c++ - UDP服务器无法发送回复，并且sendto返回-1，并且errno为22

Question

UDP客户端/服务器代码出现此问题。此代码是从https://www.geeksforgeeks.org/udp-server-client-implementation-c/复制并进行了最小限度的修改，以修复编译错误并在服务器代码中显示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] = '\0'; 
    printf("Server : %s\n", 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] = '\0'; 
    printf("Client : %s\n", buffer); 
    int str_len = 0;
    sprintf(buffer, "%s", "hello from server");
    str_len = strlen(buffer);
    printf("Sendinger buffer: <%s>, size=%d\n", buffer, (int) (str_len+1));
    n = sendto(sockfd,buffer, (int)(str_len+1), 
        MSG_CONFIRM, (const struct sockaddr *) &cliaddr, 
            len); 

    printf("n=%d, errno=%d\n", 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

我所看到的是，客户端成功地将消息发送到服务器，服务器从客户端打印了消息，并且当服务器将回复发送回客户端时，它从sendto得到-1，而errno为22。客户端保持挂起状态等待回复信息。

这是服务器的输出:

[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22是EINVAL，这意味着您传递了无效的参数。

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

也可能是传递给MSG_CONFIRM的sendto标志，在您的应用程序中没有意义。发送时传递零标志。