如果我理解正确,在Unix世界中启动一个子进程的方法是调用fork(),然后调用exec *().另外,如果我正确理解,调用fork()将会重复所有的文件描述符等等,这些都需要在子进程中关闭,除非标记为FD_CLOEXEC(因此在调用exec *()时被原子关闭).
Boost Asio需要通过调用fork()才能正常通知调用notify_fork().但是,在多线程程序中,这会产生以下几个问题：
>如果我正确理解,默认情况下,插件是由子进程继承的.它们可以设置为SOCK_CLOEXEC – 但不能直接创建*,因此如果从另一个线程创建子进程,则导致定时窗口.
> notify_fork()要求没有其他线程调用任何其他io_service函数,也没有与io_service关联的任何其他I / O对象的任何函数.这似乎不是可行的 – 所有的程序是多线程的,因为一个原因.
>如果我理解正确,fork()和exec *()之间的任何函数调用都需要异步信号安全(见fork() documentation).没有关于notify_fork()调用异步信号安全的文档.实际上,如果我看看Boost Asio的源代码(至少在版本1.54中),可能会有pthread_mutex_lock的电话,这不是异步信号安全(参见Signal Concepts,还有其他的电话是不在白名单上).
问题#1我可以通过分离子进程和套接字文件的创建来解决问题,以便我可以确保在创建的套接字和设置SOCK_CLOEXEC之间的窗口中不会创建子进程.问题#2是棘手的,我可能需要确保所有的asio处理程序线程都被阻止,做叉子,然后重新创建它们,这是最好的,最糟糕的是真的很糟糕(我的未决定时器呢？ ).问题#3似乎完全不可能正确使用.
如何在一个多线程程序中与fork()exec *()正确使用Boost Asio？
…还是我“分叉”？
请让我知道,如果我误解了任何基本概念(我提出Windows编程,而不是* nix …).
编辑：
* – 实际上可以创建直接在Linux上设置的SOCK_CLOEXEC的套接字,从2.6.27开始可用(参见socket() documentation).在Windows上,相应的标志WSA_FLAG_NO_HANDLE_INHERIT可用于Windows 7 SP 1 / Windows Server 2008 R2 SP 1(参见WSASocket() documentation). OS X似乎并不支持.
解决方法
io_service::notify_fork()在孩子中调用不安全.但是,Boost.Asio希望它可以基于
fork() support进行调用,因为这是当孩子关闭父母以前的内部文件描述符并创建新的内容时.虽然Boost.Asio显式列出了调用io_service :: notify_fork()的前提条件,保证在fork()期间内部组件的状态,但是对
implementation的简要概述则表明std :: vector :: push_back()可能会从免费商店分配内存,并且分配不能保证是异步信号安全的.
就这样说,一个可能值得考虑的解决方案是fork(),当它仍然是单线程时.子进程将保持单线程,并在父进程通过进程间通信被告知执行fork()和exec()时执行.这种分离通过在执行fork()和exec()时消除管理多个线程的状态的需要来简化问题.
以下是一个完整的示例,演示此方法,多线程服务器将通过UDP接收文件名,子进程将执行fork()和exec()来运行文件名/usr/bin/touch.为了使这个例子变得更加可读,我已经选择使用stackful coroutines.
#include <unistd.h> // execl,fork
#include <iostream>
#include <string>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/thread.hpp>
/// @brief launcher receives a command from inter-process communication,/// and will then fork,allowing the child process to return to
/// the caller.
class launcher
{
public:
launcher(boost::asio::io_service& io_service,boost::asio::local::datagram_protocol::socket& socket,std::string& command)
: io_service_(io_service),socket_(socket),command_(command)
{}
void operator()(boost::asio::yield_context yield)
{
std::vector<char> buffer;
while (command_.empty())
{
// Wait for server to write data.
std::cout << "launcher is waiting for data" << std::endl;
socket_.async_receive(boost::asio::null_buffers(),yield);
// Resize buffer and read all data.
