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Programming UNIX Sockets in C - Frequently Asked Questions: Writing Client Applications (TCP/SOCK_STREAM) Previous Next Table of Contents

3. Writing Client Applications (TCP/SOCK_STREAM)

3.1 How do I convert a string into an internet address?

If you are reading a host's address from the command line, you may not know if you have an aaa.bbb.ccc.ddd style address, or a style address. What I do with these, is first try to use it as a aaa.bbb.ccc.ddd type address, and if that fails, then do a name lookup on it. Here is an example:

/* Converts ascii text to in_addr struct.  NULL is returned if the 
   address can not be found. */
struct in_addr *atoaddr(char *address) {
  struct hostent *host;
  static struct in_addr saddr;

  /* First try it as aaa.bbb.ccc.ddd. */
  saddr.s_addr = inet_addr(address);
  if (saddr.s_addr != -1) {
    return &saddr;
  host = gethostbyname(address);
  if (host != NULL) {
    return (struct in_addr *) *host->h_addr_list;
  return NULL;

3.2 How can my client work through a firewall/proxy server?

If you are running through separate proxies for each service, you shouldn't need to do anything. If you are working through sockd, you will need to "socksify" your application. Details for doing this can be found in the package itself, which is available at:

you can get the socks faq at:

3.3 Why does connect() succeed even before my server did an accept()?

From Andrew Gierth (

Once you have done a listen() call on your socket, the kernel is primed to accept connections on it. The usual UNIX implementation of this works by immediately completing the SYN handshake for any incoming valid SYN segments (connection attempts), creating the socket for the new connection, and keeping this new socket on an internal queue ready for the accept() call. So the socket is fully open before the accept is done.

The other factor in this is the 'backlog' parameter for listen(); that defines how many of these completed connections can be queued at one time. If the specified number is exceeded, then new incoming connects are simply ignored (which causes them to be retried).

3.4 Why do I sometimes loose a server's address when using more than one server?

From Andrew Gierth (

Take a careful look at struct hostent. Notice that almost everything in it is a pointer? All these pointers will refer to statically allocated data.

For example, if you do:

    struct hostent *host = gethostbyname(hostname);

then (as you should know) a subsequent call to gethostbyname() will overwrite the structure pointed to by 'host'.

But if you do:

    struct hostent myhost;
    struct hostent *hostptr = gethostbyname(hostname);
    if (hostptr) myhost = *host;

to make a copy of the hostent before it gets overwritten, then it still gets clobbered by a subsequent call to gethostbyname(), since although myhost won't get overwritten, all the data it is pointing to will be.

You can get round this by doing a proper 'deep copy' of the hostent structure, but this is tedious. My recommendation would be to extract the needed fields of the hostent and store them in your own way.

3.5 How can I set the timeout for the connect() system call?

From Richard Stevens (

Normally you cannot change this. Solaris does let you do this, on a per-kernel basis with the ndd tcp_ip_abort_cinterval parameter.

The easiest way to shorten the connect time is with an alarm() around the call to connect(). A harder way is to use select(), after setting the socket nonblocking. Also notice that you can only shorten the connect time, there's normally no way to lengthen it.

3.6 Should I bind() a port number in my client program, or let thesystem choose one for me on the connect() call?

From Andrew Gierth (

** Let the system choose your client's port number **

The exception to this, is if the server has been written to be picky about what client ports it will allow connections from. Rlogind and rshd are the classic examples. This is usually part of a Unix-specific (and rather weak) authentication scheme; the intent is that the server allows connections only from processes with root privilege. (The weakness in the scheme is that many O/Ss (e.g. MS-DOS) allow anyone to bind any port.)

The rresvport() routine exists to help out clients that are using this scheme. It basically does the equivalent of socket() + bind(), choosing a port number in the range 512..1023.

If the server is not fussy about the client's port number, then don't try and assign it yourself in the client, just let connect() pick it for you.

If, in a client, you use the naive scheme of starting at a fixed port number and calling bind() on consecutive values until it works, then you buy yourself a whole lot of trouble:

The problem is if the server end of your connection does an active close. (E.G. client sends 'QUIT' command to server, server responds by closing the connection). That leaves the client end of the connection in CLOSED state, and the server end in TIME_WAIT state. So after the client exits, there is no trace of the connection on the client end.

Now run the client again. It will pick the same port number, since as far as it can see, it's free. But as soon as it calls connect(), the server finds that you are trying to duplicate an existing connection (although one in TIME_WAIT). It is perfectly entitled to refuse to do this, so you get, I suspect, ECONNREFUSED from connect(). (Some systems may sometimes allow the connection anyway, but you can't rely on it.)

This problem is especially dangerous because it doesn't show up unless the client and server are on different machines. (If they are the same machine, then the client won't pick the same port number as before). So you can get bitten well into the development cycle (if you do what I suspect most people do, and test client & server on the same box initially).

Even if your protocol has the client closing first, there are still ways to produce this problem (e.g. kill the server).

3.7 Why do I get "connection refused" when the server isn't running?

The connect() call will only block while it is waiting to establish a connection. When there is no server waiting at the other end, it gets notified that the connection can not be established, and gives up with the error message you see. This is a good thing, since if it were not the case clients might wait for ever for a service which just doesn't exist. Users would think that they were only waiting for the connection to be established, and then after a while give up, muttering something about crummy software under their breath.

3.8 What does one do when one does not know how much information is commingover the socket ? Is there a way to have a dynamic buffer ?

This question asked by Niranjan Perera (

When the size of the incoming data is unknown, you can either make the size of the buffer as big as the largest possible (or likely) buffer, or you can re-size the buffer on the fly during your read. When you malloc() a large buffer, most (if not all) varients of unix will only allocate address space, but not physical pages of ram. As more and more of the buffer is used, the kernel allocates physical memory. This means that malloc'ing a large buffer will not waste resources unless that memory is used, and so it is perfectly acceptable to ask for a meg of ram when you expect only a few K.

On the other hand, a more elegant solution that does not depend on the inner workings of the kernel is to use realloc() to expand the buffer as required in say 4K chunks (since 4K is the size of a page of ram on most systems). I may add something like this to sockhelp.c in the example code one day.

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