TCP socket programming error

Question

Learning socket programming, and the following is the cut/paste of error parts.

FYI, I was following this tutorial.

Undrestood gethostbyname() returns struct hostent

struct hostent *gethostbyname(const char *name);

with the following code.

  1 #include <stdio.h>
  2 #include <sys/types.h>
  3 #include <sys/socket.h>
  4 #include <netinet/in.h>
  5 #include <stdlib.h>
  6 #include <strings.h>
  7 
  8 int main(int argc, char *argv[])
  9 {
 10     int sockfd, portno, n;
 11     struct sockaddr_in serv_addr;
 12     struct hostent *server;
 13 
 14     server = gethostbyname(argv[1]);
 15 
 16     /* compose serv_addr */
 17     bzero( (char *)&serv_addr, sizeof(serv_addr) );
 18     serv_addr.sin_family = AF_INET;
 19     bcopy( (char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);
 20     serv_addr.sin_port = htons(portno);
 21 
 22     return 0;
 23 }

I have the following gcc error/warning

$gcc client2.c
client2.c: In function ‘main’:
client2.c:14: warning: assignment makes pointer from integer without a cast
client2.c:19: error: dereferencing pointer to incomplete type
client2.c:19: error: dereferencing pointer to incomplete type

Please point me what I m doing wrong?

Also: gethostbyame() and gethostbyaddr() are somehow deprecated. You might better use getnameinfo() and getaddrinfo(). — alk
– alk, Commented May 12, 2012 at 7:54

Asherah · Accepted Answer · 2012-05-12 00:25:23Z

9

Try adding this to the top:

#include <netdb.h>

It's a required include for gethostbyname. If you type man gethostbyname at the command line, you'll get the man page, which opens:

Screenie of "gethostbyname(3)" manpage.

The manpages detail which headers you need to include (under SYNOPSIS).

(Hit q to exit man.)

edited May 12, 2012 at 0:25

answered May 11, 2012 at 23:43

Asherah

19.4k5 gold badges56 silver badges74 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user405725 Over a year ago

+1. Though it would be nice to elaborate on C, include files and how did you come up with that netdb.h. Perhaps at least mentioning man gethostbyname would be good...

Asherah Over a year ago

@VladLazarenko: thanks for the feedback! Have added some information on the manpage lookup :)

sarnold · Accepted Answer · 2012-05-11 23:50:38Z

Note in the bzero(3) manpage:

   4.3BSD.  This function is deprecated (marked as LEGACY in
   POSIX.1-2001): use memset(3) in new programs.  POSIX.1-2008
   removes the specification of bzero().

You should definitely use memset(3) instead of bzero(3) -- the API is only slightly more verbose but that's not entirely horrible. Same story for bcopy(3) and memcpy(3).

memset(3) and memcpy(3) are in <string.h>, not <strings.h>, so change that too.

You're using too many casts in your code; when the prototype for a function includes void *, it'll accept any pointer type as a parameter and the compiler will correctly track the types. Adding a cast will tell the compiler that you know better -- and will, quite often, mask actual errors and warnings.

Also note that gethostbyname(3) requires the <netdb.h> header, too.

With these changes in place, your code compiles with fewer warnings:

$ CFLAGS="-Wall -Wextra" make client2
cc -Wall -Wextra    client2.c   -o client2
client2.c: In function ‘main’:
client2.c:11:25: warning: unused variable ‘n’ [-Wunused-variable]
client2.c:11:9: warning: unused variable ‘sockfd’ [-Wunused-variable]
client2.c:9:14: warning: unused parameter ‘argc’ [-Wunused-parameter]
client2.c:21:31: warning: ‘portno’ is used uninitialized in this function [-Wuninitialized]

(None of them should be a surprise; it's code in progress. Good on you for trying to build the code slowly...)

Collectives™ on Stack Overflow

TCP socket programming error

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related