This is libc.info, produced by makeinfo version 4.3 from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002
Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software Needs Free Documentation" and
"GNU Lesser General Public License", the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Creating a Socket,  Next: Closing a Socket,  Up: Open/Close Sockets

Creating a Socket
-----------------

   The primitive for creating a socket is the `socket' function,
declared in `sys/socket.h'.

 - Function: int socket (int NAMESPACE, int STYLE, int PROTOCOL)
     This function creates a socket and specifies communication style
     STYLE, which should be one of the socket styles listed in *Note
     Communication Styles::.  The NAMESPACE argument specifies the
     namespace; it must be `PF_LOCAL' (*note Local Namespace::) or
     `PF_INET' (*note Internet Namespace::).  PROTOCOL designates the
     specific protocol (*note Socket Concepts::); zero is usually right
     for PROTOCOL.

     The return value from `socket' is the file descriptor for the new
     socket, or `-1' in case of error.  The following `errno' error
     conditions are defined for this function:

    `EPROTONOSUPPORT'
          The PROTOCOL or STYLE is not supported by the NAMESPACE
          specified.

    `EMFILE'
          The process already has too many file descriptors open.

    `ENFILE'
          The system already has too many file descriptors open.

    `EACCES'
          The process does not have the privilege to create a socket of
          the specified STYLE or PROTOCOL.

    `ENOBUFS'
          The system ran out of internal buffer space.

     The file descriptor returned by the `socket' function supports both
     read and write operations.  However, like pipes, sockets do not
     support file positioning operations.

   For examples of how to call the `socket' function, see *Note Local
Socket Example::, or *Note Inet Example::.


File: libc.info,  Node: Closing a Socket,  Next: Socket Pairs,  Prev: Creating a Socket,  Up: Open/Close Sockets

Closing a Socket
----------------

   When you have finished using a socket, you can simply close its file
descriptor with `close'; see *Note Opening and Closing Files::.  If
there is still data waiting to be transmitted over the connection,
normally `close' tries to complete this transmission.  You can control
this behavior using the `SO_LINGER' socket option to specify a timeout
period; see *Note Socket Options::.

   You can also shut down only reception or transmission on a
connection by calling `shutdown', which is declared in `sys/socket.h'.

 - Function: int shutdown (int SOCKET, int HOW)
     The `shutdown' function shuts down the connection of socket
     SOCKET.  The argument HOW specifies what action to perform:

    `0'
          Stop receiving data for this socket.  If further data arrives,
          reject it.

    `1'
          Stop trying to transmit data from this socket.  Discard any
          data waiting to be sent.  Stop looking for acknowledgement of
          data already sent; don't retransmit it if it is lost.

    `2'
          Stop both reception and transmission.

     The return value is `0' on success and `-1' on failure.  The
     following `errno' error conditions are defined for this function:

    `EBADF'
          SOCKET is not a valid file descriptor.

    `ENOTSOCK'
          SOCKET is not a socket.

    `ENOTCONN'
          SOCKET is not connected.


File: libc.info,  Node: Socket Pairs,  Prev: Closing a Socket,  Up: Open/Close Sockets

Socket Pairs
------------

   A "socket pair" consists of a pair of connected (but unnamed)
sockets.  It is very similar to a pipe and is used in much the same
way.  Socket pairs are created with the `socketpair' function, declared
in `sys/socket.h'.  A socket pair is much like a pipe; the main
difference is that the socket pair is bidirectional, whereas the pipe
has one input-only end and one output-only end (*note Pipes and
FIFOs::).

 - Function: int socketpair (int NAMESPACE, int STYLE, int PROTOCOL,
          int FILEDES[2])
     This function creates a socket pair, returning the file
     descriptors in `FILEDES[0]' and `FILEDES[1]'.  The socket pair is
     a full-duplex communications channel, so that both reading and
     writing may be performed at either end.

     The NAMESPACE, STYLE and PROTOCOL arguments are interpreted as for
     the `socket' function.  STYLE should be one of the communication
     styles listed in *Note Communication Styles::.  The NAMESPACE
     argument specifies the namespace, which must be `AF_LOCAL' (*note
     Local Namespace::); PROTOCOL specifies the communications
     protocol, but zero is the only meaningful value.

     If STYLE specifies a connectionless communication style, then the
     two sockets you get are not _connected_, strictly speaking, but
     each of them knows the other as the default destination address,
     so they can send packets to each other.

     The `socketpair' function returns `0' on success and `-1' on
     failure.  The following `errno' error conditions are defined for
     this function:

    `EMFILE'
          The process has too many file descriptors open.

    `EAFNOSUPPORT'
          The specified namespace is not supported.

    `EPROTONOSUPPORT'
          The specified protocol is not supported.

    `EOPNOTSUPP'
          The specified protocol does not support the creation of
          socket pairs.


