------------------------------------------------------------------------------
--                                                                          --
--                         GNAT COMPILER COMPONENTS                         --
--                                                                          --
--                         G N A T . S O C K E T S                          --
--                                                                          --
--                                 S p e c                                  --
--                                                                          --
--                     Copyright (C) 2001-2019, AdaCore                     --
--                                                                          --
-- GNAT is free software;  you can  redistribute it  and/or modify it under --
-- terms of the  GNU General Public License as published  by the Free Soft- --
-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
--                                                                          --
-- As a special exception under Section 7 of GPL version 3, you are granted --
-- additional permissions described in the GCC Runtime Library Exception,   --
-- version 3.1, as published by the Free Software Foundation.               --
--                                                                          --
-- You should have received a copy of the GNU General Public License and    --
-- a copy of the GCC Runtime Library Exception along with this program;     --
-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
-- <http://www.gnu.org/licenses/>.                                          --
--                                                                          --
-- GNAT was originally developed  by the GNAT team at  New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc.      --
--                                                                          --
------------------------------------------------------------------------------

--  This package provides an interface to the sockets communication facility
--  provided on many operating systems. This is implemented on the following
--  platforms:

--     All native ports, with restrictions as follows

--       Multicast is available only on systems which provide support for this
--       feature, so it is not available if Multicast is not supported, or not
--       installed.

--     VxWorks cross ports fully implement this package

--     This package is not yet implemented on LynxOS or other cross ports

with Ada.Exceptions;
with Ada.Streams;
with Ada.Strings.Unbounded;
with Ada.Unchecked_Deallocation;

with Interfaces.C;

with System.OS_Constants;
with System.Storage_Elements;

package GNAT.Sockets is

   --  Sockets are designed to provide a consistent communication facility
   --  between applications. This package provides an Ada binding to the
   --  de-facto standard BSD sockets API. The documentation below covers
   --  only the specific binding provided by this package. It assumes that
   --  the reader is already familiar with general network programming and
   --  sockets usage. A useful reference on this matter is W. Richard Stevens'
   --  "UNIX Network Programming: The Sockets Networking API"
   --  (ISBN: 0131411551).

   --  GNAT.Sockets has been designed with several ideas in mind

   --  This is a system independent interface. Therefore, we try as much as
   --  possible to mask system incompatibilities. Some functionalities are not
   --  available because there are not fully supported on some systems.

   --  This is a thick binding. For instance, a major effort has been done to
   --  avoid using memory addresses or untyped ints. We preferred to define
   --  streams and enumeration types. Errors are not returned as returned
   --  values but as exceptions.

   --  This package provides a POSIX-compliant interface (between two
   --  different implementations of the same routine, we adopt the one closest
   --  to the POSIX specification). For instance, using select(), the
   --  notification of an asynchronous connect failure is delivered in the
   --  write socket set (POSIX) instead of the exception socket set (NT).

   --  The example below demonstrates various features of GNAT.Sockets:

   --  with GNAT.Sockets; use GNAT.Sockets;

   --  with Ada.Text_IO;
   --  with Ada.Exceptions; use Ada.Exceptions;

   --  procedure PingPong is

   --     Group : constant String := "239.255.128.128";
   --     --  Multicast group: administratively scoped IP address

   --     task Pong is
   --        entry Start;
   --        entry Stop;
   --     end Pong;

   --     task body Pong is
   --        Address  : Sock_Addr_Type;
   --        Server   : Socket_Type;
   --        Socket   : Socket_Type;
   --        Channel  : Stream_Access;

   --     begin
   --        --  Get an Internet address of a host (here the local host name).
   --        --  Note that a host can have several addresses. Here we get
   --        --  the first one which is supposed to be the official one.

   --        Address.Addr := Addresses (Get_Host_By_Name (Host_Name), 1);

   --        --  Get a socket address that is an Internet address and a port

   --        Address.Port := 5876;

   --        --  The first step is to create a socket. Once created, this
   --        --  socket must be associated to with an address. Usually only a
   --        --  server (Pong here) needs to bind an address explicitly. Most
   --        --  of the time clients can skip this step because the socket
   --        --  routines will bind an arbitrary address to an unbound socket.

   --        Create_Socket (Server);

   --        --  Allow reuse of local addresses

   --        Set_Socket_Option
   --          (Server,
   --           Socket_Level,
   --           (Reuse_Address, True));

   --        Bind_Socket (Server, Address);

   --        --  A server marks a socket as willing to receive connect events

   --        Listen_Socket (Server);

   --        --  Once a server calls Listen_Socket, incoming connects events
   --        --  can be accepted. The returned Socket is a new socket that
   --        --  represents the server side of the connection. Server remains
   --        --  available to receive further connections.

   --        accept Start;

   --        Accept_Socket (Server, Socket, Address);

   --        --  Return a stream associated to the connected socket

   --        Channel := Stream (Socket);

   --        --  Force Pong to block

   --        delay 0.2;

   --        --  Receive and print message from client Ping

   --        declare
   --           Message : String := String'Input (Channel);

   --        begin
   --           Ada.Text_IO.Put_Line (Message);

   --           --  Send same message back to client Ping

   --           String'Output (Channel, Message);
   --        end;

   --        Close_Socket (Server);
   --        Close_Socket (Socket);

   --        --  Part of the multicast example

   --        --  Create a datagram socket to send connectionless, unreliable
   --        --  messages of a fixed maximum length.

   --        Create_Socket (Socket, Family_Inet, Socket_Datagram);

   --        --  Allow reuse of local addresses

   --        Set_Socket_Option
   --          (Socket,
   --           Socket_Level,
   --           (Reuse_Address, True));

   --        --  Controls the live time of the datagram to avoid it being
   --        --  looped forever due to routing errors. Routers decrement
   --        --  the TTL of every datagram as it traverses from one network
   --        --  to another and when its value reaches 0 the packet is
   --        --  dropped. Default is 1.

