Question

see the following code:

accept(sockfd, (struct sockaddr*)&cliaddr, &slen);
cout << inet_ntop(AF_INET, cliaddr.sin_addr, ipv4addr, 100);

my client connects from localhost.
i get an absurd address in the output. this is not my ip address. everytime i run the code i get a different ip address. when i ping that ip address i don’t get any response.

what is the reason.

i am running suse linux on a virtual machine in windows vista.

Update:

bzero(&cliaddr, sizeof(cliaddr));
int connfd = accept(sockfd, (struct sockaddr*)&cliaddr, &slen);

if (sem_wait(&mutex) < 0)
    err_sys("sem_init error");

char ipv4addr[100];
cout << inet_ntop(AF_INET, &cliaddr.sin_addr, ipv4addr, 100) << endl;

//const char* p = inet_ntop(AF_INET, &cliaddr.sin_addr, ipv4addr, 100);
//cout << p << endl;

//cout << (void*)p << " " << (void*)ipv4addr << endl;

this returns address as 0.0.0.0

if i uncomment the lines, i get the correct address in all the lines, 127.0.0.1

Answer 1

You are missing the 4th parameter in your call to inet_ntop(). Here’s a working example:

  int sockfd, fd;
  struct sockaddr_in saddr;
  socklen_t len = sizeof( saddr );
  char addr_buf[INET_ADDRSTRLEN]; /* defined in <netinet/in.h> */

  /* ... socket(), bind(), listen() */

  bzero( &saddr, len );
  if (( fd = accept( sockfd, ( struct sockaddr* )&saddr, &len )) == -1 )
  { perror( "accept" ); exit( 1 ); } /* watch out for EINTR */

  if ( inet_ntop( AF_INET, &saddr.sin_addr, addr_buf,
    INET_ADDRSTRLEN ) == NULL )
  { perror( "inet_ntop" ); exit( 1 ); }

  printf( "accepted connection from [%s:%d]\n",
    addr_buf, ntohs( saddr.sin_port ));
  ...

Always check for errors when talking to network.