c++ - 为什么在这个 UDP 客户端/服务器示例中不需要绑定(bind)客户端套接字？

Question

在学习套接字编程时，我根据示例和阅读 Material 拼凑了以下代码。这是一个简单的 UDP 客户端/服务器示例:

客户端.cpp

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <cerrno>
#include <ctime>
#include <cstring>
#include <unistd.h>
#include <cstdlib>

int main( int argc, char* argv[] )
{
  errno = 0;
  int retval = 0;

  // Populate server sockaddr_in.
  sockaddr_in svr_si;
  svr_si.sin_family = AF_INET;
  const char* svr_addr = "127.0.0.3";
  svr_si.sin_addr.s_addr = inet_addr( svr_addr );
  svr_si.sin_port = htons( 9090 );

  sockaddr_in peer_si;                      // Peer sockaddr_in (populated by recv())
  socklen_t peer_addr_len = sizeof peer_si; // Peer sockaddr_in length

  srand( time( NULL ) );

  // Create socket.
  int sd = socket( PF_INET, SOCK_DGRAM, 0 );

  char buf[ 1024 ]; // Receive buffer
  ssize_t bytes;    // Sent/received bytes
  ssize_t msg_len;  // Length of sent message

  bool cont = true;
  while ( cont )
  {
    // Create random message of length 10 or "quit"
    if ( ! ( rand() % 7 ) )
    {
      std::strcpy( buf, "quit" );
    }
    else
    {
      msg_len = rand() % 10;
      for ( int i = 0; i < msg_len; ++i )
      {
        buf[ i ] = rand() % ('z' - 'A' + 1) + 'A';
      }
      buf[ msg_len ] = '\0';
    }
    std::cout << "Client sending \"" << buf << "\" to ip(" << svr_addr <<
                 ") port(" << ntohs( svr_si.sin_port ) << ")." << std::endl;

    // Send message to server.
    if ( -1 == ( bytes = sendto( sd, buf, strlen( buf ), 0,
                                 (struct sockaddr*)&svr_si,
                                 sizeof( svr_si ) ) ) )
    {
      std::cout << "send() failed." << std::endl;
    }

    // Receive (blocking) on unbound socket; save sender address to peer_si.
    bytes = recvfrom( sd, buf, sizeof buf - 1, 0, (sockaddr*)&peer_si,
                      &peer_addr_len );
    if ( bytes >= 0 )
    {
      buf[ bytes ] = '\0';

      std::cout << "Client received " << bytes << " bytes from addr(" <<
        inet_ntoa( peer_si.sin_addr ) << ") port(" << peer_si.sin_port <<
        "): [" << buf << "]" << std::endl;
      std::cout << "Server acked." << std::endl;
      if ( ! std::strcmp( buf, "quit" ) )
      {
        std::cout << "Client quitting." << std::endl;
        cont = false;
      }
    }
    else
    {
      std::cout << "recvfrom() failed: " << errno << "(" << strerror( errno )
        << ")." << std::endl;
      retval = 1;
      cont = false;
    }
  }

  if ( -1 != sd )
  {
    close( sd );
  }

  return retval;
}

server.cpp

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <cerrno>
#include <cstring>
#include <unistd.h>

int main( int argc, char* argv[] )
{
  errno = 0;
  int retval = 0;

  // Create a socket.
  int sd = socket( PF_INET, SOCK_DGRAM, 0 );

  // Initialize a sockaddr_in with this' (the server's) address.
  sockaddr_in si;
  si.sin_family = AF_INET;
  si.sin_addr.s_addr = inet_addr( "127.0.0.3" );
  si.sin_port = htons( 9090 );

  sockaddr_in peer_si;                        // Peer sockaddr_in
  socklen_t peer_addr_len = sizeof peer_si;   // Peer sockaddr length
  memset( &peer_si, 0, peer_addr_len );

