telnet 127.0.0.1 5555
创建到服务器的连接。 helgrind
正确指出存在数据竞争,因为 runTx
和 runRx
都试图异步访问同一个流:
==16188== Possible data race during read of size 1 at 0x1FFEFFF1CC by thread #1
==16188== Locks held: none
... many more lines elided
并发.cpp
#include <asio.hpp>
#include <iostream>
#include <fstream>
#include <thread>
#include <array>
#include <chrono>
class Console {
public:
Console() :
want_quit{false},
want_reset{false}
{}
bool getQuitValue() const { return want_quit; }
int run(std::istream *in, std::ostream *out);
bool wantReset() const { return want_reset; }
private:
int runTx(std::istream *in);
int runRx(std::ostream *out);
bool want_quit;
bool want_reset;
};
int Console::runTx(std::istream *in) {
static const std::array<std::string, 3> cmds{
"quit", "one", "two",
};
std::string command;
while (!want_quit && !want_reset && *in >> command) {
if (command == cmds.front()) {
want_quit = true;
}
if (std::find(cmds.cbegin(), cmds.cend(), command) == cmds.cend()) {
want_reset = true;
std::cout << "unknown command [" << command << "]\n";
} else {
std::cout << command << '\n';
}
}
return 0;
}
int Console::runRx(std::ostream *out) {
for (int i=0; !(want_reset || want_quit); ++i) {
(*out) << "This is message number " << i << '\n';
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
out->flush();
}
return 0;
}
int Console::run(std::istream *in, std::ostream *out) {
want_reset = false;
std::thread t1{&Console::runRx, this, out};
int status = runTx(in);
t1.join();
return status;
}
int main()
{
Console con;
asio::io_service ios;
// IPv4 address, port 5555
asio::ip::tcp::acceptor acceptor(ios,
asio::ip::tcp::endpoint{asio::ip::tcp::v4(), 5555});
while (!con.getQuitValue()) {
asio::ip::tcp::iostream stream;
acceptor.accept(*stream.rdbuf());
con.run(&stream, &stream);
if (con.wantReset()) {
std::cout << "resetting\n";
}
}
}
最佳答案
是的,您正在共享作为流基础的套接字,没有同步
Sidenote, same with the boolean flags, which can easily be "fixed" by changing:
std::atomic_bool want_quit; std::atomic_bool want_reset;
如何解决
说实话,我觉得没有什么好的解决办法。您自己说过:操作是异步的,因此如果您尝试同步执行它们,就会遇到麻烦。
你可以试着想想黑客。如果我们基于相同的底层套接字(文件描述符)创建一个单独的流对象会怎样。这不会非常容易,因为这样的流不是 Asio 的一部分。
但我们可以使用 Boost Iostreams 破解一个:
#define BOOST_IOSTREAMS_USE_DEPRECATED
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
// .... later:
// HACK: procure a _separate `ostream` to prevent the race, using the same fd
namespace bio = boost::iostreams;
bio::file_descriptor_sink fds(stream.rdbuf()->native_handle(), false); // close_on_exit flag is deprecated
bio::stream<bio::file_descriptor_sink> hack_ostream(fds);
con.run(stream, hack_ostream);
事实上,这在没有竞争的情况下运行(在同一个套接字上同时读取和写入 are fine,只要您不共享包装它们的非线程安全 Asio 对象)。
我推荐的是:
不要那样做。这是一个kludge。您使事情复杂化,显然是为了避免使用异步代码。我会咬紧牙关。
将 IO 机制从服务逻辑中分离出来并不太费力。您最终将摆脱随机限制(您可以考虑与多个客户端打交道,您可以完全不使用任何线程,等等)。
如果您想了解一些中间立场,请查看堆栈协程 (http://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/reference/spawn.html)
list
仅供引用
Note I refactored to remove the need for pointers. You're not transferring ownership, so a reference will do. In case you didn't know how to pass the reference to a
bind
/std::thread
constructor, the trick is in thestd::ref
you'll see.[For stress testing I have greatly reduced the delays.]
#include <boost/asio.hpp>
#include <iostream>
#include <fstream>
#include <thread>
#include <array>
#include <chrono>
class Console {
public:
Console() :
want_quit{false},
want_reset{false}
{}
bool getQuitValue() const { return want_quit; }
int run(std::istream &in, std::ostream &out);
bool wantReset() const { return want_reset; }
private:
int runTx(std::istream &in);
int runRx(std::ostream &out);
std::atomic_bool want_quit;
std::atomic_bool want_reset;
};
int Console::runTx(std::istream &in) {
static const std::array<std::string, 3> cmds{
{"quit", "one", "two"},
};
std::string command;
while (!want_quit && !want_reset && in >> command) {
if (command == cmds.front()) {
want_quit = true;
}
if (std::find(cmds.cbegin(), cmds.cend(), command) == cmds.cend()) {
want_reset = true;
std::cout << "unknown command [" << command << "]\n";
} else {
std::cout << command << '\n';
}
}
return 0;
}
int Console::runRx(std::ostream &out) {
for (int i=0; !(want_reset || want_quit); ++i) {
out << "This is message number " << i << '\n';
std::this_thread::sleep_for(std::chrono::milliseconds(1));
out.flush();
}
return 0;
}
int Console::run(std::istream &in, std::ostream &out) {
want_reset = false;
std::thread t1{&Console::runRx, this, std::ref(out)};
int status = runTx(in);
t1.join();
return status;
}
#define BOOST_IOSTREAMS_USE_DEPRECATED
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
int main()
{
Console con;
boost::asio::io_service ios;
// IPv4 address, port 5555
boost::asio::ip::tcp::acceptor acceptor(ios, boost::asio::ip::tcp::endpoint{boost::asio::ip::tcp::v4(), 5555});
while (!con.getQuitValue()) {
boost::asio::ip::tcp::iostream stream;
acceptor.accept(*stream.rdbuf());
{
// HACK: procure a _separate `ostream` to prevent the race, using the same fd
namespace bio = boost::iostreams;
bio::file_descriptor_sink fds(stream.rdbuf()->native_handle(), false); // close_on_exit flag is deprecated
bio::stream<bio::file_descriptor_sink> hack_ostream(fds);
con.run(stream, hack_ostream);
}
if (con.wantReset()) {
std::cout << "resetting\n";
}
}
}
测试:
netcat localhost 5555 <<<quit
This is message number 0
This is message number 1
This is message number 2
和
commands=( one two one two one two one two one two one two one two three )
while sleep 0.1; do echo ${commands[$(($RANDOM%${#commands}))]}; done | (while netcat localhost 5555; do sleep 1; done)
无限期地运行,偶尔会重置连接(当发送命令“三”时)。
关于c++ - 如何避免与 `asio::ip::tcp::iostream` 的数据竞争?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/48117493/