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

4. Writing Server Applications (TCP/SOCK_STREAM)

4.1 How come I get "address already in use" from bind()?

You get this when the address is already in use. (Oh, you figured that much out?) The most common reason for this is that you have stopped your server, and then re-started it right away. The sockets that were used by the first incarnation of the server are still active. This is further explained in 2.7 Please explain the TIME_WAIT state., and 2.5 How do I properly close a socket?.

4.2 Why don't my sockets close?

When you issue the close() system call, you are closing your interface to the socket, not the socket itself. It is up to the kernel to close the socket. Sometimes, for really technical reasons, the socket is kept alive for a few minutes after you close it. It is normal, for example for the socket to go into a TIME_WAIT state, on the server side, for a few minutes. People have reported ranges from 20 seconds to 4 minutes to me. The official standard says that it should be 4 minutes. On my Linux system it is about 2 minutes. This is explained in great detail in 2.7 Please explain the TIME_WAIT state..

4.3 How can I make my server a daemon?

There are two approaches you can take here. The first is to use inetd to do all the hard work for you. The second is to do all the hard work yourself.

If you use inetd, you simply use stdin, stdout, or stderr for your socket. (These three are all created with dup() from the real socket) You can use these as you would a socket in your code. The inetd process will even close the socket for you when you are done.

If you wish to write your own server, there is a detailed explanation in "Unix Network Programming" by Richard Stevens (see 1.5 Where can I get source code for the book [book title]?). I also picked up this posting from comp.unix.programmer, by Nikhil Nair ( You may want to add code to ignore SIGPIPE, because if this signal is not dealt with, it will cause your application to exit. (Thanks to for pointing this out).

I worked all this lot out from the GNU C Library Manual (on-line
documentation).  Here's some code I wrote - you can adapt it as necessary:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/wait.h>

/* Global variables */
volatile sig_atomic_t keep_going = 1; /* controls program termination */

/* Function prototypes: */
void termination_handler (int signum); /* clean up before termination */

main (void)

  if (chdir (HOME_DIR))         /* change to directory containing data 
                                    files */
     fprintf (stderr, "`%s': ", HOME_DIR);
     perror (NULL);
     exit (1);

   /* Become a daemon: */
   switch (fork ())
     case -1:                    /* can't fork */
       perror ("fork()");
       exit (3);
     case 0:                     /* child, process becomes a daemon: */
       close (STDIN_FILENO);
       close (STDOUT_FILENO);
       close (STDERR_FILENO);
       if (setsid () == -1)      /* request a new session (job control) */
           exit (4);
     default:                    /* parent returns to calling process: */
       return 0;

   /* Establish signal handler to clean up before termination: */
   if (signal (SIGTERM, termination_handler) == SIG_IGN)
     signal (SIGTERM, SIG_IGN);
   signal (SIGINT, SIG_IGN);
   signal (SIGHUP, SIG_IGN);

   /* Main program loop */
   while (keep_going)
   return 0;

termination_handler (int signum)
  keep_going = 0;
  signal (signum, termination_handler);

4.4 How can I listen on more than one port at a time?

The best way to do this is with the select() call. This tells the kernel to let you know when a socket is available for use. You can have one process do i/o with multiple sockets with this call. If you want to wait for a connect on sockets 4, 6 and 10 you might execute the following code snippet:

fd_set socklist;

FD_ZERO(&socklist); /* Always clear the structure first. */
FD_SET(4, &socklist);
FD_SET(6, &socklist);
FD_SET(10, &socklist);
if (select(11, NULL, &socklist, NULL, NULL) < 0)

The kernel will notify us as soon as a file descriptor which is less than 11 (the first parameter to select()), and is a member of our socklist becomes available for writing. See the man page on select() for more details.

4.5 What exactly does SO_REUSEADDR do?

This socket option tells the kernel that even if this port is busy, go ahead and reuse it anyway. It is useful if your server has been shut down, and then restarted right away while sockets are still active on its port. You should be aware that if any unexpected data comes in, it may confuse your server, but while this is possible, it is not likely.