   --        Set_Socket_Option
   --          (Socket,
   --           IP_Protocol_For_IP_Level,
   --           (Multicast_TTL, 1));

   --        --  Want the data you send to be looped back to your host

   --        Set_Socket_Option
   --          (Socket,
   --           IP_Protocol_For_IP_Level,
   --           (Multicast_Loop, True));

   --        --  If this socket is intended to receive messages, bind it
   --        --  to a given socket address.

   --        Address.Addr := Any_Inet_Addr;
   --        Address.Port := 55505;

   --        Bind_Socket (Socket, Address);

   --        --  Join a multicast group

   --        --  Portability note: On Windows, this option may be set only
   --        --  on a bound socket.

   --        Set_Socket_Option
   --          (Socket,
   --           IP_Protocol_For_IP_Level,
   --           (Add_Membership, Inet_Addr (Group), Any_Inet_Addr));

   --        --  If this socket is intended to send messages, provide the
   --        --  receiver socket address.

   --        Address.Addr := Inet_Addr (Group);
   --        Address.Port := 55506;

   --        Channel := Stream (Socket, Address);

   --        --  Receive and print message from client Ping

   --        declare
   --           Message : String := String'Input (Channel);

   --        begin
   --           --  Get the address of the sender

   --           Address := Get_Address (Channel);
   --           Ada.Text_IO.Put_Line (Message & " from " & Image (Address));

   --           --  Send same message back to client Ping

   --           String'Output (Channel, Message);
   --        end;

   --        Close_Socket (Socket);

   --        accept Stop;

   --     exception when E : others =>
   --        Ada.Text_IO.Put_Line
   --          (Exception_Name (E) & ": " & Exception_Message (E));
   --     end Pong;

   --     task Ping is
   --        entry Start;
   --        entry Stop;
   --     end Ping;

   --     task body Ping is
   --        Address  : Sock_Addr_Type;
   --        Socket   : Socket_Type;
   --        Channel  : Stream_Access;

   --     begin
   --        accept Start;

   --        --  See comments in Ping section for the first steps

   --        Address.Addr := Addresses (Get_Host_By_Name (Host_Name), 1);
   --        Address.Port := 5876;
   --        Create_Socket (Socket);

   --        Set_Socket_Option
   --          (Socket,
   --           Socket_Level,
   --           (Reuse_Address, True));

   --        --  Force Ping to block

   --        delay 0.2;

   --        --  If the client's socket is not bound, Connect_Socket will
   --        --  bind to an unused address. The client uses Connect_Socket to
   --        --  create a logical connection between the client's socket and
   --        --  a server's socket returned by Accept_Socket.

   --        Connect_Socket (Socket, Address);

   --        Channel := Stream (Socket);

   --        --  Send message to server Pong

   --        String'Output (Channel, "Hello world");

   --        --  Force Ping to block

   --        delay 0.2;

   --        --  Receive and print message from server Pong

   --        Ada.Text_IO.Put_Line (String'Input (Channel));
   --        Close_Socket (Socket);

   --        --  Part of multicast example. Code similar to Pong's one

   --        Create_Socket (Socket, Family_Inet, Socket_Datagram);

   --        Set_Socket_Option
   --          (Socket,
   --           Socket_Level,
   --           (Reuse_Address, True));

   --        Set_Socket_Option
   --          (Socket,
   --           IP_Protocol_For_IP_Level,
   --           (Multicast_TTL, 1));

   --        Set_Socket_Option
   --          (Socket,
   --           IP_Protocol_For_IP_Level,
   --           (Multicast_Loop, True));

   --        Address.Addr := Any_Inet_Addr;
   --        Address.Port := 55506;

   --        Bind_Socket (Socket, Address);

   --        Set_Socket_Option
   --          (Socket,
   --           IP_Protocol_For_IP_Level,
   --           (Add_Membership, Inet_Addr (Group), Any_Inet_Addr));

   --        Address.Addr := Inet_Addr (Group);
   --        Address.Port := 55505;

   --        Channel := Stream (Socket, Address);

   --        --  Send message to server Pong

   --        String'Output (Channel, "Hello world");

   --        --  Receive and print message from server Pong

   --        declare
   --           Message : String := String'Input (Channel);

   --        begin
   --           Address := Get_Address (Channel);
   --           Ada.Text_IO.Put_Line (Message & " from " & Image (Address));
   --        end;

   --        Close_Socket (Socket);

   --        accept Stop;

   --     exception when E : others =>
   --        Ada.Text_IO.Put_Line
   --          (Exception_Name (E) & ": " & Exception_Message (E));
   --     end Ping;

   --  begin
   --     Initialize;
   --     Ping.Start;
   --     Pong.Start;
   --     Ping.Stop;
   --     Pong.Stop;
   --     Finalize;
   --  end PingPong;

   package SOSC renames System.OS_Constants;
   --  Renaming used to provide short-hand notations throughout the sockets
   --  binding. Note that System.OS_Constants is an internal unit, and the
   --  entities declared therein are not meant for direct access by users,
   --  including through this renaming.

   use type Interfaces.C.int;
   --  Need visibility on "-" operator so that we can write -1

   procedure Initialize;
   pragma Obsolescent
     (Entity  => Initialize,
      Message => "explicit initialization is no longer required");
   --  Initialize must be called before using any other socket routines.
   --  Note that this operation is a no-op on UNIX platforms, but applications
   --  should make sure to call it if portability is expected: some platforms
   --  (such as Windows) require initialization before any socket operation.
   --  This is now a no-op (initialization and finalization are done
   --  automatically).