File: libc.info,  Node: Connections,  Next: Datagrams,  Prev: Open/Close Sockets,  Up: Sockets

Using Sockets with Connections
==============================

   The most common communication styles involve making a connection to a
particular other socket, and then exchanging data with that socket over
and over.  Making a connection is asymmetric; one side (the "client")
acts to request a connection, while the other side (the "server") makes
a socket and waits for the connection request.

* Menu:

* Connecting::    	     What the client program must do.
* Listening::		     How a server program waits for requests.
* Accepting Connections::    What the server does when it gets a request.
* Who is Connected::	     Getting the address of the
				other side of a connection.
* Transferring Data::        How to send and receive data.
* Byte Stream Example::	     An example program: a client for communicating
			      over a byte stream socket in the Internet namespace.
* Server Example::	     A corresponding server program.
* Out-of-Band Data::         This is an advanced feature.


File: libc.info,  Node: Connecting,  Next: Listening,  Up: Connections

Making a Connection
-------------------

   In making a connection, the client makes a connection while the
server waits for and accepts the connection.  Here we discuss what the
client program must do with the `connect' function, which is declared in
`sys/socket.h'.

 - Function: int connect (int SOCKET, struct sockaddr *ADDR, socklen_t
          LENGTH)
     The `connect' function initiates a connection from the socket with
     file descriptor SOCKET to the socket whose address is specified by
     the ADDR and LENGTH arguments.  (This socket is typically on
     another machine, and it must be already set up as a server.)
     *Note Socket Addresses::, for information about how these
     arguments are interpreted.

     Normally, `connect' waits until the server responds to the request
     before it returns.  You can set nonblocking mode on the socket
     SOCKET to make `connect' return immediately without waiting for
     the response.  *Note File Status Flags::, for information about
     nonblocking mode.

     The normal return value from `connect' is `0'.  If an error
     occurs, `connect' returns `-1'.  The following `errno' error
     conditions are defined for this function:

    `EBADF'
          The socket SOCKET is not a valid file descriptor.

    `ENOTSOCK'
          File descriptor SOCKET is not a socket.

    `EADDRNOTAVAIL'
          The specified address is not available on the remote machine.

    `EAFNOSUPPORT'
          The namespace of the ADDR is not supported by this socket.

    `EISCONN'
          The socket SOCKET is already connected.

    `ETIMEDOUT'
          The attempt to establish the connection timed out.

    `ECONNREFUSED'
          The server has actively refused to establish the connection.

    `ENETUNREACH'
          The network of the given ADDR isn't reachable from this host.

    `EADDRINUSE'
          The socket address of the given ADDR is already in use.

    `EINPROGRESS'
          The socket SOCKET is non-blocking and the connection could
          not be established immediately.  You can determine when the
          connection is completely established with `select'; *note
          Waiting for I/O::.  Another `connect' call on the same
          socket, before the connection is completely established, will
          fail with `EALREADY'.

    `EALREADY'
          The socket SOCKET is non-blocking and already has a pending
          connection in progress (see `EINPROGRESS' above).

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.


File: libc.info,  Node: Listening,  Next: Accepting Connections,  Prev: Connecting,  Up: Connections

Listening for Connections
-------------------------

   Now let us consider what the server process must do to accept
connections on a socket.  First it must use the `listen' function to
enable connection requests on the socket, and then accept each incoming
connection with a call to `accept' (*note Accepting Connections::).
Once connection requests are enabled on a server socket, the `select'
function reports when the socket has a connection ready to be accepted
(*note Waiting for I/O::).

   The `listen' function is not allowed for sockets using
connectionless communication styles.

   You can write a network server that does not even start running
until a connection to it is requested.  *Note Inetd Servers::.

   In the Internet namespace, there are no special protection mechanisms
for controlling access to a port; any process on any machine can make a
connection to your server.  If you want to restrict access to your
server, make it examine the addresses associated with connection
requests or implement some other handshaking or identification protocol.

   In the local namespace, the ordinary file protection bits control
who has access to connect to the socket.

 - Function: int listen (int SOCKET, unsigned int N)
     The `listen' function enables the socket SOCKET to accept
     connections, thus making it a server socket.

     The argument N specifies the length of the queue for pending
     connections.  When the queue fills, new clients attempting to
     connect fail with `ECONNREFUSED' until the server calls `accept' to
     accept a connection from the queue.

     The `listen' function returns `0' on success and `-1' on failure.
     The following `errno' error conditions are defined for this
     function:

    `EBADF'
          The argument SOCKET is not a valid file descriptor.

    `ENOTSOCK'
          The argument SOCKET is not a socket.

    `EOPNOTSUPP'
          The socket SOCKET does not support this operation.


File: libc.info,  Node: Accepting Connections,  Next: Who is Connected,  Prev: Listening,  Up: Connections

Accepting Connections
---------------------

   When a server receives a connection request, it can complete the
connection by accepting the request.  Use the function `accept' to do
this.

   A socket that has been established as a server can accept connection
requests from multiple clients.  The server's original socket _does not
become part of the connection_; instead, `accept' makes a new socket
which participates in the connection.  `accept' returns the descriptor
for this socket.  The server's original socket remains available for
listening for further connection requests.