  // Bind address to socket.
  if ( ! bind( sd, (sockaddr*)&si, sizeof si ) )
  {
    char buf[ 1024 ];                           // Receive buffer
    ssize_t bytes;                              // Bytes received
    bool cont = true;

    while ( cont )
    {
      // Receive (blocking) on bound socket.
      bytes = recvfrom( sd, buf, sizeof buf - 1, 0, (sockaddr*)&peer_si,
                        &peer_addr_len );
      if ( bytes >= 0 )
      {
        buf[ bytes ] = '\0';

        std::cout << "Server received " << bytes << " bytes from addr(" <<
                     inet_ntoa( peer_si.sin_addr ) << ") port(" <<
                     peer_si.sin_port << ") family(" << peer_si.sin_family <<
                     "): [" << buf << "]" << std::endl;
        std::cout << "Server acking." << std::endl;

        if ( ! std::strcmp( buf, "quit" ) )
        {
          std::cout << "Server quitting." << std::endl;
          cont = false;
        }
        else
        {
          // Send "ack" back to whatever was read into peer_si.
          bytes = sendto( sd, "ack", 3, 0, (struct sockaddr*)&peer_si,
                          peer_addr_len );
          if ( 3 != bytes )
          {
            std::cout << "sendto() failed: " << errno << "(" <<
                         strerror( errno ) << ")" << std::endl;
            retval = 1;
            cont = false;
          }
        }
      }
      else
      {
        std::cout << "recvfrom() failed: " << errno << "(" << strerror( errno )
          << ")." << std::endl;
        retval = 1;
        cont = false;
      }
    }
  }
  else
  {
    std::cout << "bind() failed: " << errno << "(" << strerror( errno ) << ")."
              << std::endl;
    retval = 1;
  }

  if ( -1 != sd )
  {
    if ( 0 != peer_si.sin_addr.s_addr )
    {
      sendto( sd, "quit", 4, 0, (struct sockaddr*)&peer_si, peer_addr_len );
    }
    close( sd );
  }

  return retval;
}

编译执行可以按如下方式进行:

>g++ --version
g++ (GCC) 4.8.3 20140911 (Red Hat 4.8.3-7)
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

>g++ -g client.cpp -o client
>g++ -g server.cpp -o server

>./server &
[1] 19106
>./client 
Client sending "vw" to ip(127.0.0.3) port(9090).
Server received 2 bytes from addr(127.0.0.1) port(12004) family(2): [vw]
Server acking.
Client received 3 bytes from addr(127.0.0.3) port(33315): [ack]
Server acked.
Client sending "M" to ip(127.0.0.3) port(9090).
Server received 1 bytes from addr(127.0.0.1) port(12004) family(2): [M]
Server acking.
Client received 3 bytes from addr(127.0.0.3) port(33315): [ack]
Server acked.
Client sending "quit" to ip(127.0.0.3) port(9090).
Server received 4 bytes from addr(127.0.0.1) port(12004) family(2): [quit]
Server acking.
Server quitting.
Client received 4 bytes from addr(127.0.0.3) port(33315): [quit]
Server acked.
Client quitting.
[1]+  Done                    ./server

我的问题:

似乎不需要 bind() 客户端上的套接字 - 为什么？您会在 client.cpp 中注意到，我所做的只是在发送消息之前创建套接字。服务器似乎可以很好地接收消息，而且能够将消息发送回它从 recvfrom() 填充的地址获得的地址。

客户端套接字的 bind() 看似不必要与自动将套接字绑定(bind)到 INADDR_ANY 有什么关系吗？如果是这样，为什么客户端套接字绑定(bind)的结果 IP 地址恰好是 127.0.0.1？它可能是一个不同的“可用”IP，例如127.0.0.2?

Answer 1

第一次发送时，如有必要，会自动绑定(bind)。也许先发送或接收，不确定。

Why does the resulting IP address that the client's socket is bound to happen to be 127.0.0.1

因为您要发送到本地地址。它将根据到达目的地的静态 IP 路由表选择正确的本地 IP 地址。

Could it ever be a different "available" IP, e.g. 127.0.0.2

帮不上忙，抱歉。