   procedure Initialize (Process_Blocking_IO : Boolean);
   pragma Obsolescent
     (Entity  => Initialize,
      Message => "passing a parameter to Initialize is no longer supported");
   --  Previous versions of GNAT.Sockets used to require the user to indicate
   --  whether socket I/O was process- or thread-blocking on the platform.
   --  This property is now determined automatically when the run-time library
   --  is built. The old version of Initialize, taking a parameter, is kept
   --  for compatibility reasons, but this interface is obsolete (and if the
   --  value given is wrong, an exception will be raised at run time).
   --  This is now a no-op (initialization and finalization are done
   --  automatically).

   procedure Finalize;
   pragma Obsolescent
     (Entity  => Finalize,
      Message => "explicit finalization is no longer required");
   --  After Finalize is called it is not possible to use any routines
   --  exported in by this package. This procedure is idempotent.
   --  This is now a no-op (initialization and finalization are done
   --  automatically).

   type Socket_Type is private;
   --  Sockets are used to implement a reliable bi-directional point-to-point,
   --  stream-based connections between hosts. No_Socket provides a special
   --  value to denote uninitialized sockets.

   No_Socket : constant Socket_Type;

   type Selector_Type is limited private;
   type Selector_Access is access all Selector_Type;
   --  Selector objects are used to wait for i/o events to occur on sockets

   Null_Selector : constant Selector_Type;
   --  The Null_Selector can be used in place of a normal selector without
   --  having to call Create_Selector if the use of Abort_Selector is not
   --  required.

   --  Timeval_Duration is a subtype of Standard.Duration because the full
   --  range of Standard.Duration cannot be represented in the equivalent C
   --  structure (struct timeval). Moreover, negative values are not allowed
   --  to avoid system incompatibilities.

   Immediate : constant Duration := 0.0;

   Forever : constant Duration :=
               Duration'Min
                 (Duration'Last,
                  (if SOSC."=" (SOSC.Target_OS, SOSC.Windows)
                   then Duration (2 ** 32 / 1000)
                   else 1.0 * SOSC.MAX_tv_sec));
   --  Largest possible Duration that is also a valid value for the OS type
   --  used for socket timeout.

   subtype Timeval_Duration is Duration range Immediate .. Forever;

   subtype Selector_Duration is Timeval_Duration;
   --  Timeout value for selector operations

   type Selector_Status is (Completed, Expired, Aborted);
   --  Completion status of a selector operation, indicated as follows:
   --    Complete: one of the expected events occurred
   --    Expired:  no event occurred before the expiration of the timeout
   --    Aborted:  an external action cancelled the wait operation before
   --              any event occurred.

   Socket_Error : exception;
   --  There is only one exception in this package to deal with an error during
   --  a socket routine. Once raised, its message contains a string describing
   --  the error code.

   function Image (Socket : Socket_Type) return String;
   --  Return a printable string for Socket

   function To_Ada (Fd : Integer) return Socket_Type with Inline;
   --  Convert a file descriptor to Socket_Type. This is useful when a socket
   --  file descriptor is obtained from an external library call.

   function To_C (Socket : Socket_Type) return Integer with Inline;
   --  Return a file descriptor to be used by external subprograms. This is
   --  useful for C functions that are not yet interfaced in this package.

   type Family_Type is (Family_Inet, Family_Inet6, Family_Unix, Family_Unspec);
   --  Address family (or protocol family) identifies the communication domain
   --  and groups protocols with similar address formats.
   --  The order of the enumeration elements should not be changed unilaterally
   --  because the IPv6_TCP_Preferred routine rely on it.

   subtype Family_Inet_4_6 is Family_Type range Family_Inet .. Family_Inet6;

   type Mode_Type is (Socket_Stream, Socket_Datagram, Socket_Raw);
   --  Stream sockets provide connection-oriented byte streams. Datagram
   --  sockets support unreliable connectionless message-based communication.
   --  Raw sockets provide raw network-protocol access.
   --  The order of the enumeration elements should not be changed unilaterally
   --  because the IPv6_TCP_Preferred routine relies on it.

   type Shutmode_Type is (Shut_Read, Shut_Write, Shut_Read_Write);
   --  When a process closes a socket, the policy is to retain any data queued
   --  until either a delivery or a timeout expiration (in this case, the data
   --  are discarded). Finer control is available through shutdown. With
   --  Shut_Read, no more data can be received from the socket. With_Write, no
   --  more data can be transmitted. Neither transmission nor reception can be
   --  performed with Shut_Read_Write.

   type Port_Type is range 0 .. 16#ffff#;
   --  TCP/UDP port number

   Any_Port : constant Port_Type;
   --  All ports

   No_Port : constant Port_Type;
   --  Uninitialized port number

   type Inet_Addr_Comp_Type is mod 2 ** 8;
   --  Octet for Internet address

   Inet_Addr_Bytes_Length : constant array (Family_Inet_4_6) of Natural :=
     (Family_Inet => 4, Family_Inet6 => 16);

   type Inet_Addr_Bytes is array (Natural range <>) of Inet_Addr_Comp_Type;

   subtype Inet_Addr_V4_Type is
     Inet_Addr_Bytes (1 ..  Inet_Addr_Bytes_Length (Family_Inet));
   subtype Inet_Addr_V6_Type is
     Inet_Addr_Bytes (1 ..  Inet_Addr_Bytes_Length (Family_Inet6));

   subtype Inet_Addr_VN_Type is Inet_Addr_Bytes;
   --  For backwards compatibility

   type Inet_Addr_Type (Family : Family_Inet_4_6 := Family_Inet) is record
      case Family is
         when Family_Inet =>
            Sin_V4 : Inet_Addr_V4_Type := (others => 0);

         when Family_Inet6 =>
            Sin_V6 : Inet_Addr_V6_Type := (others => 0);

      end case;
   end record;

   --  An Internet address depends on an address family (IPv4 contains 4 octets
   --  and IPv6 contains 16 octets).

