从套接字接收线程中的消息

Question

我正在编写一个套接字程序并在一个线程中接收一条消息。但是我遇到了分段错误。 当我直接接收而不启动线程时，没有这样的问题（显示在注释部分）。

在下面的代码中，我直接接收并将其发送给客户端。部分代码如下所示：

if (acceptor->start() == 0)
    {
        while (1)
        {
            stream = acceptor->accept();
            if (stream != NULL)
            {
                /*
                ssize_t len;
                char line[256];
                while ((len = stream->receive(line, sizeof(line))) > 0) {
                    line[len] = 0;
                    printf("received - %s\n", line);
                    stream->send(line, len);
                */    
                pthread_t sniffer_thread;
                if( pthread_create( &sniffer_thread, NULL, connection_handler,NULL) < 0)
                {
                     perror("could not create thread");
                     return 1;
                }
                //Now join the thread , so that we dont terminate before the thread
                pthread_join( sniffer_thread , NULL);
             }
                delete stream;
          }
       }
    exit(0);

现在，我在线程函数中收到了相同的信息。它显示分段错误。

代码如下所示。

void *connection_handler(void *arg)
{
    TCPStream* stream = NULL;
    ssize_t len;
    char line[256];
    while ((len = stream->receive(line, sizeof(line))) > 0)
    {
        line[len] = 0;
        printf("received - %s\n", line);
        stream->send(line, len);
    }
}

Valgrind 输出的一部分是

==5163== Memcheck, a memory error detector
==5163== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==5163== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==5163== Command: ./appdownload 9999 192.5.60
==5163== 
==5163== Thread 2:
==5163== Invalid read of size 4
==5163==    at 0x401975: TCPStream::receive(char*, unsigned long, int) (tcpstream.cpp:30)
==5163==    by 0x40177B: connection_handler(void*) (appdownload.cpp:68)
==5163==    by 0x4E3F181: start_thread (pthread_create.c:312)
==5163==    by 0x566947C: clone (clone.S:111)
==5163==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==5163== 
==5163== 
==5163== Process terminating with default action of signal 11 (SIGSEGV)
==5163==  Access not within mapped region at address 0x0
==5163==    at 0x401975: TCPStream::receive(char*, unsigned long, int) (tcpstream.cpp:30)
==5163==    by 0x40177B: connection_handler(void*) (appdownload.cpp:68)
==5163==    by 0x4E3F181: start_thread (pthread_create.c:312)
==5163==    by 0x566947C: clone (clone.S:111)
==5163==  If you believe this happened as a result of a stack
==5163==  overflow in you`enter code here`r program's main thread (unlikely but
==5163==  possible), you can try to increase the size of the
==5163==  main thread stack using the --main-stacksize= flag.
==5163==  The main thread stack size used in this run was 8388608.

Answer 1

TCPStream* stream = NULL; // HERE
ssize_t len;
char line[256];
while ((len = stream->receive(line, sizeof(line))) > 0) // HERE

您在 NULL 指针上调用 receive。 Valgrind 告诉你：

==5163== Thread 2:
==5163== Invalid read of size 4
==5163==    at 0x401975: TCPStream::receive(char*, unsigned long, int) (tcpstream.cpp:30) // HERE
==5163==    by 0x40177B: connection_handler(void*) (appdownload.cpp:68)
==5163==    by 0x4E3F181: start_thread (pthread_create.c:312)
==5163==    by 0x566947C: clone (clone.S:111)
==5163==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
              //  ^- HERE

如果你需要将TCPStream*从主线程传递给新的线程函数，你可以使用pthread_create的第四个参数：

pthread_create() 函数在调用中启动一个新线程 过程。新线程通过调用 start_routine() 开始执行； arg 作为 start_routine() 的唯一参数传递。 （来源：man pthread_create）

主线程：

TCPStream *stream = acceptor->accept();
(...)
pthread_create( &sniffer_thread, NULL, connection_handler, stream)
                                                      //   ^^^^^^

工作线程：

void *connection_handler(void *arg)
{
    TCPStream* stream = (TCPStream *)arg;
    //                  ^^^^^^^^^^^^^^^^

Answer 2

在显示 valgrind 跟踪之后，很明显您只是在空指针上调用 receive。

---

嗯，让我们看看。

char line[256];                                              // 256 bytes space
while ((len = stream->receive(line, sizeof(line))) > 0)      // receive up to 256 bytes
{
    line[len] = 0;                                           // put 257th byte in

我想您可以为 0 多使用一个字节 - 尝试接收最多 sizeof(line)-1。

或者只使用std::vector 然后push_back 那个0。