It has been pointed out that "A socket is a 5 tuple (proto, local addr, local port, remote addr, remote port). SO_REUSEADDR just says that you can reuse local addresses. The 5 tuple still must be unique!" by Michael Hunter ( This is true, and this is why it is very unlikely that unexpected data will ever be seen by your server. The danger is that such a 5 tuple is still floating around on the net, and while it is bouncing around, a new connection from the same client, on the same system, happens to get the same remote port. This is explained by Richard Stevens in 2.7 Please explain the TIME_WAIT state..

4.6 What exactly does SO_LINGER do?

On some unixes this does nothing. On others, it instructs the kernel to abort tcp connections instead of closing them properly. This can be dangerous. If you are not clear on this, see 2.7 Please explain the TIME_WAIT state..

4.7 What exactly does SO_KEEPALIVE do?

From Andrew Gierth (

The SO_KEEPALIVE option causes a packet (called a 'keepalive probe') to be sent to the remote system if a long time (by default, more than 2 hours) passes with no other data being sent or received. This packet is designed to provoke an ACK response from the peer. This enables detection of a peer which has become unreachable (e.g. powered off or disconnected from the net). See 2.8 Why does it take so long to detect that the peer died? for further discussion.

Note that the figure of 2 hours comes from RFC1122, "Requirements for Internet Hosts". The precise value should be configurable, but I've often found this to be difficult. The only implementation I know of that allows the keepalive interval to be set per-connection is SVR4.2.

4.8 How can I bind() to a port number < 1024?

From Andrew Gierth (

The restriction on access to ports < 1024 is part of a (fairly weak) security scheme particular to UNIX. The intention is that servers (for example rlogind, rshd) can check the port number of the client, and if it is < 1024, assume the request has been properly authorised at the client end.

The practical upshot of this, is that binding a port number < 1024 is reserved to processes having an effective UID == root.

This can, occasionally, itself present a security problem, e.g. when a server process needs to bind a well-known port, but does not itself need root access (news servers, for example). This is often solved by creating a small program which simply binds the socket, then restores the real userid and exec()s the real server. This program can then be made setuid root.

4.9 How do I get my server to find out the client's address / hostname?

From Andrew Gierth (

After accept()ing a connection, use getpeername() to get the address of the client. The client's address is of course, also returned on the accept(), but it is essential to initialise the address-length parameter before the accept call for this will work.

Jari Kokko ( has offered the following code to determine the client address:

int t;
int len;
struct sockaddr_in sin;
struct hostent *host;

len = sizeof sin;
if (getpeername(t, (struct sockaddr *) &sin, &len) < 0)
else {
        if ((host = gethostbyaddr((char *) &sin.sin_addr,
                                  sizeof sin.sin_addr,
                                  AF_INET)) == NULL)
        else printf("remote host is '%s'\n", host->h_name);

4.10 How should I choose a port number for my server?

The list of registered port assignments can be found in STD 2 or RFC 1700. Choose one that isn't already registered, and isn't in /etc/services on your system. It is also a good idea to let users customize the port number in case of conflicts with other un-registered port numbers in other servers. The best way of doing this is hardcoding a service name, and using getservbyname() to lookup the actual port number. This method allows users to change the port your server binds to by simply editing the /etc/services file.

4.11 What is the difference between SO_REUSEADDR and SO_REUSEPORT?

SO_REUSEADDR allows your server to bind to an address which is in a TIME_WAIT state. It does not allow more than one server to bind to the same address. It was mentioned that use of this flag can create a security risk because another server can bind to a the same port, by binding to a specific address as opposed to INADDR_ANY. The SO_REUSEPORT flag allows multiple processes to bind to the same address provided all of them use the SO_REUSEPORT option.