   Any_Inet_Addr       : constant Inet_Addr_Type;
   --  Wildcard enabling all addresses to use with bind

   Any_Inet6_Addr      : constant Inet_Addr_Type;
   --  Idem for IPV6 socket

   No_Inet_Addr        : constant Inet_Addr_Type;
   --  Uninitialized inet address

   Broadcast_Inet_Addr : constant Inet_Addr_Type;
   --  Broadcast destination address in the current network

   Loopback_Inet_Addr  : constant Inet_Addr_Type;
   --  Loopback address to the local host

   Loopback_Inet6_Addr : constant Inet_Addr_Type;
   --  IPv6 Loopback address to the local host

   --  Useful constants for multicast addresses

   Unspecified_Group_Inet_Addr  : constant Inet_Addr_Type;
   --  IPv4 multicast mask with prefix length 4

   Unspecified_Group_Inet6_Addr : constant Inet_Addr_Type;
   --  IPv6 multicast mask with prefix length 16

   All_Hosts_Group_Inet_Addr    : constant Inet_Addr_Type;
   --  Multicast group addresses all hosts on the same network segment

   All_Hosts_Group_Inet6_Addr   : constant Inet_Addr_Type;
   --  Idem for IPv6 protocol

   All_Routers_Group_Inet_Addr  : constant Inet_Addr_Type;
   --  Multicast group addresses all routers on the same network segment

   All_Routers_Group_Inet6_Addr : constant Inet_Addr_Type;
   --  Idem for IPv6 protocol

   IPv4_To_IPv6_Prefix : constant Inet_Addr_Bytes :=
     (1 .. 10 => 0, 11 .. 12 => 255);
   --  Prefix for IPv4 mapped to IPv6 addresses

   --  Functions to handle masks and prefixes

   function Mask
     (Family : Family_Inet_4_6;
      Length : Natural;
      Host   : Boolean := False) return Inet_Addr_Type;
   --  Return an address mask of the given family with the given prefix length.
   --  If Host is False, this is a network mask (i.e. network bits are 1,
   --  and host bits are 0); if Host is True, this is a host mask (i.e.
   --  network bits are 0, and host bits are 1).

   function "and" (Addr, Mask : Inet_Addr_Type) return Inet_Addr_Type;
   function "or" (Net, Host : Inet_Addr_Type) return Inet_Addr_Type;
   function "not" (Mask : Inet_Addr_Type) return Inet_Addr_Type;
   --  Bit-wise operations on inet addresses (both operands must have the
   --  same address family).

   type Sock_Addr_Type (Family : Family_Type := Family_Inet) is record
      case Family is
         when Family_Unix =>
            Name : Ada.Strings.Unbounded.Unbounded_String;
         when Family_Inet_4_6 =>
            Addr : Inet_Addr_Type (Family);
            Port : Port_Type;
         when Family_Unspec =>
            null;
      end case;
   end record;
   pragma No_Component_Reordering (Sock_Addr_Type);
   --  Socket addresses fully define a socket connection with protocol family,
   --  an Internet address and a port. No_Sock_Addr provides a special value
   --  for uninitialized socket addresses.

   No_Sock_Addr : constant Sock_Addr_Type;
   --  Uninitialized socket address

   function Is_IPv4_Address (Name : String) return Boolean;
   --  Return true when Name is an IPv4 address in dotted quad notation

   function Is_IPv6_Address (Name : String) return Boolean;
   --  Return true when Name is an IPv6 address in numeric format

   function Image (Value : Inet_Addr_Type) return String;
   --  Return an image of an Internet address. IPv4 notation consists in 4
   --  octets in decimal format separated by dots. IPv6 notation consists in
   --  8 hextets in hexadecimal format separated by colons.

   function Image (Value : Sock_Addr_Type) return String;
   --  Return socket address image. Network socket address image will be with
   --  a port image separated by a colon.

   function Inet_Addr (Image : String) return Inet_Addr_Type;
   --  Convert address image from numbers-dots-and-colons notation into an
   --  inet address.

   function Unix_Socket_Address (Addr : String) return Sock_Addr_Type;
   --  Convert unix local socket name to Sock_Addr_Type

   function Network_Socket_Address
     (Addr : Inet_Addr_Type; Port : Port_Type) return Sock_Addr_Type;
   --  Create network socket address

   --  Host entries provide complete information on a given host: the official
   --  name, an array of alternative names or aliases and array of network
   --  addresses.

   type Host_Entry_Type
     (Aliases_Length, Addresses_Length : Natural) is private;

   function Official_Name (E : Host_Entry_Type) return String;
   --  Return official name in host entry

   function Aliases_Length (E : Host_Entry_Type) return Natural;
   --  Return number of aliases in host entry

   function Addresses_Length (E : Host_Entry_Type) return Natural;
   --  Return number of addresses in host entry

   function Aliases
     (E : Host_Entry_Type;
      N : Positive := 1) return String;
   --  Return N'th aliases in host entry. The first index is 1

   function Addresses
     (E : Host_Entry_Type;
      N : Positive := 1) return Inet_Addr_Type;
   --  Return N'th addresses in host entry. The first index is 1

   Host_Error : exception;
   --  Exception raised by the two following procedures. Once raised, its
   --  message contains a string describing the error code. This exception is
   --  raised when an host entry cannot be retrieved.