   The number of pending connection requests on a server socket is
finite.  If connection requests arrive from clients faster than the
server can act upon them, the queue can fill up and additional requests
are refused with an `ECONNREFUSED' error.  You can specify the maximum
length of this queue as an argument to the `listen' function, although
the system may also impose its own internal limit on the length of this
queue.

 - Function: int accept (int SOCKET, struct sockaddr *ADDR, socklen_t
          *LENGTH_PTR)
     This function is used to accept a connection request on the server
     socket SOCKET.

     The `accept' function waits if there are no connections pending,
     unless the socket SOCKET has nonblocking mode set.  (You can use
     `select' to wait for a pending connection, with a nonblocking
     socket.)  *Note File Status Flags::, for information about
     nonblocking mode.

     The ADDR and LENGTH-PTR arguments are used to return information
     about the name of the client socket that initiated the connection.
     *Note Socket Addresses::, for information about the format of the
     information.

     Accepting a connection does not make SOCKET part of the
     connection.  Instead, it creates a new socket which becomes
     connected.  The normal return value of `accept' is the file
     descriptor for the new socket.

     After `accept', the original socket SOCKET remains open and
     unconnected, and continues listening until you close it.  You can
     accept further connections with SOCKET by calling `accept' again.

     If an error occurs, `accept' returns `-1'.  The following `errno'
     error conditions are defined for this function:

    `EBADF'
          The SOCKET argument is not a valid file descriptor.

    `ENOTSOCK'
          The descriptor SOCKET argument is not a socket.

    `EOPNOTSUPP'
          The descriptor SOCKET does not support this operation.

    `EWOULDBLOCK'
          SOCKET has nonblocking mode set, and there are no pending
          connections immediately available.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.

   The `accept' function is not allowed for sockets using
connectionless communication styles.


File: libc.info,  Node: Who is Connected,  Next: Transferring Data,  Prev: Accepting Connections,  Up: Connections

Who is Connected to Me?
-----------------------

 - Function: int getpeername (int SOCKET, struct sockaddr *ADDR,
          socklen_t *LENGTH-PTR)
     The `getpeername' function returns the address of the socket that
     SOCKET is connected to; it stores the address in the memory space
     specified by ADDR and LENGTH-PTR.  It stores the length of the
     address in `*LENGTH-PTR'.

     *Note Socket Addresses::, for information about the format of the
     address.  In some operating systems, `getpeername' works only for
     sockets in the Internet domain.

     The return value is `0' on success and `-1' on error.  The
     following `errno' error conditions are defined for this function:

    `EBADF'
          The argument SOCKET is not a valid file descriptor.

    `ENOTSOCK'
          The descriptor SOCKET is not a socket.

    `ENOTCONN'
          The socket SOCKET is not connected.

    `ENOBUFS'
          There are not enough internal buffers available.


File: libc.info,  Node: Transferring Data,  Next: Byte Stream Example,  Prev: Who is Connected,  Up: Connections

Transferring Data
-----------------

   Once a socket has been connected to a peer, you can use the ordinary
`read' and `write' operations (*note I/O Primitives::) to transfer
data.  A socket is a two-way communications channel, so read and write
operations can be performed at either end.

   There are also some I/O modes that are specific to socket operations.
In order to specify these modes, you must use the `recv' and `send'
functions instead of the more generic `read' and `write' functions.
The `recv' and `send' functions take an additional argument which you
can use to specify various flags to control special I/O modes.  For
example, you can specify the `MSG_OOB' flag to read or write
out-of-band data, the `MSG_PEEK' flag to peek at input, or the
`MSG_DONTROUTE' flag to control inclusion of routing information on
output.

* Menu:

* Sending Data::		Sending data with `send'.
* Receiving Data::		Reading data with `recv'.
* Socket Data Options::		Using `send' and `recv'.


File: libc.info,  Node: Sending Data,  Next: Receiving Data,  Up: Transferring Data

Sending Data
............

   The `send' function is declared in the header file `sys/socket.h'.
If your FLAGS argument is zero, you can just as well use `write'
instead of `send'; see *Note I/O Primitives::.  If the socket was
connected but the connection has broken, you get a `SIGPIPE' signal for
any use of `send' or `write' (*note Miscellaneous Signals::).

 - Function: int send (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)
     The `send' function is like `write', but with the additional flags
     FLAGS.  The possible values of FLAGS are described in *Note Socket
     Data Options::.

     This function returns the number of bytes transmitted, or `-1' on
     failure.  If the socket is nonblocking, then `send' (like `write')
     can return after sending just part of the data.  *Note File Status
     Flags::, for information about nonblocking mode.

     Note, however, that a successful return value merely indicates that
     the message has been sent without error, not necessarily that it
     has been received without error.

     The following `errno' error conditions are defined for this
     function:

    `EBADF'
          The SOCKET argument is not a valid file descriptor.

    `EINTR'
          The operation was interrupted by a signal before any data was
          sent.  *Note Interrupted Primitives::.

