UDP 服务器无法发送回复，发送返回 -1，errno 为 22

Question

我在UDP客户端/服务器代码中遇到了这个问题。此代码是从 https://www.geeksforgeeks.org/udp-server-client-implementation-c/复制的，并进行了最低限度的修改，以修复编译错误并在服务器代码中打印 errno。

// Client side implementation of UDP client-server model 
#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <string.h> 
#include <sys/types.h> 
#include <sys/socket.h> 
#include <arpa/inet.h> 
#include <netinet/in.h> 
#define PORT     8080 
#define MAXLINE 1024 
// Driver code 
int main() { 
int sockfd; 
char buffer[MAXLINE]; 
char *hello = "Hello from client"; 
struct sockaddr_in   servaddr; 
// Creating socket file descriptor 
if ( (sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
perror("socket creation failed"); 
exit(EXIT_FAILURE); 
} 
memset(&servaddr, 0, sizeof(servaddr)); 
// Filling server information 
servaddr.sin_family = AF_INET; 
servaddr.sin_port = htons(PORT); 
servaddr.sin_addr.s_addr = INADDR_ANY; 
int n; socklen_t len;
sendto(sockfd, (const char *)hello, strlen(hello), 
MSG_CONFIRM, (const struct sockaddr *) &servaddr, 
sizeof(servaddr)); 
printf("Hello message sent.n"); 
n = recvfrom(sockfd, (char *)buffer, MAXLINE, 
MSG_WAITALL, (struct sockaddr *) &servaddr, 
&len); 
buffer[n] = ''; 
printf("Server : %sn", buffer); 
close(sockfd); 
return 0; 
} 

// Server side implementation of UDP client-server model 
#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <string.h> 
#include <sys/types.h> 
#include <sys/socket.h> 
#include <arpa/inet.h> 
#include <netinet/in.h> 
#include <errno.h>
#define PORT     8080 
#define MAXLINE 1024 
// Driver code 
int main() { 
int sockfd; 
char buffer[MAXLINE]; 
struct sockaddr_in servaddr, cliaddr; 
// Creating socket file descriptor 
if ( (sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
perror("socket creation failed"); 
exit(EXIT_FAILURE); 
} 
memset(&servaddr, 0, sizeof(servaddr)); 
memset(&cliaddr, 0, sizeof(cliaddr)); 
// Filling server information 
servaddr.sin_family = AF_INET; // IPv4 
servaddr.sin_addr.s_addr = INADDR_ANY; 
servaddr.sin_port = htons(PORT); 
// Bind the socket with the server address 
if ( bind(sockfd, (const struct sockaddr *)&servaddr, 
sizeof(servaddr)) < 0 ) 
{ 
perror("bind failed"); 
exit(EXIT_FAILURE); 
} 
int n; socklen_t len;
n = recvfrom(sockfd, (char *)buffer, MAXLINE, 
MSG_WAITALL, ( struct sockaddr *) &cliaddr, 
&len); 
buffer[n] = ''; 
printf("Client : %sn", buffer); 
int str_len = 0;
sprintf(buffer, "%s", "hello from server");
str_len = strlen(buffer);
printf("Sendinger buffer: <%s>, size=%dn", buffer, (int) (str_len+1));
n = sendto(sockfd,buffer, (int)(str_len+1), 
MSG_CONFIRM, (const struct sockaddr *) &cliaddr, 
len); 
printf("n=%d, errno=%dn", n, errno);
printf("Hello message sent.n"); 
return 0; 
} 

我用简单的命令编译了服务器和客户端：

g++ udpserv.cpp -o udpserv
g++ client.cpp -o client

我看到的是客户端成功地将消息发送到服务器，服务器打印来自客户端的消息，当服务器将回复发送回客户端时，它从 sendto 获得 -1，errno 是 22。客户端保持挂起状态，等待回复消息。

这是来自服务器的输出：

[user@localhost udpserver]$ ./udpserv 
Client : Hello from client
Sendinger buffer: <hello from server>, size=18
n=-1, errno=22
Hello message sent.
[user@localhost udpserver]$ 

这是客户端的输出：

[user@localhost client]$./client 
Hello message sent.

Answer 1

错误22EINVAL，这意味着您传递了一个无效的参数。

在这种情况下，无效参数可能是未初始化的len参数：您需要在调用recvfrom之前将其初始化为sizeof cliaddr。

它也可能是您传递给sendto的MSG_CONFIRM标志，这在您的应用程序中毫无意义。发送时传递零标志。