   function Get_Host_By_Address
     (Address : Inet_Addr_Type;
      Family  : Family_Type := Family_Inet) return Host_Entry_Type;
   --  Return host entry structure for the given Inet address. Note that no
   --  result will be returned if there is no mapping of this IP address to a
   --  host name in the system tables (host database, DNS or otherwise).

   function Get_Host_By_Name
     (Name : String) return Host_Entry_Type;
   --  Return host entry structure for the given host name. Here name is
   --  either a host name, or an IP address. If Name is an IP address, this
   --  is equivalent to Get_Host_By_Address (Inet_Addr (Name)).

   function Host_Name return String;
   --  Return the name of the current host

   type Service_Entry_Type (Aliases_Length : Natural) is private;
   --  Service entries provide complete information on a given service: the
   --  official name, an array of alternative names or aliases and the port
   --  number.

   function Official_Name (S : Service_Entry_Type) return String;
   --  Return official name in service entry

   function Port_Number (S : Service_Entry_Type) return Port_Type;
   --  Return port number in service entry

   function Protocol_Name (S : Service_Entry_Type) return String;
   --  Return Protocol in service entry (usually UDP or TCP)

   function Aliases_Length (S : Service_Entry_Type) return Natural;
   --  Return number of aliases in service entry

   function Aliases
     (S : Service_Entry_Type;
      N : Positive := 1) return String;
   --  Return N'th aliases in service entry (the first index is 1)

   function Get_Service_By_Name
     (Name     : String;
      Protocol : String) return Service_Entry_Type;
   --  Return service entry structure for the given service name

   function Get_Service_By_Port
     (Port     : Port_Type;
      Protocol : String) return Service_Entry_Type;
   --  Return service entry structure for the given service port number

   Service_Error : exception;
   --  Comment required ???

   --  Errors are described by an enumeration type. There is only one exception
   --  Socket_Error in this package to deal with an error during a socket
   --  routine. Once raised, its message contains the error code between
   --  brackets and a string describing the error code.

   --  The name of the enumeration constant documents the error condition
   --  Note that on some platforms, a single error value is used for both
   --  EWOULDBLOCK and EAGAIN. Both errors are therefore always reported as
   --  Resource_Temporarily_Unavailable.

   type Error_Type is
     (Success,
      Permission_Denied,
      Address_Already_In_Use,
      Cannot_Assign_Requested_Address,
      Address_Family_Not_Supported_By_Protocol,
      Operation_Already_In_Progress,
      Bad_File_Descriptor,
      Software_Caused_Connection_Abort,
      Connection_Refused,
      Connection_Reset_By_Peer,
      Destination_Address_Required,
      Bad_Address,
      Host_Is_Down,
      No_Route_To_Host,
      Operation_Now_In_Progress,
      Interrupted_System_Call,
      Invalid_Argument,
      Input_Output_Error,
      Transport_Endpoint_Already_Connected,
      Too_Many_Symbolic_Links,
      Too_Many_Open_Files,
      Message_Too_Long,
      File_Name_Too_Long,
      Network_Is_Down,
      Network_Dropped_Connection_Because_Of_Reset,
      Network_Is_Unreachable,
      No_Buffer_Space_Available,
      Protocol_Not_Available,
      Transport_Endpoint_Not_Connected,
      Socket_Operation_On_Non_Socket,
      Operation_Not_Supported,
      Protocol_Family_Not_Supported,
      Protocol_Not_Supported,
      Protocol_Wrong_Type_For_Socket,
      Cannot_Send_After_Transport_Endpoint_Shutdown,
      Socket_Type_Not_Supported,
      Connection_Timed_Out,
      Too_Many_References,
      Resource_Temporarily_Unavailable,
      Broken_Pipe,
      Unknown_Host,
      Host_Name_Lookup_Failure,
      Non_Recoverable_Error,
      Unknown_Server_Error,
      Cannot_Resolve_Error);

   --  Get_Socket_Options and Set_Socket_Options manipulate options associated
   --  with a socket. Options may exist at multiple protocol levels in the
   --  communication stack. Socket_Level is the uppermost socket level.

   type Level_Type is
     (Socket_Level,
      IP_Protocol_For_IP_Level,
      IP_Protocol_For_IPv6_Level,
      IP_Protocol_For_UDP_Level,
      IP_Protocol_For_TCP_Level,
      IP_Protocol_For_ICMP_Level,
      IP_Protocol_For_IGMP_Level,
      IP_Protocol_For_RAW_Level);

   --  There are several options available to manipulate sockets. Each option
   --  has a name and several values available. Most of the time, the value
   --  is a boolean to enable or disable this option. Each socket option is
   --  provided with an appropriate C name taken from the sockets API comments.
   --  The C name can be used to find a detailed description in the OS-specific
   --  documentation. The options are grouped by main Level_Type value, which
   --  can be used together with this option in calls to the Set_Socket_Option
   --  and Get_Socket_Option routines. Note that some options can be used with
   --  more than one level.

   type Option_Name is
     (Generic_Option,
      --  Can be used to set/get any socket option via an OS-specific option
      --  code with an integer value.

      ------------------
      -- Socket_Level --
      ------------------

      Keep_Alive,      -- SO_KEEPALIVE
      --  Enable sending of keep-alive messages on connection-oriented sockets

      Reuse_Address,   -- SO_REUSEADDR
      --  Enable binding to an address and port already in use

      Broadcast,       -- SO_BROADCAST
      --  Enable sending broadcast datagrams on the socket

      Send_Buffer,     -- SO_SNDBUF
      --  Set/get the maximum socket send buffer in bytes

      Receive_Buffer,  -- SO_RCVBUF
      --  Set/get the maximum socket receive buffer in bytes

      Linger,          -- SO_LINGER
      --  When enabled, a Close_Socket or Shutdown_Socket will wait until all
      --  queued messages for the socket have been successfully sent or the
      --  linger timeout has been reached.

