c - gethostbyaddr : Success 上出现错误

Question

我正在用c语言构建一个多客户端UDP服务器，但是当我尝试从系统连接到我的服务器时，我收到此错误gethostbyaddr上的错误:成功请找到下面的服务器代码。我已经尝试了类似问题的解决方案( gethostbyaddr() returns NULL but errno result in SUCCESS )，但我无法让它工作。任何帮助将不胜感激

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

#define BUFSIZE 1024

/*
* error - wrapper for perror
*/
 void error(char *msg) {
  perror(msg);
  exit(1);
}

int main(int argc, char **argv) {
  int sockfd; /* socket */
  int portno; /* port to listen on */
  int clientlen; /* byte size of client's address */
  struct sockaddr_in serveraddr; /* server's addr */
  struct sockaddr_in clientaddr; /* client addr */
  struct hostent *hostp; /* client host info */
  char buf[BUFSIZE]; /* message buf */
  char *hostaddrp; /* dotted decimal host addr string */
  int optval; /* flag value for setsockopt */
  int n; /* message byte size */
  FILE *fp; /* file variable */
  char str[10];
  int i = 0;
  char userlist[10];
  int array_size;
  char line[256];
  int cred,flag;

  /*
  * check command line arguments
  */
  if (argc != 2) {
     fprintf(stderr, "usage: %s <port>\n", argv[0]);
     exit(1);
  }
  portno = atoi(argv[1]);

  /*
  * socket: create the parent socket
  */
  sockfd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sockfd < 0)
    error("ERROR opening socket");

  /* setsockopt: Handy debugging trick that lets
  * us rerun the server immediately after we kill it;
  * otherwise we have to wait about 20 secs.
  * Eliminates "ERROR on binding: Address already in use" error.
  */
  optval = 1;
  setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR,
        (const void *)&optval , sizeof(int));

  /*
  * build the server's Internet address
  */
  bzero((char *) &serveraddr, sizeof(serveraddr));
  memset(&serveraddr,0,sizeof(serveraddr));
  serveraddr.sin_family = AF_INET;
  serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
  serveraddr.sin_port = htons((unsigned short)portno);

  /*
  * bind: associate the parent socket with a port
  */
  if (bind(sockfd, (struct sockaddr *) &serveraddr,
     sizeof(serveraddr)) < 0)
     error("ERROR on binding");

  /*
  * main loop: wait for a datagram, then echo it
  */
  clientlen = sizeof(clientaddr);
  while (1) {

   /*
   * recvfrom: receive a UDP datagram from a client
   */
   bzero(buf, BUFSIZE);
   n = recvfrom(sockfd, buf, BUFSIZE, 0,
     (struct sockaddr *) &clientaddr, &clientlen);
   if (n < 0)
      error("ERROR in recvfrom");

   /*
 * gethostbyaddr: determine who sent the datagram
 */
hostp = gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr,
          sizeof(clientaddr.sin_addr.s_addr), AF_INET);
if (hostp == NULL)
  error("ERROR on gethostbyaddr");
hostaddrp = inet_ntoa(clientaddr.sin_addr);
if (hostaddrp == NULL)
  error("ERROR on inet_ntoa\n");
printf("server received datagram from %s (%s)\n",
   hostp->h_name, hostaddrp);
printf("server received %d/%d bytes: %s\n", strlen(buf), n, buf);

fp = fopen("users.txt", "r");
while (fgets(line, sizeof(line), fp)) {
  //printf("%s\n",line);
  cred = strncmp(buf,line,strlen(line)-1);
  //printf("%d",strlen(line)-1);
  if(cred == 0){
    printf("Authenticated....");
    flag = 1;
    break;
  }
  else{
    printf("Invalid username/password");
  }
}
 fclose(fp);

Answer 1

gethostbyaddr() 需要一个指向 struct in_addr 的指针作为第一个参数，对于您显示的代码，该参数将为 &clientaddr.sin_addr 。

形成相关(Linux)文档(man gethostbyaddr):

[...] The host address argument is a pointer to a struct of a type depending on the address type, for example a struct in_addr * (probably obtained via a call to inet_addr(3)) for address type AF_INET.

<小时/>

gethostbyaddr() 在 h_errno 中设置错误代码，而不是在 errno 中。

形成相关(Linux)文档(man gethostbyaddr):

RETURN VALUE

[...[ The gethostbyname() and gethostbyaddr() functions return the hostent structure or a null pointer if an error occurs. On error, the h_errno variable holds an error number.

手册页中也给出了可能的错误代码:

ERRORS

The variable h_errno can have the following values:

• HOST_NOT_FOUND

  The specified host is unknown.

• NO_ADDRESS or NO_DATA

  The requested name is valid but does not have an IP address.

• NO_RECOVERY

  A nonrecoverable name server error occurred.

• TRY_AGAIN

  A temporary error occurred on an authoritative name server. Try again later.