C++ UDP。为什么recvfrom()没有阻塞?
我正在使用 UDP 编写一些简单的客户端/服务器代码。程序工作正常,但如果我只启动客户端,recvfrom方法不会阻塞。但是,当我删除 sendto 方法时,recvfrom 开始阻塞。知道发生了什么事吗?
这是客户端代码:
int server_length; /* Length of server struct */
char send_buffer[256] = "hi"; /* Data to send */
time_t current_time; /* Time received */
while(true)
{
/* Tranmsit data to get time */
server_length = sizeof(struct sockaddr_in);
if (sendto(m_oSocket, send_buffer, (int)strlen(send_buffer) + 1, 0, (struct sockaddr *)&m_oServer, server_length) == -1)
{
fprintf(stderr, "Error transmitting data.\n");
continue;
}
/* Receive time */
if (recvfrom(m_oSocket, (char *)¤t_time, (int)sizeof(current_time), 0, (struct sockaddr *)&m_oServer, &server_length) < 0)
{
fprintf(stderr, "Error receiving data.\n");
continue;
}
/* Display time */
printf("Current time: %s\n", ctime(¤t_time));
Sleep(1000);
}
这是初始化:
unsigned short m_iPortnumber;
struct sockaddr_in m_oServer;
struct sockaddr_in m_oClient;
SOCKET m_oSocket;
WSADATA w; /* Used to open Windows connection */
int a1, a2, a3, a4; /* Server address components in xxx.xxx.xxx.xxx form */
a1 = 192;
a2 = 168;
a3 = 2;
a4 = 14;
m_iPortnumber = 52685;
/* Open windows connection */
if (WSAStartup(0x0101, &w) != 0)
{
fprintf(stderr, "Could not open Windows connection.\n");
exit(0);
}
/* Open a datagram socket */
m_oSocket = socket(AF_INET, SOCK_DGRAM, 0);
if (m_oSocket == INVALID_SOCKET)
{
fprintf(stderr, "Could not create socket.\n");
WSACleanup();
exit(0);
}
/* Clear out server struct */
memset((void *)&m_oServer, '\0', sizeof(struct sockaddr_in));
/* Set family and port */
m_oServer.sin_family = AF_INET;
m_oServer.sin_port = htons(m_iPortnumber);
/* Set server address */
m_oServer.sin_addr.S_un.S_un_b.s_b1 = (unsigned char)a1;
m_oServer.sin_addr.S_un.S_un_b.s_b2 = (unsigned char)a2;
m_oServer.sin_addr.S_un.S_un_b.s_b3 = (unsigned char)a3;
m_oServer.sin_addr.S_un.S_un_b.s_b4 = (unsigned char)a4;
/* Clear out client struct */
memset((void *)&m_oClient, '\0', sizeof(struct sockaddr_in));
/* Set family and port */
m_oClient.sin_family = AF_INET;
m_oClient.sin_addr.s_addr=INADDR_ANY;
m_oClient.sin_port = htons(0);
/* Bind local address to socket */
if (bind(m_oSocket, (struct sockaddr *)&m_oClient, sizeof(struct sockaddr_in)) == -1)
{
fprintf(stderr, "Cannot bind address to socket.\n");
closesocket(m_oSocket);
WSACleanup();
exit(0);
}
I am writing some simple client/server code using UDP. The program works fine, but if I only start the client, the recvfrom method does not block. However, when I remove the sendto method, recvfrom starts to block. Any idea of what is going on?