      Error,           -- SO_ERROR
      --  Get and clear the pending socket error integer code

      Send_Timeout,    -- SO_SNDTIMEO
      --  Specify sending timeout until reporting an error

      Receive_Timeout, -- SO_RCVTIMEO
      --  Specify receiving timeout until reporting an error

      Busy_Polling,    -- SO_BUSY_POLL
      --  Sets the approximate time in microseconds to busy poll on a blocking
      --  receive when there is no data.

      -------------------------------
      -- IP_Protocol_For_TCP_Level --
      -------------------------------

      No_Delay, -- TCP_NODELAY
      --  Disable the Nagle algorithm. This means that output buffer content
      --  is always sent as soon as possible, even if there is only a small
      --  amount of data.

      ------------------------------
      -- IP_Protocol_For_IP_Level --
      ------------------------------

      Add_Membership_V4,   -- IP_ADD_MEMBERSHIP
      --  Join a multicast group

      Drop_Membership_V4,  -- IP_DROP_MEMBERSHIP
      --  Leave a multicast group

      Multicast_If_V4,     -- IP_MULTICAST_IF
      --  Set/Get outgoing interface for sending multicast packets

      Multicast_Loop_V4,   -- IP_MULTICAST_LOOP
      --  This boolean option determines whether sent multicast packets should
      --  be looped back to the local sockets.

      Multicast_TTL,       -- IP_MULTICAST_TTL
      --  Set/Get the time-to-live of sent multicast packets

      Receive_Packet_Info, -- IP_PKTINFO
      --  Receive low-level packet info as ancillary data

      --------------------------------
      -- IP_Protocol_For_IPv6_Level --
      --------------------------------

      Add_Membership_V6,   -- IPV6_ADD_MEMBERSHIP
      --  Join IPv6 multicast group

      Drop_Membership_V6,  -- IPV6_DROP_MEMBERSHIP
      --  Leave IPv6 multicast group

      Multicast_If_V6,     -- IPV6_MULTICAST_IF
      --  Set/Get outgoing interface index for sending multicast packets

      Multicast_Loop_V6,   -- IPV6_MULTICAST_LOOP
      --  This boolean option determines whether sent multicast IPv6 packets
      --  should be looped back to the local sockets.

      IPv6_Only,           -- IPV6_V6ONLY
      --  Restricted to IPv6 communications only

      Multicast_Hops       -- IPV6_MULTICAST_HOPS
      --  Set the multicast hop limit for the IPv6 socket
     );

   subtype Specific_Option_Name is
     Option_Name range Keep_Alive .. Option_Name'Last;

   Add_Membership  : Option_Name renames Add_Membership_V4;
   Drop_Membership : Option_Name renames Drop_Membership_V4;
   Multicast_If    : Option_Name renames Multicast_If_V4;
   Multicast_Loop  : Option_Name renames Multicast_Loop_V4;

   type Option_Type (Name : Option_Name := Keep_Alive) is record
      case Name is
         when Generic_Option =>
            Optname : Interfaces.C.int := -1;
            Optval  : Interfaces.C.int;

         when Keep_Alive          |
              Reuse_Address       |
              Broadcast           |
              Linger              |
              No_Delay            |
              Receive_Packet_Info |
              IPv6_Only           |
              Multicast_Loop_V4   |
              Multicast_Loop_V6   =>
            Enabled : Boolean;

            case Name is
               when Linger    =>
                  Seconds : Natural;
               when others    =>
                  null;
            end case;

         when Busy_Polling    =>
            Microseconds : Natural;

         when Send_Buffer     |
              Receive_Buffer  =>
            Size : Natural;

         when Error           =>
            Error : Error_Type;

         when Add_Membership_V4  |
              Add_Membership_V6  |
              Drop_Membership_V4 |
              Drop_Membership_V6 =>
            Multicast_Address : Inet_Addr_Type;
            case Name is
               when Add_Membership_V4  |
                    Drop_Membership_V4 =>
                  Local_Interface : Inet_Addr_Type;
               when others =>
                  Interface_Index : Natural;
            end case;

         when Multicast_If_V4 =>
            Outgoing_If : Inet_Addr_Type;

         when Multicast_If_V6 =>
            Outgoing_If_Index : Natural;

         when Multicast_TTL  =>
            Time_To_Live : Natural;

         when Multicast_Hops =>
            Hop_Limit : Integer range -1 .. 255;

         when Send_Timeout |
              Receive_Timeout =>
            Timeout : Timeval_Duration;

      end case;
   end record;

   --  There are several controls available to manipulate sockets. Each option
   --  has a name and several values available. These controls differ from the
   --  socket options in that they are not specific to sockets but are
   --  available for any device.

   type Request_Name is
     (Non_Blocking_IO,  --  Cause a caller not to wait on blocking operations
      N_Bytes_To_Read); --  Return the number of bytes available to read

   type Request_Type (Name : Request_Name := Non_Blocking_IO) is record
      case Name is
         when Non_Blocking_IO =>
            Enabled : Boolean;

         when N_Bytes_To_Read =>
            Size : Natural;

      end case;
   end record;

   --  A request flag allows specification of the type of message transmissions
   --  or receptions. A request flag can be combination of zero or more
   --  predefined request flags.

   type Request_Flag_Type is private;

   No_Request_Flag : constant Request_Flag_Type;
   --  This flag corresponds to the normal execution of an operation

   Process_Out_Of_Band_Data : constant Request_Flag_Type;
   --  This flag requests that the receive or send function operates on
   --  out-of-band data when the socket supports this notion (e.g.
   --  Socket_Stream).

