CODEWITHSUNDEEP
csockets
allenhwkim
allenhwkim

Reputation: 27748

TCP socket programming error

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?

Upvotes: 2

Views: 1207

Answers (2)

Talya
Talya

Reputation: 19347

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.)

Upvotes: 9

sarnold
sarnold

Reputation: 104080

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...)

Upvotes: 5

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