    `ENOTSOCK'
          The descriptor SOCKET is not a socket.

    `EMSGSIZE'
          The socket type requires that the message be sent atomically,
          but the message is too large for this to be possible.

    `EWOULDBLOCK'
          Nonblocking mode has been set on the socket, and the write
          operation would block.  (Normally `send' blocks until the
          operation can be completed.)

    `ENOBUFS'
          There is not enough internal buffer space available.

    `ENOTCONN'
          You never connected this socket.

    `EPIPE'
          This socket was connected but the connection is now broken.
          In this case, `send' generates a `SIGPIPE' signal first; if
          that signal is ignored or blocked, or if its handler returns,
          then `send' fails with `EPIPE'.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.


File: libc.info,  Node: Receiving Data,  Next: Socket Data Options,  Prev: Sending Data,  Up: Transferring Data

Receiving Data
..............

   The `recv' function is declared in the header file `sys/socket.h'.
If your FLAGS argument is zero, you can just as well use `read' instead
of `recv'; see *Note I/O Primitives::.

 - Function: int recv (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)
     The `recv' function is like `read', but with the additional flags
     FLAGS.  The possible values of FLAGS are described in *Note Socket
     Data Options::.

     If nonblocking mode is set for SOCKET, and no data are available to
     be read, `recv' fails immediately rather than waiting.  *Note File
     Status Flags::, for information about nonblocking mode.

     This function returns the number of bytes received, or `-1' on
     failure.  The following `errno' error conditions are defined for
     this function:

    `EBADF'
          The SOCKET argument is not a valid file descriptor.

    `ENOTSOCK'
          The descriptor SOCKET is not a socket.

    `EWOULDBLOCK'
          Nonblocking mode has been set on the socket, and the read
          operation would block.  (Normally, `recv' blocks until there
          is input available to be read.)

    `EINTR'
          The operation was interrupted by a signal before any data was
          read.  *Note Interrupted Primitives::.

    `ENOTCONN'
          You never connected this socket.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.


File: libc.info,  Node: Socket Data Options,  Prev: Receiving Data,  Up: Transferring Data

Socket Data Options
...................

   The FLAGS argument to `send' and `recv' is a bit mask.  You can
bitwise-OR the values of the following macros together to obtain a
value for this argument.  All are defined in the header file
`sys/socket.h'.

 - Macro: int MSG_OOB
     Send or receive out-of-band data.  *Note Out-of-Band Data::.

 - Macro: int MSG_PEEK
     Look at the data but don't remove it from the input queue.  This is
     only meaningful with input functions such as `recv', not with
     `send'.

 - Macro: int MSG_DONTROUTE
     Don't include routing information in the message.  This is only
     meaningful with output operations, and is usually only of interest
     for diagnostic or routing programs.  We don't try to explain it
     here.


File: libc.info,  Node: Byte Stream Example,  Next: Server Example,  Prev: Transferring Data,  Up: Connections

Byte Stream Socket Example
--------------------------

   Here is an example client program that makes a connection for a byte
stream socket in the Internet namespace.  It doesn't do anything
particularly interesting once it has connected to the server; it just
sends a text string to the server and exits.

   This program uses `init_sockaddr' to set up the socket address; see
*Note Inet Example::.

     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netdb.h>
     
     #define PORT            5555
     #define MESSAGE         "Yow!!! Are we having fun yet?!?"
     #define SERVERHOST      "mescaline.gnu.org"
     
     void
     write_to_server (int filedes)
     {
       int nbytes;
     
       nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
       if (nbytes < 0)
         {
           perror ("write");
           exit (EXIT_FAILURE);
         }
     }
     
     
     int
     main (void)
     {
       extern void init_sockaddr (struct sockaddr_in *name,
                                  const char *hostname,
                                  uint16_t port);
       int sock;
       struct sockaddr_in servername;
     
       /* Create the socket. */
       sock = socket (PF_INET, SOCK_STREAM, 0);
       if (sock < 0)
         {
           perror ("socket (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Connect to the server. */
       init_sockaddr (&servername, SERVERHOST, PORT);
       if (0 > connect (sock,
                        (struct sockaddr *) &servername,
                        sizeof (servername)))
         {
           perror ("connect (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Send data to the server. */
       write_to_server (sock);
       close (sock);
       exit (EXIT_SUCCESS);
     }


File: libc.info,  Node: Server Example,  Next: Out-of-Band Data,  Prev: Byte Stream Example,  Up: Connections

Byte Stream Connection Server Example
-------------------------------------

   The server end is much more complicated.  Since we want to allow
multiple clients to be connected to the server at the same time, it
would be incorrect to wait for input from a single client by simply
calling `read' or `recv'.  Instead, the right thing to do is to use
`select' (*note Waiting for I/O::) to wait for input on all of the open
sockets.  This also allows the server to deal with additional
connection requests.

   This particular server doesn't do anything interesting once it has
gotten a message from a client.  It does close the socket for that
client when it detects an end-of-file condition (resulting from the
client shutting down its end of the connection).

