HTML中forks的含义,doc/html/boost_asio/example/cpp03/fork/daemon.cpp

最新推荐文章于 2025-06-17 13:47:51 发布

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#HTML中forks的含义

这个示例展示了如何使用Boost.Asio库在C++中实现一个UDP日间服务器。服务器接收请求并发送当前时间作为响应。代码涉及信号处理、守护进程化、错误处理和I/O上下文操作。

doc/html/boost_asio/example/cpp03/fork/daemon.cpp

// daemon.cpp

// ~~~~~~~~~~

// Distributed under the Boost Software License, Version 1.0. (See accompanying

// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#include

using boost::asio::ip::udp;

class udp_daytime_server

{

public:

udp_daytime_server(boost::asio::io_context& io_context)

: socket_(io_context, udp::endpoint(udp::v4(), 13))

{

start_receive();

}

private:

void start_receive()

{

socket_.async_receive_from(

boost::asio::buffer(recv_buffer_), remote_endpoint_,

boost::bind(&udp_daytime_server::handle_receive, this, _1));

}

void handle_receive(const boost::system::error_code& ec)

{

if (!ec)

{

using namespace std; // For time_t, time and ctime;

time_t now = time(0);

std::string message = ctime(&now);

boost::system::error_code ignored_ec;

socket_.send_to(boost::asio::buffer(message),

remote_endpoint_, 0, ignored_ec);

}

start_receive();

}

udp::socket socket_;

udp::endpoint remote_endpoint_;

boost::array recv_buffer_;

};

int main()

{

try

{

boost::asio::io_context io_context;

// Initialise the server before becoming a daemon. If the process is

// started from a shell, this means any errors will be reported back to the

// user.

udp_daytime_server server(io_context);

// Register signal handlers so that the daemon may be shut down. You may

// also want to register for other signals, such as SIGHUP to trigger a

// re-read of a configuration file.

boost::asio::signal_set signals(io_context, SIGINT, SIGTERM);

signals.async_wait(

boost::bind(&boost::asio::io_context::stop, &io_context));

// Inform the io_context that we are about to become a daemon. The

// io_context cleans up any internal resources, such as threads, that may

// interfere with forking.

io_context.notify_fork(boost::asio::io_context::fork_prepare);

// Fork the process and have the parent exit. If the process was started

// from a shell, this returns control to the user. Forking a new process is

// also a prerequisite for the subsequent call to setsid().

if (pid_t pid = fork())

{

if (pid > 0)

{

// We're in the parent process and need to exit.

// When the exit() function is used, the program terminates without

// invoking local variables' destructors. Only global variables are

// destroyed. As the io_context object is a local variable, this means

// we do not have to call:

// io_context.notify_fork(boost::asio::io_context::fork_parent);

// However, this line should be added before each call to exit() if

// using a global io_context object. An additional call:

// io_context.notify_fork(boost::asio::io_context::fork_prepare);

// should also precede the second fork().

exit(0);

}

else

{

syslog(LOG_ERR | LOG_USER, "First fork failed: %m");

return 1;

}

// Make the process a new session leader. This detaches it from the

// terminal.

setsid();

// A process inherits its working directory from its parent. This could be

// on a mounted filesystem, which means that the running daemon would

// prevent this filesystem from being unmounted. Changing to the root

// directory avoids this problem.

chdir("/");

// The file mode creation mask is also inherited from the parent process.

// We don't want to restrict the permissions on files created by the

// daemon, so the mask is cleared.

umask(0);

// A second fork ensures the process cannot acquire a controlling terminal.

if (pid_t pid = fork())

{

if (pid > 0)

{

exit(0);

}

else

{

syslog(LOG_ERR | LOG_USER, "Second fork failed: %m");

return 1;

}

// Close the standard streams. This decouples the daemon from the terminal

// that started it.

close(0);

close(1);

close(2);

// We don't want the daemon to have any standard input.

if (open("/dev/null", O_RDONLY) < 0)

{

syslog(LOG_ERR | LOG_USER, "Unable to open /dev/null: %m");

return 1;

}

// Send standard output to a log file.

const char* output = "/tmp/asio.daemon.out";

const int flags = O_WRONLY | O_CREAT | O_APPEND;

const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;

if (open(output, flags, mode) < 0)

{

syslog(LOG_ERR | LOG_USER, "Unable to open output file %s: %m", output);

return 1;

}

// Also send standard error to the same log file.

if (dup(1) < 0)

{

syslog(LOG_ERR | LOG_USER, "Unable to dup output descriptor: %m");

return 1;

}

// Inform the io_context that we have finished becoming a daemon. The

// io_context uses this opportunity to create any internal file descriptors

// that need to be private to the new process.

io_context.notify_fork(boost::asio::io_context::fork_child);

// The io_context can now be used normally.

syslog(LOG_INFO | LOG_USER, "Daemon started");

io_context.run();

syslog(LOG_INFO | LOG_USER, "Daemon stopped");

}

catch (std::exception& e)

{

syslog(LOG_ERR | LOG_USER, "Exception: %s", e.what());

std::cerr << "Exception: " << e.what() << std::endl;

}