   Peek_At_Incoming_Data : constant Request_Flag_Type;
   --  This flag causes the receive operation to return data from the beginning
   --  of the receive queue without removing that data from the queue. A
   --  subsequent receive call will return the same data.

   Wait_For_A_Full_Reception : constant Request_Flag_Type;
   --  This flag requests that the operation block until the full request is
   --  satisfied. However, the call may still return less data than requested
   --  if a signal is caught, an error or disconnect occurs, or the next data
   --  to be received is of a different type than that returned. Note that
   --  this flag depends on support in the underlying sockets implementation,
   --  and is not supported under Windows.

   Send_End_Of_Record : constant Request_Flag_Type;
   --  This flag indicates that the entire message has been sent and so this
   --  terminates the record.

   function "+" (L, R : Request_Flag_Type) return Request_Flag_Type;
   --  Combine flag L with flag R

   type Stream_Element_Reference is access all Ada.Streams.Stream_Element;

   type Vector_Element is record
      Base   : Stream_Element_Reference;
      Length : Interfaces.C.size_t;
   end record;

   type Vector_Type is array (Integer range <>) of Vector_Element;

   type Address_Info is record
      Addr  : Sock_Addr_Type;
      Mode  : Mode_Type  := Socket_Stream;
      Level : Level_Type := IP_Protocol_For_IP_Level;
   end record;

   type Address_Info_Array is array (Positive range <>) of Address_Info;

   function Get_Address_Info
     (Host         : String;
      Service      : String;
      Family       : Family_Type := Family_Unspec;
      Mode         : Mode_Type   := Socket_Stream;
      Level        : Level_Type  := IP_Protocol_For_IP_Level;
      Numeric_Host : Boolean     := False;
      Passive      : Boolean     := False;
      Unknown      : access procedure
        (Family, Mode, Level, Length : Integer) := null)
      return Address_Info_Array;
   --  Returns available addresses for the Host and Service names.
   --  If Family is Family_Unspec, all available protocol families returned.
   --  Service is the name of service as defined in /etc/services or port
   --  number in string representation.
   --  If Unknown procedure access specified it will be called in case of
   --  unknown family found.
   --  Numeric_Host flag suppresses any potentially lengthy network host
   --  address lookups, and Host have to represent numerical network address in
   --  this case.
   --  If Passive is True and Host is empty then the returned socket addresses
   --  will be suitable for binding a socket that will accept connections.
   --  The returned socket address will contain  the  "wildcard  address".
   --  The wildcard address is used by applications (typically servers) that
   --  intend to accept connections on any of the hosts's network addresses.
   --  If Host is not empty, then the Passive flag is ignored.
   --  If Passive is False, then the returned socket addresses will be suitable
   --  for use with connect, sendto, or sendmsg.  If Host is empty, then the
   --  network address will be set  to  the  loopback  interface  address;
   --  this is used by applications that intend to communicate with peers
   --  running on the same host.

   procedure Sort
     (Addr_Info : in out Address_Info_Array;
      Compare   : access function (Left, Right : Address_Info) return Boolean);
   --  Sort address info array in order defined by compare function

   function IPv6_TCP_Preferred (Left, Right : Address_Info) return Boolean;
   --  To use with Sort to order where IPv6 and TCP addresses first

   type Host_Service (Host_Length, Service_Length : Natural) is record
      Host    : String (1 .. Host_Length);
      Service : String (1 .. Service_Length);
   end record;

   function Get_Name_Info
     (Addr         : Sock_Addr_Type;
      Numeric_Host : Boolean := False;
      Numeric_Serv : Boolean := False) return Host_Service;
   --  Returns host and service names by the address and port.
   --  If Numeric_Host is True, then the numeric form of the hostname is
   --  returned. When Numeric_Host is False, this will still happen in case the
   --  host name cannot be determined.
   --  If Numenric_Serv is True, then the numeric form of the service address
   --  (port number) is returned.  When Numenric_Serv is False, this will still
   --  happen in case the service's name cannot be determined.

   procedure Create_Socket
     (Socket : out Socket_Type;
      Family : Family_Type := Family_Inet;
      Mode   : Mode_Type   := Socket_Stream;
      Level  : Level_Type  := IP_Protocol_For_IP_Level);
   --  Create an endpoint for communication. Raises Socket_Error on error.

   procedure Create_Socket_Pair
     (Left   : out Socket_Type;
      Right  : out Socket_Type;
      Family : Family_Type := Family_Unspec;
      Mode   : Mode_Type   := Socket_Stream;
      Level  : Level_Type  := IP_Protocol_For_IP_Level);
   --  Create two connected sockets. Raises Socket_Error on error.
   --  If Family is unspecified, it creates Family_Unix sockets on UNIX and
   --  Family_Inet sockets on non UNIX platforms.

   procedure Accept_Socket
     (Server  : Socket_Type;
      Socket  : out Socket_Type;
      Address : out Sock_Addr_Type);
   --  Extracts the first connection request on the queue of pending
   --  connections, creates a new connected socket with mostly the same
   --  properties as Server, and allocates a new socket. The returned Address
   --  is filled in with the address of the connection. Raises Socket_Error on
   --  error. Note: if Server is a non-blocking socket, whether or not this
   --  aspect is inherited by Socket is platform-dependent.

   procedure Accept_Socket
     (Server   : Socket_Type;
      Socket   : out Socket_Type;
      Address  : out Sock_Addr_Type;
      Timeout  : Selector_Duration;
      Selector : access Selector_Type := null;
      Status   : out Selector_Status);
   --  Accept a new connection on Server using Accept_Socket, waiting no longer
   --  than the given timeout duration. Status is set to indicate whether the
   --  operation completed successfully, timed out, or was aborted. If Selector
   --  is not null, the designated selector is used to wait for the socket to
   --  become available, else a private selector object is created by this
   --  procedure and destroyed before it returns.