   This program uses `make_socket' to set up the socket address; see
*Note Inet Example::.

     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netdb.h>
     
     #define PORT    5555
     #define MAXMSG  512
     
     int
     read_from_client (int filedes)
     {
       char buffer[MAXMSG];
       int nbytes;
     
       nbytes = read (filedes, buffer, MAXMSG);
       if (nbytes < 0)
         {
           /* Read error. */
           perror ("read");
           exit (EXIT_FAILURE);
         }
       else if (nbytes == 0)
         /* End-of-file. */
         return -1;
       else
         {
           /* Data read. */
           fprintf (stderr, "Server: got message: `%s'\n", buffer);
           return 0;
         }
     }
     
     int
     main (void)
     {
       extern int make_socket (uint16_t port);
       int sock;
       fd_set active_fd_set, read_fd_set;
       int i;
       struct sockaddr_in clientname;
       size_t size;
     
       /* Create the socket and set it up to accept connections. */
       sock = make_socket (PORT);
       if (listen (sock, 1) < 0)
         {
           perror ("listen");
           exit (EXIT_FAILURE);
         }
     
       /* Initialize the set of active sockets. */
       FD_ZERO (&active_fd_set);
       FD_SET (sock, &active_fd_set);
     
       while (1)
         {
           /* Block until input arrives on one or more active sockets. */
           read_fd_set = active_fd_set;
           if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
             {
               perror ("select");
               exit (EXIT_FAILURE);
             }
     
           /* Service all the sockets with input pending. */
           for (i = 0; i < FD_SETSIZE; ++i)
             if (FD_ISSET (i, &read_fd_set))
               {
                 if (i == sock)
                   {
                     /* Connection request on original socket. */
                     int new;
                     size = sizeof (clientname);
                     new = accept (sock,
                                   (struct sockaddr *) &clientname,
                                   &size);
                     if (new < 0)
                       {
                         perror ("accept");
                         exit (EXIT_FAILURE);
                       }
                     fprintf (stderr,
                              "Server: connect from host %s, port %hd.\n",
                              inet_ntoa (clientname.sin_addr),
                              ntohs (clientname.sin_port));
                     FD_SET (new, &active_fd_set);
                   }
                 else
                   {
                     /* Data arriving on an already-connected socket. */
                     if (read_from_client (i) < 0)
                       {
                         close (i);
                         FD_CLR (i, &active_fd_set);
                       }
                   }
               }
         }
     }


File: libc.info,  Node: Out-of-Band Data,  Prev: Server Example,  Up: Connections

Out-of-Band Data
----------------

   Streams with connections permit "out-of-band" data that is delivered
with higher priority than ordinary data.  Typically the reason for
sending out-of-band data is to send notice of an exceptional condition.
To send out-of-band data use `send', specifying the flag `MSG_OOB'
(*note Sending Data::).

   Out-of-band data are received with higher priority because the
receiving process need not read it in sequence; to read the next
available out-of-band data, use `recv' with the `MSG_OOB' flag (*note
Receiving Data::).  Ordinary read operations do not read out-of-band
data; they read only ordinary data.

   When a socket finds that out-of-band data are on their way, it sends
a `SIGURG' signal to the owner process or process group of the socket.
You can specify the owner using the `F_SETOWN' command to the `fcntl'
function; see *Note Interrupt Input::.  You must also establish a
handler for this signal, as described in *Note Signal Handling::, in
order to take appropriate action such as reading the out-of-band data.

   Alternatively, you can test for pending out-of-band data, or wait
until there is out-of-band data, using the `select' function; it can
wait for an exceptional condition on the socket.  *Note Waiting for
I/O::, for more information about `select'.

   Notification of out-of-band data (whether with `SIGURG' or with
`select') indicates that out-of-band data are on the way; the data may
not actually arrive until later.  If you try to read the out-of-band
data before it arrives, `recv' fails with an `EWOULDBLOCK' error.

   Sending out-of-band data automatically places a "mark" in the stream
of ordinary data, showing where in the sequence the out-of-band data
"would have been".  This is useful when the meaning of out-of-band data
is "cancel everything sent so far".  Here is how you can test, in the
receiving process, whether any ordinary data was sent before the mark:

     success = ioctl (socket, SIOCATMARK, &atmark);

   The `integer' variable ATMARK is set to a nonzero value if the
socket's read pointer has reached the "mark".

