Warning: pointer type mismatch

godna · 11-07-2014, 11:01 PM

Hi all, I'm new programming in C, so I had the next message in my code:

Dual.c:88:20: warning: pointer type mismatch in conditional expression [enabled by default] : &clientSa.sin6.sin6.sin6_addr,

Any help would be great

Code:

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int main(int argc,const char **argv)
{
  int serverPort = 1234;
  int rc;
  union {
    struct sockaddr_in sin;
    struct sockaddr_in6 sin6;
  } serverSa; 
  union {
    struct sockaddr_in sin;
    struct sockaddr_in6 sin6;
  } clientSa;

  int clientSaSize = sizeof(clientSa);
  int on = 1;
  int family;
  socklen_t serverSaSize;
  int c;
  char buf[INET6_ADDRSTRLEN];

  int s = socket(PF_INET6,SOCK_STREAM,0);
  if (s < 0)
  {
    fprintf(stderr, "IPv6 not active, falling back to IPv4...\n");
    s = socket(PF_INET,SOCK_STREAM,0);
    if (s < 0)
    {
      perror("socket failed");
      exit (1);
    }
    family = AF_INET;
    serverSaSize = sizeof(struct sockaddr_in);
  }
 else  /* got a v6 socket */
  {
    family = AF_INET6;
    serverSaSize = sizeof(struct sockaddr_in6);
  }
  printf("socket descriptor is
 rc = setsockopt(s,SOL_SOCKET,SO_REUSEADDR,&on,sizeof on);

  /* initialize the server's sockaddr */
  memset(&serverSa,0,sizeof(serverSa));
  switch(family)
  {
    case AF_INET:
      serverSa.sin.sin_family = AF_INET;
      serverSa.sin.sin_addr.s_addr = htonl(INADDR_ANY);
      serverSa.sin.sin_port = htons(serverPort);
      break;
    case AF_INET6:
      serverSa.sin6.sin6_family = AF_INET6;
      serverSa.sin6.sin6_addr = in6addr_any;
      serverSa.sin6.sin6_port = htons(serverPort);
  }
  rc = bind(s,(struct sockaddr *)&serverSa,serverSaSize);
  if (rc < 0)
  {
    perror("bind failed");
    exit(1);
  }
  rc = listen(s,10);
  if (rc < 0)
  {
    perror("listen failed");
    exit(1);
  }
  rc = accept(s,(struct sockaddr *)&clientSa,&clientSaSize);
  if (rc < 0)
  {
    perror("accept failed");
    exit(1);
  }
  c = rc;
  printf("Client address is: %s\n",
       inet_ntop(clientSa.sin.sin_family,
                 clientSa.sin.sin_family == AF_INET
                   ? &clientSa.sin.sin_addr
                   : &clientSa.sin6.sin6_addr,
                 buf, sizeof(buf)));

  if(clientSa.sin.sin_family == AF_INET6
    && ! IN6_IS_ADDR_V4MAPPED(&clientSa.sin6.sin6_addr))
    printf("Client is v6\n");
  else
    printf("Client is v4\n");

  rc = write(c,"hello\n",6);
  close (s);
  close (c);
  return 0;
}

business_kid · 11-08-2014, 05:13 AM

Usual cause of that one is that you have defined a variable, and used it as a pointer. The exact error message would give you a line number. M$ compilers just fix it and shut up, but the gnu gcc has gone to lengths to minimize all the C errors which result from sloppy coding, and can cause bugs.

When one is new that is a major pita. Try this link
http://www.tutorialspoint.com/cprogr...c_pointers.htm

johnsfine · 11-08-2014, 07:08 AM

The ? : construct requires that its second and third arguments be the same data type.
(A ? B : C) requires B and C to be the same type and only a limited amount of automatic casting will occur to get them to match. Otherwise it is the programmer's responsibility.

The inet_ntop function expects its second parameter to be void*, which so far as I recall would normally be automatically cast from another pointer type.

So the confusing issue might be that automatic casts in C are always determined bottom up, using no top down information. The fact that the result of (A ? B : C) would be cast to void* does not inform the compiler that each of B and C should be cast to void*. Instead the compiler first tries to automatically cast B and C to the same pointer type. Only if that succeeds would it cast the result to void*.

The simple solution is to force the casts to void* by adding the code I showed here in red.

Code:

                 clientSa.sin.sin_family == AF_INET
                   ? (void*) &clientSa.sin.sin_addr
                   : (void*) &clientSa.sin6.sin6_addr,

NevemTeve · 11-08-2014, 10:49 AM

Let's note that there is an union sockaddr, which az union of different sockaddr types.