Here is the client side code:
int server_length; /* Length of server struct */
char send_buffer[256] = "hi"; /* Data to send */
time_t current_time; /* Time received */
while(true)
{
/* Tranmsit data to get time */
server_length = sizeof(struct sockaddr_in);
if (sendto(m_oSocket, send_buffer, (int)strlen(send_buffer) + 1, 0, (struct sockaddr *)&m_oServer, server_length) == -1)
{
fprintf(stderr, "Error transmitting data.\n");
continue;
}
/* Receive time */
if (recvfrom(m_oSocket, (char *)¤t_time, (int)sizeof(current_time), 0, (struct sockaddr *)&m_oServer, &server_length) < 0)
{
fprintf(stderr, "Error receiving data.\n");
continue;
}
/* Display time */
printf("Current time: %s\n", ctime(¤t_time));
Sleep(1000);
}
And here is the initialization:
unsigned short m_iPortnumber;
struct sockaddr_in m_oServer;
struct sockaddr_in m_oClient;
SOCKET m_oSocket;
WSADATA w; /* Used to open Windows connection */
int a1, a2, a3, a4; /* Server address components in xxx.xxx.xxx.xxx form */
a1 = 192;
a2 = 168;
a3 = 2;
a4 = 14;
m_iPortnumber = 52685;
/* Open windows connection */
if (WSAStartup(0x0101, &w) != 0)
{
fprintf(stderr, "Could not open Windows connection.\n");
exit(0);
}
/* Open a datagram socket */
m_oSocket = socket(AF_INET, SOCK_DGRAM, 0);
if (m_oSocket == INVALID_SOCKET)
{
fprintf(stderr, "Could not create socket.\n");
WSACleanup();
exit(0);
}
/* Clear out server struct */
memset((void *)&m_oServer, '\0', sizeof(struct sockaddr_in));
/* Set family and port */
m_oServer.sin_family = AF_INET;
m_oServer.sin_port = htons(m_iPortnumber);
/* Set server address */
m_oServer.sin_addr.S_un.S_un_b.s_b1 = (unsigned char)a1;
m_oServer.sin_addr.S_un.S_un_b.s_b2 = (unsigned char)a2;
m_oServer.sin_addr.S_un.S_un_b.s_b3 = (unsigned char)a3;
m_oServer.sin_addr.S_un.S_un_b.s_b4 = (unsigned char)a4;
/* Clear out client struct */
memset((void *)&m_oClient, '\0', sizeof(struct sockaddr_in));
/* Set family and port */
m_oClient.sin_family = AF_INET;
m_oClient.sin_addr.s_addr=INADDR_ANY;
m_oClient.sin_port = htons(0);
/* Bind local address to socket */
if (bind(m_oSocket, (struct sockaddr *)&m_oClient, sizeof(struct sockaddr_in)) == -1)
{
fprintf(stderr, "Cannot bind address to socket.\n");
closesocket(m_oSocket);
WSACleanup();
exit(0);
}
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评论(3)
sendto
失败的原因有很多种。有些(例如 arp 失败)会在sendto
期间导致错误。其他的,例如ICMP port unreachable
,可能会在您下次使用套接字时报告。您的
recvfrom
调用实际上可能正在获取响应您的传出数据包而发送的 ICMP 数据包。第二个
recvfrom
是否按预期阻塞?There are a variety of ways that
sendto
can fail. Some, such as arp failure, will cause an error duringsendto
. Other, such asICMP port unreachable
, may be reported when you next use the socket.Your
recvfrom
call could actually be fetching the ICMP packet sent in response to your outgoing packet.Does a second
recvfrom
block as expected?套接字需要设置为 BLOCKING/NON-BLOCKING。
设置阻止
设置非阻塞
Socket required to be set BLOCKING/NON-BLOCKING.
Set BLOCKING
Set NON-BLOCKING
看起来您正在以相同的方式设置服务器套接字和客户端套接字。对于服务器来说,初始化看起来不错,但对于客户端来说,您需要绑定到端口 0。
事实上,对于这两个客户端,您都可以执行 INADDR_ANY (IP 0.0.0.0),它不会绑定到特定接口,而是允许正确端口上的任何连接。
It looks like you're setting up the server socket and the client socket the same way. The initialization looks good for a server, but for the client, you'll want to bind to port 0.
In fact, for both of them you can do INADDR_ANY (IP 0.0.0.0), which doesn't bind to a specific interface, but instead allows any connection on the correct port.