   Here's a function to discard any ordinary data preceding the
out-of-band mark:

     int
     discard_until_mark (int socket)
     {
       while (1)
         {
           /* This is not an arbitrary limit; any size will do.  */
           char buffer[1024];
           int atmark, success;
     
           /* If we have reached the mark, return.  */
           success = ioctl (socket, SIOCATMARK, &atmark);
           if (success < 0)
             perror ("ioctl");
           if (result)
             return;
     
           /* Otherwise, read a bunch of ordinary data and discard it.
              This is guaranteed not to read past the mark
              if it starts before the mark.  */
           success = read (socket, buffer, sizeof buffer);
           if (success < 0)
             perror ("read");
         }
     }

   If you don't want to discard the ordinary data preceding the mark,
you may need to read some of it anyway, to make room in internal system
buffers for the out-of-band data.  If you try to read out-of-band data
and get an `EWOULDBLOCK' error, try reading some ordinary data (saving
it so that you can use it when you want it) and see if that makes room.
Here is an example:

     struct buffer
     {
       char *buf;
       int size;
       struct buffer *next;
     };
     
     /* Read the out-of-band data from SOCKET and return it
        as a `struct buffer', which records the address of the data
        and its size.
     
        It may be necessary to read some ordinary data
        in order to make room for the out-of-band data.
        If so, the ordinary data are saved as a chain of buffers
        found in the `next' field of the value.  */
     
     struct buffer *
     read_oob (int socket)
     {
       struct buffer *tail = 0;
       struct buffer *list = 0;
     
       while (1)
         {
           /* This is an arbitrary limit.
              Does anyone know how to do this without a limit?  */
     #define BUF_SZ 1024
           char *buf = (char *) xmalloc (BUF_SZ);
           int success;
           int atmark;
     
           /* Try again to read the out-of-band data.  */
           success = recv (socket, buf, BUF_SZ, MSG_OOB);
           if (success >= 0)
             {
               /* We got it, so return it.  */
               struct buffer *link
                 = (struct buffer *) xmalloc (sizeof (struct buffer));
               link->buf = buf;
               link->size = success;
               link->next = list;
               return link;
             }
     
           /* If we fail, see if we are at the mark.  */
           success = ioctl (socket, SIOCATMARK, &atmark);
           if (success < 0)
             perror ("ioctl");
           if (atmark)
             {
               /* At the mark; skipping past more ordinary data cannot help.
                  So just wait a while.  */
               sleep (1);
               continue;
             }
     
           /* Otherwise, read a bunch of ordinary data and save it.
              This is guaranteed not to read past the mark
              if it starts before the mark.  */
           success = read (socket, buf, BUF_SZ);
           if (success < 0)
             perror ("read");
     
           /* Save this data in the buffer list.  */
           {
             struct buffer *link
               = (struct buffer *) xmalloc (sizeof (struct buffer));
             link->buf = buf;
             link->size = success;
     
             /* Add the new link to the end of the list.  */
             if (tail)
               tail->next = link;
             else
               list = link;
             tail = link;
           }
         }
     }


File: libc.info,  Node: Datagrams,  Next: Inetd,  Prev: Connections,  Up: Sockets

Datagram Socket Operations
==========================

   This section describes how to use communication styles that don't use
connections (styles `SOCK_DGRAM' and `SOCK_RDM').  Using these styles,
you group data into packets and each packet is an independent
communication.  You specify the destination for each packet
individually.

   Datagram packets are like letters: you send each one independently
with its own destination address, and they may arrive in the wrong
order or not at all.

   The `listen' and `accept' functions are not allowed for sockets
using connectionless communication styles.

* Menu:

* Sending Datagrams::    Sending packets on a datagram socket.
* Receiving Datagrams::  Receiving packets on a datagram socket.
* Datagram Example::     An example program: packets sent over a
                           datagram socket in the local namespace.
* Example Receiver::	 Another program, that receives those packets.


File: libc.info,  Node: Sending Datagrams,  Next: Receiving Datagrams,  Up: Datagrams

Sending Datagrams
-----------------

   The normal way of sending data on a datagram socket is by using the
`sendto' function, declared in `sys/socket.h'.

   You can call `connect' on a datagram socket, but this only specifies
a default destination for further data transmission on the socket.
When a socket has a default destination you can use `send' (*note
Sending Data::) or even `write' (*note I/O Primitives::) to send a
packet there.  You can cancel the default destination by calling
`connect' using an address format of `AF_UNSPEC' in the ADDR argument.
*Note Connecting::, for more information about the `connect' function.

 - Function: int sendto (int SOCKET, void *BUFFER. size_t SIZE, int
          FLAGS, struct sockaddr *ADDR, socklen_t LENGTH)
     The `sendto' function transmits the data in the BUFFER through the
     socket SOCKET to the destination address specified by the ADDR and
     LENGTH arguments.  The SIZE argument specifies the number of bytes
     to be transmitted.

     The FLAGS are interpreted the same way as for `send'; see *Note
     Socket Data Options::.

     The return value and error conditions are also the same as for
     `send', but you cannot rely on the system to detect errors and
     report them; the most common error is that the packet is lost or
     there is no-one at the specified address to receive it, and the
     operating system on your machine usually does not know this.

     It is also possible for one call to `sendto' to report an error
     owing to a problem related to a previous call.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.


File: libc.info,  Node: Receiving Datagrams,  Next: Datagram Example,  Prev: Sending Datagrams,  Up: Datagrams

Receiving Datagrams
-------------------

   The `recvfrom' function reads a packet from a datagram socket and
also tells you where it was sent from.  This function is declared in
`sys/socket.h'.

