c++ - 在 recvfrom 系统调用期间取消 C++11 std::thread？

Question

我正在使用 C++11 std::thread。它的主循环包括一个阻塞的 recvfrom() 调用，用于监听到达 DATAGRAM 套接字的 UDP 数据包，以及一些复杂的代码，用于解析消息并在此过程中操纵 STL 容器的加载。

线程属于一个类(helloexchange)，由构造函数启动，应该在析构函数中取消。出于显而易见的原因，我不想强​​行终止线程，因为这可能会破坏部分位于类外部的数据结构。

当使用 pthread 而不是 std::thread 时，有 pthread_cancel 方法，它与 pthread_setcancelstate，提供了我需要的所有功能:它只会在某些系统调用中阻塞时取消线程，并且可以对某些部分完全禁用取消。一旦再次启用，就会执行取消。这是工作 pthread 代码的完整示例:

#include <arpa/inet.h>
#include <unistd.h>
#include <iostream>
#include <sys/socket.h>
#include <sys/types.h>
#include <cstdio>
#include <pthread.h>
#include <net/if.h>
#include <ifaddrs.h>

int sock;

void *tfun(void *arg) {
    std::cout << "Thread running" << std::endl;
    while(true) {
        char buf[256];
        struct sockaddr_in addr;
        addr.sin_family = AF_INET;
        socklen_t addrlen = sizeof(addr);
        //allow cancelling the thread
        pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);

        //perform the blocking recvfrom syscall
        int size = recvfrom(sock, (void *) buf, sizeof(buf), 0,
            (struct sockaddr *) &addr, &addrlen);

        //disallow cancelling the thread
        pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);

        if(size < 0) {
            perror("Could not receive packet");
            return NULL;
        } else {
            //process the packet in the most complex ways
            //you could imagine.
            std::cout << "Packet received: " << size << " bytes";
            std::cout << std::endl;
        }
    }
    return NULL;
}


int main() {
    //open datagram socket
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if(sock < 0) {
        perror("Could not open socket");
        return 1;
    }
    //bind socket to port
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(1337);
    addr.sin_addr.s_addr = 0;
    if(bind(sock, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        perror("Could not bind datagram socket");
        return 2;
    }
    //create the listener thread
    pthread_t t;
    if(pthread_create(&t, NULL, tfun, NULL) != 0) {
        perror("Could not thread");
        return 3;
    };
    //wait
    std::cin.get();
    //cancel the listener thread. pthread_cancel does not block.
    std::cout << "Cancelling thread" << std::endl;
    if(pthread_cancel(t) != 0) {
        perror("Could not cancel thread");
    }
    //join (blocks until the thread has actually cancelled).
    std::cout << "Joining thread" << std::endl;
    if(pthread_join(t, NULL) != 0) {
        perror("Could not join thread");
    } else {
        std::cout << "Join successful" << std::endl;
    }
    //close socket
    if(close(sock) != 0) {
        perror("Could not close socket");
    };
}

但是，std::thread不支持cancel，std::this_thread也不支持setcancelstate (您会找到一个引用 here )。但是，它确实支持 native_handle，它返回内部使用的 pthread_t id。不过，将 pthread_cancel() 发送到线程的 native 句柄的明显方法会导致段错误:

#include <iostream>
#include <thread>
#include <cstdio>

#include <arpa/inet.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <pthread.h>
#include <net/if.h>
#include <ifaddrs.h>

int sock;

void tfun() {
    std::cout << "Thread running" << std::endl;
    while(true) {
        char buf[256];
        struct sockaddr_in addr;
        addr.sin_family = AF_INET;
        socklen_t addrlen = sizeof(addr);

        //perform the blocking recvfrom syscall
        int size = recvfrom(sock, (void *) buf, sizeof(buf), 0,
            (struct sockaddr *) &addr, &addrlen);

        if(size < 0) {
            perror("Could not receive packet");
            return;
        } else {
            //process the packet in the most complex ways
            //you could imagine.
            std::cout << "Packet received: " << size << " bytes";
            std::cout << std::endl;
        }
    }
    return;
}

int main() {
    //open datagram socket
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if(sock < 0) {
        perror("Could not open socket");
        return 1;
    }
    //bind socket to port
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(1337);
    addr.sin_addr.s_addr = 0;
    if(bind(sock, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        perror("Could not bind datagram socket");
        return 2;
    }
    //the listener thread
    std::thread *t = new std::thread(&tfun);
    //wait
    std::cin.get();
    //cancel the listener thread. pthread_cancel does not block.
    std::cout << "Cancelling thread" << std::endl;
    if(pthread_cancel(t->native_handle()) != 0) {
        perror("Could not cancel thread");
    }
    //join (blocks until the thread has actually cancelled).
    std::cout << "Joining thread" << std::endl;
    t->join();
    delete t;
    //close socket
    if(close(sock) != 0) {
        perror("Could not close socket");
    };
}

结果:

(gdb) run
Starting program: /tmp/test/test-dbg 
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[New Thread 0x7ffff6550700 (LWP 11329)]
Thread running

Cancelling thread
Joining thread

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff6550700 (LWP 11329)]
0x00007ffff6e67b45 in __gnu_cxx::__verbose_terminate_handler() () from /usr/lib/libstdc++.so.6

有什么方法可以在 std::thread 在系统调用中阻塞时取消它吗？

编辑

我不是在寻求跨平台解决方案；符合 POSIX 标准的解决方案就足够了。

Answer 1

我将根据众所周知的 self-pipe trick 提出一个解决方法用于解锁 select(2)并避开整个困惑的线程取消业务。

因为您知道 socket(7) 的 IP 地址和端口你的线程正在阻塞，只是 sendto(2)从您的主线程向它发送一些众所周知的哨兵数据包，这表明是时候跳出循环了。

这样您就不必破坏 std::thread 抽象并且可以保持合理的可移植性。

编辑0:

如果您不喜欢解决方法，请将其称为技术 :)