From Richard Stevens (

This is a newer flag that appeared in the 4.4BSD multicasting code (although that code was from elsewhere, so I am not sure just who invented the new SO_REUSEPORT flag).

What this flag lets you do is rebind a port that is already in use, but only if all users of the port specify the flag. I believe the intent is for multicasting apps, since if you're running the same app on a host, all need to bind the same port. But the flag may have other uses. For example the following is from a post in February:

From Stu Friedberg (

SO_REUSEPORT is also useful for eliminating the try-10-times-to-bind hack in ftpd's data connection setup routine. Without SO_REUSEPORT, only one ftpd thread can bind to TCP (lhost, lport, INADDR_ANY, 0) in preparation for connecting back to the client. Under conditions of heavy load, there are more threads colliding here than the try-10-times hack can accomodate. With SO_REUSEPORT, things work nicely and the hack becomes unnecessary.

I have also heard that DEC OSF supports the flag. Also note that under 4.4BSD, if you are binding a multicast address, then SO_REUSEADDR is condisered the same as SO_REUSEPORT (p. 731 of "TCP/IP Illustrated, Volume 2"). I think under Solaris you just replace SO_REUSEPORT with SO_REUSEADDR.

From a later Stevens posting, with minor editing:

Basically SO_REUSEPORT is a BSD'ism that arose when multicasting was added, even thought it was not used in the original Steve Deering code. I believe some BSD-derived systems may also include it (OSF, now Digital Unix, perhaps?). SO_REUSEPORT lets you bind the same address *and* port, but only if all the binders have specified it. But when binding a multicast address (its main use), SO_REUSEADDR is considered identical to SO_REUSEPORT (p. 731, "TCP/IP Illustrated, Volume 2"). So for portability of multicasting applications I always use SO_REUSEADDR.

4.12 How can I write a multi-homed server?

The original question was actually from Shankar Ramamoorthy (

I want to run a server on a multi-homed host. The host is part of two networks and has two ethernet cards. I want to run a server on this machine, binding to a pre-determined port number. I want clients on either subnet to be able to send broadcast packates to the port and have the server receive them.

And answered by Andrew Gierth (

Your first question in this scenario is, do you need to know which subnet the packet came from? I'm not at all sure that this can be reliably determined in all cases.

If you don't really care, then all you need is one socket bound to INADDR_ANY. That simplifies things greatly.

If you do care, then you have to bind multiple sockets. You are obviously attempting to do this in your code as posted, so I'll assume you do.

I was hoping that something like the following would work. Will it? This is on Sparcs running Solaris 2.4/2.5.

I don't have access to Solaris, but I'll comment based on my experience with other Unixes.

[Shankar's original code omitted]

What you are doing is attempting to bind all the current hosts unicast addresses as listed in hosts/NIS/DNS. This may or may not reflect reality, but much more importantly, neglects the broadcast addresses. It seems to be the case in the majority of implementations that a socket bound to a unicast address will not see incoming packets with broadcast addresses as their destinations.

The approach I've taken is to use SIOCGIFCONF to retrieve the list of active network interfaces, and SIOCGIFFLAGS and SIOCGIFBRDADDR to identify broadcastable interfaces and get the broadcast addresses. Then I bind to each unicast address, each broadcast address, and to INADDR_ANY as well. That last is necessary to catch packets that are on the wire with INADDR_BROADCAST in the destination. (SO_REUSEADDR is necessary to bind INADDR_ANY as well as the specific addresses.)

This gives me very nearly what I want. The wrinkles are:

4.13 How can I read only one character at a time?

This question is usually asked by people who are testing their server with telnet, and want it to process their keystrokes one character at a time. The correct technique is to use a psuedo terminal (pty). More on that in a minute.

According to Roger Espel Llima (, you can have your server send a sequence of control characters: 0xff 0xfb 0x01 0xff 0xfb 0x03 0xff 0xfd 0x0f3, which translates to IAC WILL ECHO IAC WILL SUPPRESS-GO-AHEAD IAC DO SUPPRESS-GO-AHEAD. For more information on what this means, check out std8, std28 and std29. Roger also gave the following tips:

Use of a pty would also be the correct way to execute a child process and pass the i/o to a socket.