 - Function: int recvfrom (int SOCKET, void *BUFFER, size_t SIZE, int
          FLAGS, struct sockaddr *ADDR, socklen_t *LENGTH-PTR)
     The `recvfrom' function reads one packet from the socket SOCKET
     into the buffer BUFFER.  The SIZE argument specifies the maximum
     number of bytes to be read.

     If the packet is longer than SIZE bytes, then you get the first
     SIZE bytes of the packet and the rest of the packet is lost.
     There's no way to read the rest of the packet.  Thus, when you use
     a packet protocol, you must always know how long a packet to
     expect.

     The ADDR and LENGTH-PTR arguments are used to return the address
     where the packet came from.  *Note Socket Addresses::.  For a
     socket in the local domain the address information won't be
     meaningful, since you can't read the address of such a socket
     (*note Local Namespace::).  You can specify a null pointer as the
     ADDR argument if you are not interested in this information.

     The FLAGS are interpreted the same way as for `recv' (*note Socket
     Data Options::).  The return value and error conditions are also
     the same as for `recv'.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.

   You can use plain `recv' (*note Receiving Data::) instead of
`recvfrom' if you don't need to find out who sent the packet (either
because you know where it should come from or because you treat all
possible senders alike).  Even `read' can be used if you don't want to
specify FLAGS (*note I/O Primitives::).


File: libc.info,  Node: Datagram Example,  Next: Example Receiver,  Prev: Receiving Datagrams,  Up: Datagrams

Datagram Socket Example
-----------------------

   Here is a set of example programs that send messages over a datagram
stream in the local namespace.  Both the client and server programs use
the `make_named_socket' function that was presented in *Note Local
Socket Example::, to create and name their sockets.

   First, here is the server program.  It sits in a loop waiting for
messages to arrive, bouncing each message back to the sender.
Obviously this isn't a particularly useful program, but it does show
the general ideas involved.

     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <sys/socket.h>
     #include <sys/un.h>
     
     #define SERVER  "/tmp/serversocket"
     #define MAXMSG  512
     
     int
     main (void)
     {
       int sock;
       char message[MAXMSG];
       struct sockaddr_un name;
       size_t size;
       int nbytes;
     
       /* Remove the filename first, it's ok if the call fails */
       unlink (SERVER);
     
       /* Make the socket, then loop endlessly. */
       sock = make_named_socket (SERVER);
       while (1)
         {
           /* Wait for a datagram. */
           size = sizeof (name);
           nbytes = recvfrom (sock, message, MAXMSG, 0,
                              (struct sockaddr *) & name, &size);
           if (nbytes < 0)
             {
               perror ("recfrom (server)");
               exit (EXIT_FAILURE);
             }
     
           /* Give a diagnostic message. */
           fprintf (stderr, "Server: got message: %s\n", message);
     
           /* Bounce the message back to the sender. */
           nbytes = sendto (sock, message, nbytes, 0,
                            (struct sockaddr *) & name, size);
           if (nbytes < 0)
             {
               perror ("sendto (server)");
               exit (EXIT_FAILURE);
             }
         }
     }


File: libc.info,  Node: Example Receiver,  Prev: Datagram Example,  Up: Datagrams

Example of Reading Datagrams
----------------------------

   Here is the client program corresponding to the server above.

   It sends a datagram to the server and then waits for a reply.  Notice
that the socket for the client (as well as for the server) in this
example has to be given a name.  This is so that the server can direct
a message back to the client.  Since the socket has no associated
connection state, the only way the server can do this is by referencing
the name of the client.

     #include <stdio.h>
     #include <errno.h>
     #include <unistd.h>
     #include <stdlib.h>
     #include <sys/socket.h>
     #include <sys/un.h>
     
     #define SERVER  "/tmp/serversocket"
     #define CLIENT  "/tmp/mysocket"
     #define MAXMSG  512
     #define MESSAGE "Yow!!! Are we having fun yet?!?"
     
     int
     main (void)
     {
       extern int make_named_socket (const char *name);
       int sock;
       char message[MAXMSG];
       struct sockaddr_un name;
       size_t size;
       int nbytes;
     
       /* Make the socket. */
       sock = make_named_socket (CLIENT);
     
       /* Initialize the server socket address. */
       name.sun_family = AF_LOCAL;
       strcpy (name.sun_path, SERVER);
       size = strlen (name.sun_path) + sizeof (name.sun_family);
     
       /* Send the datagram. */
       nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
                        (struct sockaddr *) & name, size);
       if (nbytes < 0)
         {
           perror ("sendto (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Wait for a reply. */
       nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
       if (nbytes < 0)
         {
           perror ("recfrom (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Print a diagnostic message. */
       fprintf (stderr, "Client: got message: %s\n", message);
     
       /* Clean up. */
       remove (CLIENT);
       close (sock);
     }

   Keep in mind that datagram socket communications are unreliable.  In
this example, the client program waits indefinitely if the message
never reaches the server or if the server's response never comes back.
It's up to the user running the program to kill and restart it if
desired.  A more automatic solution could be to use `select' (*note
Waiting for I/O::) to establish a timeout period for the reply, and in
case of timeout either re-send the message or shut down the socket and
exit.


