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Getting port number of socket after bind with sin_port=0

Posted on 2004-04-09
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Last Modified: 2012-05-04
How do I retrieve the random port number after I do a bind on a socket?

    me.sin_family = AF_INET;
    me.sin_addr.s_addr = htonl(INADDR_ANY);
    me.sin_port = htons(0);
    rc = bind(sd, (struct sockaddr *) &me, sizeof(me))

    printf("Sending on port %d\n", random_port);

In other words, how do I get random_port?
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Question by:iemonslice
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Kdo earned 100 total points
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Hi iemonslice,

Call getsockname().

getsockname() will store the assigned address and port into the sockaddr structure."


Good Luck,
Kent
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by:iemonslice
Comment Utility
When I do a getsockname(), I get the following error:

getsockname: Bad address

What does the above mean?

Below is the code segment if you need to see it:
===============================================
    if (getsockname(sd, (struct sockaddr *) &me, (int *)sizeof(me)) == -1){
        perror("getsockname");
      exit(1);
    }
    printf("Listening on port %d\n", me.sin_port);
===============================================
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Expert Comment

by:Kdo
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Hi iemonslice,

Based on your first call, you need to pass 'rc', not 'sd'.

    if (getsockname(rc, (struct sockaddr *) &me, (int *)sizeof(me)) == -1){
        perror("getsockname");
     exit(1);
    }
    printf("Listening on port %d\n", me.sin_port);


Kent
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by:iemonslice
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I thought rc was the correct value to pass in, but when I tried it earlier, it gave me an error:

getsockname: Socket operation on non-socket

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by:Kdo
Comment Utility
Hi iemonslice,

Hmm.....   My mistake.  Your code should take this sequence:

  SocketId = socket ();
  bind (SocketId, ...);
  getsockname (SocketId, ...);


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Author Comment

by:iemonslice
Comment Utility
That's the way I have it right now. I'll paste the section in it's entirety below so it's easier than having to look above.

With the below I still have the getsockname: Bad address error.
 
===============================================
    if ((sd = socket(AF_INET, SOCK_DGRAM, 0)) == -1){
        perror("socket");
      exit(1);
    }

    me.sin_family = AF_INET;
    me.sin_addr.s_addr = htonl(INADDR_ANY);
    me.sin_port = htons(0);
    if ((bind(sd, (struct sockaddr *) &me, sizeof(me))) == -1){
        perror("bind");
        exit(1);
    }

    if (getsockname(sd, (struct sockaddr *) &me, (int *)sizeof(me)) == -1){
        perror("getsockname");
      exit(1);
    }
===============================================
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LVL 45

Expert Comment

by:Kdo
Comment Utility
Hi iemonslice,

Print the address that is in 'me'.



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by:iemonslice
Comment Utility
Do you mean me.sin_addr.s_addr? If so, printing it out right after the bind outputs 0.
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Author Comment

by:iemonslice
Comment Utility
Anyone have any ideas about this problem of mine?
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Author Comment

by:iemonslice
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Ah the problem was the last parameter of getsockname().

Instead of sizeof(me), a pointer to the size worked.

socklen_t slen = sizeof(me);
getsock..(x,x,&slent);
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Expert Comment

by:Kdo
Comment Utility
Cool.

Where does this stand now?
Kent
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Author Comment

by:iemonslice
Comment Utility
I actually knew about getsockname() before posting on experts-exchange, but going back to my first post, I never mentioned getsockname() and my problem. You did tell me the correct answer so you get the points. :)
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