我在学习套接字编程时从示例和阅读材料中拼凑了以下代码。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 ] = ' ';
}
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 ] = ' ';
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;
}
服务器.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 ] = ' ';
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;
}
编译和执行可以按如下方式完成：
>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：
似乎没有必要在客户端上bind()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，此外，它还能够将消息发送回它从recvfrom()填充的地址获得的地址。
看似没有必要bind()客户端套接字与自动将套接字绑定到INADDR_ANY有关吗？如果是这样，为什么客户端套接字必然生成的 IP 地址恰好是127.0.0.1？它是否可能是一个不同的"可用"IP，例如127.0.0.2？
如有必要，在第一次发送时会自动绑定。也许先发送或接收，不确定。
为什么客户端套接字绑定的结果 IP 地址恰好是 127.0.0.1
因为您要发送到本地地址。它将根据到目标的静态 IP 路由表选择正确的本地 IP 地址。
它可能是不同的"可用"IP，例如127.0.0.2
没办法，对不起。