c - linux c 中的 sendmsg() 调用出错

Question

我在 UDP 套接字编程中遇到了一些问题。我想使用 sendmsg() 和 recvmsg() API 以便能够在客户端和服务器之间进行通信。客户端代码返回错误号 105 {ENOBUFS}。我不知道它显示此错误的原因是什么，因为根据这些 API 的描述，它是书面的

ENOBUFS

The  output  queue for a network interface was full.  This generally indicates that the interface has stopped sending, but may be caused by transient congestion.  (Normally, this does  not  occur in Linux. Packets are just silently dropped when a device queue overflows.)

所以，这个错误并不常见，那么我也面临着这个问题。如果有人遇到类似的问题，请分享一些信息。我正在寻找解决方案的提示，而不是现成的解决方案。另外，请不要使用替代 API。需要以下代码才能工作。

服务器代码

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <string.h>
#include <errno.h>

#define PORT            32000
#define BUFSIZE         8192

int main()
{
    int sockfd,n;
    struct sockaddr_in servaddr,cliaddr;
    socklen_t len;
    char mesg[1000];
    char message_control_array[BUFSIZE];
    int count = 1;
    struct iovec io;
    int ret = 0;
    int ttlval = 255;

    /* Messageheader structure to pass to recvmsg call*/
    struct msghdr header;

    /* This structure will contain the individual ancillary messages*/
    struct cmsghdr *cmsg;

    /* Filling up the control data related variables.*/
    header.msg_name         = &servaddr;
    header.msg_namelen      = sizeof (struct sockaddr_in);
    header.msg_control      = message_control_array;
    header.msg_controllen   = BUFSIZE;
    header.msg_iov          = &io;
    header.msg_iov->iov_base= mesg;
    header.msg_iov->iov_len = 1000;
    header.msg_iovlen       = 1;

    /*create UDP socket*/
    if ((sockfd=socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP)) < 0)
    {
            printf("Error 1: Socket\n");
            exit(1);
    }

    ret = setsockopt (sockfd, IPPROTO_IP, IP_TTL, &ttlval, sizeof(ttlval));
    if ( ret == -1)
    {
            printf("setsockopt failing for IP_TTL, errno %d\n", errno);
    }

    bzero(&servaddr,sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
    servaddr.sin_port=htons(PORT);

    if ((bind(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr))) == -1)
    {
            printf("Error 2:bind\n");
            exit(1);
    }
    while (1)
{
  //len = sizeof(cliaddr);
  //if ((n = recvfrom(sockfd,mesg,1000,0,(struct sockaddr *)&cliaddr,&len)) == -1)
  if ((n = recvmsg(sockfd,&header, 0)) == -1)
    {
            printf("recvmsg error!!!!!!!!!!!!!\n\n");
    }
    /*Send the received packet back to the sender [client]*/
    //sendto(sockfd,mesg,n,0,(struct sockaddr *)&cliaddr,sizeof(cliaddr));

    printf("-------------------------------------------------------\n");
    mesg[n] = 0;
    printf("Received the following:\n");
    printf("%s\n",mesg);
    printf("The info from the msghdr structure\n");
    printf("The source address is %s\n", inet_ntoa(servaddr.sin_addr.s_addr));
    printf("The source address length is %d\n", (int)header.msg_namelen);
    for (cmsg = CMSG_FIRSTHDR(&header); cmsg != NULL ; cmsg = CMSG_NXTHDR(&header, cmsg))
    {
            printf("number %d cmsg structure\n", count++);
            //if
    }
  printf("-------------------------------------------------------\n");
}
return 0;
}

客户端代码

/* Sample UDP client */

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <errno.h>
#include <netinet/in.h>

#define PORT 3200
#define BUFSIZE 512

int main(int argc, char**argv)
{
    int sockfd,n;
    struct sockaddr_in servaddr,cliaddr;
    char sendline[BUFSIZE];
    char recvline[BUFSIZE];
    int ret = 0;
    int ttlval = 255;
            /*required for sending ancillary data using sendmsg() call*/
            struct msghdr header;
    struct cmsghdr *cmsg;
    struct iovec io;

    if (argc != 2)
    {
            printf("usage:  %s <Server IP address>\n", argv[0]);
            exit(1);
    }

    sockfd=socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);

    ret = setsockopt (sockfd, IPPROTO_IP, IP_TTL, &ttlval, sizeof (ttlval));
    if (ret == -1)
    {
            printf("setsockopt failing for IP_TTL , errno %d\n", errno);
    }

    bzero(&servaddr,sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr=inet_addr(argv[1]);
    servaddr.sin_port=htons(PORT);

    header.msg_name         = &servaddr;
    header.msg_namelen      = sizeof (struct sockaddr_in);
    header.msg_iov          = &io;
    header.msg_iov->iov_base= sendline;
    header.msg_iov->iov_len = BUFSIZE;
    header.msg_iovlen       = 1;

    while (1)
    {
 tryagain:printf("Enter the string to be sent\n");
     //scanf("%s", sendline);
     if ((fgets(sendline, 100, stdin)) == NULL)
     {
             printf("error in reading from stdin\n\n");
             goto tryagain;
     }
    header.msg_iov->iov_len = strlen(sendline) - 1;
    printf("kddkdd %d %d %s\n", sizeof(sendline), strlen(sendline), sendline);
     //sendto(sockfd,sendline,strlen(sendline),0,(struct sockaddr *)&servaddr,sizeof(servaddr));
     if ((sendmsg(sockfd, &header, 0)) == -1)
     {
             printf("Error in sendmsg,\t [errno = %d]\n", errno);
     }
#if 0
     if ((n=recvfrom(sockfd,recvline,10000,0,NULL,NULL)) == -1)
     {
             printf("recvfrom error\n");
             exit(1);
     }
     recvline[n]=0;
     fputs(recvline,stdout);
     printf("packet sent\n\n");
#endif
    }
    return 0;
}

输出是这样的

[root@localhost udp]# ./client 30.5.42.194
Enter the string to be sent
sourav
Error in sendmsg,        [errno = 105]
Enter the string to be sent
  
  

我使用的是 RHEL (Red Hat 4.1.2-48)，内核为 2.6.18-194.el5，gcc 版本为 4.1.2

Answer 1

您没有正确初始化 msghdr 结构。您需要在声明时这样做:

struct msghdr header = { 0 };

或以后:

header.msg_controllen = 0;
header.msg_control = NULL;
header.msg_flags = 0;

否则 msg_controllen 内容就是垃圾。