I'll add pty stuff to the list of example source I'd like to add to the faq. If someone has some source they'd like to contribute (without copyright) to the faq which demonstrates use of pty's, please email me!

4.14 I'm trying to exec() a program from my server, and attach my socket's IO to it, but I'm not getting all the data across. Why?

If the program you are running uses printf(), etc (streams from stdio.h) you have to deal with two buffers. The kernel buffers all socket IO, and this is explained in section 2.11. The second buffer is the one that is causing you grief. This is the stdio buffer, and the problem was well explained by Andrew:

(The short answer to this question is that you want to use a pty rather than a socket; the remainder of this article is an attempt to explain why.)

Firstly, the socket buffer controlled by setsockopt() has absolutly nothing to do with stdio buffering. Setting it to 1 is guaranteed to be the Wrong Thing(tm).

Perhaps the following diagram might make things a little clearer:

        Process A                   Process B
    +---------------------+     +---------------------+
    |                     |     |                     |
    |    mainline code    |     |    mainline code    |
    |         |           |     |         ^           |
    |         v           |     |         |           |
    |      fputc()        |     |      fgetc()        |
    |         |           |     |         ^           |
    |         v           |     |         |           |
    |    +-----------+    |     |    +-----------+    |
    |    | stdio     |    |     |    | stdio     |    |
    |    | buffer    |    |     |    | buffer    |    |
    |    +-----------+    |     |    +-----------+    |
    |         |           |     |         ^           |
    |         |           |     |         |           |
    |      write()        |     |       read()        |
    |         |           |     |         |           |
    +-------- | ----------+     +-------- | ----------+
              |                           |                  User space
  ------------|-------------------------- | ---------------------------
              |                           |                Kernel space
              v                           |
         +-----------+               +-----------+
         | socket    |               | socket    |
         | buffer    |               | buffer    |
         +-----------+               +-----------+
              |                           ^
              v                           |
      (AF- and protocol-          (AF- and protocol-
       dependent code)             dependent code)

Assuming these two processes are communicating with each other (I've deliberately omitted the actual comms mechanisms, which aren't really relevent), you can see that data written by process A to it's stdio buffer is completely inaccessible to process B. Only once the decision is made to flush that buffer to the kernel (via write()) can the data actually be delivered to the other process.

The only guaranteed way to affect the buffering within process A is to change the code. However, the default buffering for stdout is controlled by whether the underlying FD refers to a terminal or not; generally, output to terminals is line-buffered, and output to non-terminals (including but not limited to files, pipes, sockets, non-tty devices, etc.) is fully buffered. So the desired effect can usually be achieved by using a pty device; this, for example, is what the 'expect' program does.

Since the stdio buffer (and the FILE structure, and everything else related to stdio) is user-level data, it is not preserved across an exec() call, hence trying to use setvbuf() before the exec is ineffective.

A couple of alternate solutions were proposed by Roger Espel Llima (

If it's an option, you can use some standalone program that will just run something inside a pty and buffer its input/output. I've seen a package by the name pty.tar.gz that did that; you could search around for it with archie or AltaVista.

Another option (**warning, evil hack**) , if you're on a system that supports this (SunOS, Solaris, Linux ELF do; I don't know about others) is to, on your main program, putenv() the name of a shared executable (*.so) in LD_PRELOAD, and then in that .so redefine some commonly used libc function that the program you're exec'ing is known to use early. There you can 'get control' on the running program, and the first time you get it, do a setbuf(stdout, NULL) on the program's behalf, and then call the original libc function with a dlopen() + dlsym(). And you keep the dlsym() value on a static var, so you can just call that the following times.

(Editors note: I still haven't done an expample for how to do pty's, but I hope I will be able to do one after I finish the non-blocking example code.)

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