File: libc.info,  Node: Inetd,  Next: Socket Options,  Prev: Datagrams,  Up: Sockets

The `inetd' Daemon
==================

   We've explained above how to write a server program that does its own
listening.  Such a server must already be running in order for anyone
to connect to it.

   Another way to provide a service on an Internet port is to let the
daemon program `inetd' do the listening.  `inetd' is a program that
runs all the time and waits (using `select') for messages on a
specified set of ports.  When it receives a message, it accepts the
connection (if the socket style calls for connections) and then forks a
child process to run the corresponding server program.  You specify the
ports and their programs in the file `/etc/inetd.conf'.

* Menu:

* Inetd Servers::
* Configuring Inetd::


File: libc.info,  Node: Inetd Servers,  Next: Configuring Inetd,  Up: Inetd

`inetd' Servers
---------------

   Writing a server program to be run by `inetd' is very simple.  Each
time someone requests a connection to the appropriate port, a new server
process starts.  The connection already exists at this time; the socket
is available as the standard input descriptor and as the standard
output descriptor (descriptors 0 and 1) in the server process.  Thus
the server program can begin reading and writing data right away.
Often the program needs only the ordinary I/O facilities; in fact, a
general-purpose filter program that knows nothing about sockets can
work as a byte stream server run by `inetd'.

   You can also use `inetd' for servers that use connectionless
communication styles.  For these servers, `inetd' does not try to accept
a connection since no connection is possible.  It just starts the
server program, which can read the incoming datagram packet from
descriptor 0.  The server program can handle one request and then exit,
or you can choose to write it to keep reading more requests until no
more arrive, and then exit.  You must specify which of these two
techniques the server uses when you configure `inetd'.


File: libc.info,  Node: Configuring Inetd,  Prev: Inetd Servers,  Up: Inetd

Configuring `inetd'
-------------------

   The file `/etc/inetd.conf' tells `inetd' which ports to listen to
and what server programs to run for them.  Normally each entry in the
file is one line, but you can split it onto multiple lines provided all
but the first line of the entry start with whitespace.  Lines that
start with `#' are comments.

   Here are two standard entries in `/etc/inetd.conf':

     ftp	stream	tcp	nowait	root	/libexec/ftpd	ftpd
     talk	dgram	udp	wait	root	/libexec/talkd	talkd

   An entry has this format:

     SERVICE STYLE PROTOCOL WAIT USERNAME PROGRAM ARGUMENTS

   The SERVICE field says which service this program provides.  It
should be the name of a service defined in `/etc/services'.  `inetd'
uses SERVICE to decide which port to listen on for this entry.

   The fields STYLE and PROTOCOL specify the communication style and
the protocol to use for the listening socket.  The style should be the
name of a communication style, converted to lower case and with `SOCK_'
deleted--for example, `stream' or `dgram'.  PROTOCOL should be one of
the protocols listed in `/etc/protocols'.  The typical protocol names
are `tcp' for byte stream connections and `udp' for unreliable
datagrams.

   The WAIT field should be either `wait' or `nowait'.  Use `wait' if
STYLE is a connectionless style and the server, once started, handles
multiple requests as they come in.  Use `nowait' if `inetd' should
start a new process for each message or request that comes in.  If
STYLE uses connections, then WAIT *must* be `nowait'.

   USER is the user name that the server should run as.  `inetd' runs
as root, so it can set the user ID of its children arbitrarily.  It's
best to avoid using `root' for USER if you can; but some servers, such
as Telnet and FTP, read a username and password themselves.  These
servers need to be root initially so they can log in as commanded by
the data coming over the network.

   PROGRAM together with ARGUMENTS specifies the command to run to
start the server.  PROGRAM should be an absolute file name specifying
the executable file to run.  ARGUMENTS consists of any number of
whitespace-separated words, which become the command-line arguments of
PROGRAM.  The first word in ARGUMENTS is argument zero, which should by
convention be the program name itself (sans directories).

   If you edit `/etc/inetd.conf', you can tell `inetd' to reread the
file and obey its new contents by sending the `inetd' process the
`SIGHUP' signal.  You'll have to use `ps' to determine the process ID
of the `inetd' process as it is not fixed.


File: libc.info,  Node: Socket Options,  Next: Networks Database,  Prev: Inetd,  Up: Sockets

Socket Options
==============

   This section describes how to read or set various options that modify
the behavior of sockets and their underlying communications protocols.

   When you are manipulating a socket option, you must specify which
"level" the option pertains to.  This describes whether the option
applies to the socket interface, or to a lower-level communications
protocol interface.

* Menu:

* Socket Option Functions::     The basic functions for setting and getting
                                 socket options.
* Socket-Level Options::        Details of the options at the socket level.