buffer.resize(socket_.available());
socket_.receive(boost::asio::buffer(buffer));
io_service_.notify_fork(boost::asio::io_service::fork_prepare);
if (fork() == 0) // child
{
io_service_.notify_fork(boost::asio::io_service::fork_child);
command_.assign(buffer.begin(),buffer.end());
}
else // parent
{
io_service_.notify_fork(boost::asio::io_service::fork_parent);
}
}
}
private:
boost::asio::io_service& io_service_;
boost::asio::local::datagram_protocol::socket& socket_;
std::string& command_;
};
using boost::asio::ip::udp;
/// @brief server reads filenames from UDP and then uses
/// inter-process communication to delegate forking and exec
/// to the child launcher process.
class server
{
public:
server(boost::asio::io_service& io_service,short port)
: io_service_(io_service),launcher_socket_(socket),socket_(boost::make_shared<udp::socket>(
boost::ref(io_service),udp::endpoint(udp::v4(),port)))
{}
void operator()(boost::asio::yield_context yield)
{
udp::endpoint sender_endpoint;
std::vector<char> buffer;
for (;;)
{
std::cout << "server is waiting for data" << std::endl;
// Wait for data to become available.
socket_->async_receive_from(boost::asio::null_buffers(),sender_endpoint,yield);
// Resize buffer and read all data.
buffer.resize(socket_->available());
socket_->receive_from(boost::asio::buffer(buffer),sender_endpoint);
std::cout << "server got data: ";
std::cout.write(&buffer[0],buffer.size());
std::cout << std::endl;
// Write filename to launcher.
launcher_socket_.async_send(boost::asio::buffer(buffer),yield);
}
}
private:
boost::asio::io_service& io_service_;
boost::asio::local::datagram_protocol::socket& launcher_socket_;
// To be used as a coroutine,server must be copyable,so make socket_
// copyable.
boost::shared_ptr<udp::socket> socket_;
};
int main(int argc,char* argv[])
{
std::string filename;
// Try/catch provides exception handling,but also allows for the lifetime
// of the io_service and its IO objects to be controlled.
try
{
if (argc != 2)
{
std::cerr << "Usage: <port>\n";
return 1;
}
boost::thread_group threads;
boost::asio::io_service io_service;
// Create two connected sockets for inter-process communication.
boost::asio::local::datagram_protocol::socket parent_socket(io_service);
boost::asio::local::datagram_protocol::socket child_socket(io_service);
boost::asio::local::connect_pair(parent_socket,child_socket);
io_service.notify_fork(boost::asio::io_service::fork_prepare);
if (fork() == 0) // child
{
io_service.notify_fork(boost::asio::io_service::fork_child);
parent_socket.close();
boost::asio::spawn(io_service,launcher(io_service,child_socket,filename));
}
else // parent
{
io_service.notify_fork(boost::asio::io_service::fork_parent);
child_socket.close();
boost::asio::spawn(io_service,server(io_service,parent_socket,std::atoi(argv[1])));
// Spawn additional threads.
for (std::size_t i = 0; i < 3; ++i)
{
threads.create_thread(
boost::bind(&boost::asio::io_service::run,&io_service));
}
}
io_service.run();
threads.join_all();
}
catch (std::exception& e)
{
std::cerr << "Exception: " << e.what() << "\n";
}
// Now that the io_service and IO objects have been destroyed,all internal
// Boost.Asio file descriptors have been closed,so the execl should be
// in a clean state. If the filename has been set,then exec touch.
if (!filename.empty())
{
std::cout << "creating file: " << filename << std::endl;
execl("/usr/bin/touch","touch",filename.c_str(),static_cast<char*>(0));
}
}
1号航站楼：
$ls a.out example.cpp $./a.out 12345 server is waiting for data launcher is waiting for data server got data: a server is waiting for data launcher is waiting for data creating file: a server got data: b server is waiting for data launcher is waiting for data creating file: b server got data: c server is waiting for data launcher is waiting for data creating file: c ctrl + c $ls a a.out b c example.cpp
2号航站楼：
$nc -u 127.0.0.1 12345 actrl + dbctrl + dcctrl + d
版权声明：本文内容由互联网用户自发贡献，该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容， 请发送邮件至 dio@foxmail.com 举报，一经查实，本站将立刻删除。