   procedure Bind_Socket
     (Socket  : Socket_Type;
      Address : Sock_Addr_Type);
   --  Once a socket is created, assign a local address to it. Raise
   --  Socket_Error on error.

   procedure Close_Socket (Socket : Socket_Type);
   --  Close a socket and more specifically a non-connected socket

   procedure Connect_Socket
     (Socket : Socket_Type;
      Server : Sock_Addr_Type);
   --  Make a connection to another socket which has the address of Server.
   --  Raises Socket_Error on error.

   procedure Connect_Socket
     (Socket   : Socket_Type;
      Server   : Sock_Addr_Type;
      Timeout  : Selector_Duration;
      Selector : access Selector_Type := null;
      Status   : out Selector_Status);
   --  Connect Socket to the given Server address using Connect_Socket, waiting
   --  no longer than the given timeout duration. Status is set to indicate
   --  whether the operation completed successfully, timed out, or was aborted.
   --  If Selector is not null, the designated selector is used to wait for the
   --  socket to become available, else a private selector object is created
   --  by this procedure and destroyed before it returns. If Timeout is 0.0,
   --  no attempt is made to detect whether the connection has succeeded; it
   --  is up to the user to determine this using Check_Selector later on.

   procedure Control_Socket
     (Socket  : Socket_Type;
      Request : in out Request_Type);
   --  Obtain or set parameter values that control the socket. This control
   --  differs from the socket options in that they are not specific to sockets
   --  but are available for any device.

   function Get_Peer_Name (Socket : Socket_Type) return Sock_Addr_Type;
   --  Return the peer or remote socket address of a socket. Raise
   --  Socket_Error on error.

   function Get_Socket_Name (Socket : Socket_Type) return Sock_Addr_Type;
   --  Return the local or current socket address of a socket. Return
   --  No_Sock_Addr on error (e.g. socket closed or not locally bound).

   function Get_Socket_Option
     (Socket  : Socket_Type;
      Level   : Level_Type;
      Name    : Option_Name;
      Optname : Interfaces.C.int := -1) return Option_Type;
   --  Get the options associated with a socket. Raises Socket_Error on error.
   --  Optname identifies specific option when Name is Generic_Option.

   procedure Listen_Socket
     (Socket : Socket_Type;
      Length : Natural := 15);
   --  To accept connections, a socket is first created with Create_Socket,
   --  a willingness to accept incoming connections and a queue Length for
   --  incoming connections are specified. Raise Socket_Error on error.
   --  The queue length of 15 is an example value that should be appropriate
   --  in usual cases. It can be adjusted according to each application's
   --  particular requirements.

   procedure Receive_Socket
     (Socket : Socket_Type;
      Item   : out Ada.Streams.Stream_Element_Array;
      Last   : out Ada.Streams.Stream_Element_Offset;
      Flags  : Request_Flag_Type := No_Request_Flag);
   --  Receive message from Socket. Last is the index value such that Item
   --  (Last) is the last character assigned. Note that Last is set to
   --  Item'First - 1 when the socket has been closed by peer. This is not
   --  an error, and no exception is raised in this case unless Item'First
   --  is Stream_Element_Offset'First, in which case Constraint_Error is
   --  raised. Flags allows control of the reception. Raise Socket_Error on
   --  error.

   procedure Receive_Socket
     (Socket : Socket_Type;
      Item   : out Ada.Streams.Stream_Element_Array;
      Last   : out Ada.Streams.Stream_Element_Offset;
      From   : out Sock_Addr_Type;
      Flags  : Request_Flag_Type := No_Request_Flag);
   --  Receive message from Socket. If Socket is not connection-oriented, the
   --  source address From of the message is filled in. Last is the index
   --  value such that Item (Last) is the last character assigned. Flags
   --  allows control of the reception. Raises Socket_Error on error.

   procedure Receive_Vector
     (Socket : Socket_Type;
      Vector : Vector_Type;
      Count  : out Ada.Streams.Stream_Element_Count;
      Flags  : Request_Flag_Type := No_Request_Flag);
   --  Receive data from a socket and scatter it into the set of vector
   --  elements Vector. Count is set to the count of received stream elements.
   --  Flags allow control over reception.

   function Resolve_Exception
     (Occurrence : Ada.Exceptions.Exception_Occurrence) return Error_Type;
   --  When Socket_Error or Host_Error are raised, the exception message
   --  contains the error code between brackets and a string describing the
   --  error code. Resolve_Error extracts the error code from an exception
   --  message and translate it into an enumeration value.

   procedure Send_Socket
     (Socket : Socket_Type;
      Item   : Ada.Streams.Stream_Element_Array;
      Last   : out Ada.Streams.Stream_Element_Offset;
      To     : access Sock_Addr_Type;
      Flags  : Request_Flag_Type := No_Request_Flag);
   pragma Inline (Send_Socket);
   --  Transmit a message over a socket. For a datagram socket, the address
   --  is given by To.all. For a stream socket, To must be null. Last
   --  is the index value such that Item (Last) is the last character
   --  sent. Note that Last is set to Item'First - 1 if the socket has been
   --  closed by the peer (unless Item'First is Stream_Element_Offset'First,
   --  in which case Constraint_Error is raised instead). This is not an error,
   --  and Socket_Error is not raised in that case. Flags allows control of the
   --  transmission. Raises exception Socket_Error on error. Note: this
   --  subprogram is inlined because it is also used to implement the two
   --  variants below.

   procedure Send_Socket
     (Socket : Socket_Type;
      Item   : Ada.Streams.Stream_Element_Array;
      Last   : out Ada.Streams.Stream_Element_Offset;
      Flags  : Request_Flag_Type := No_Request_Flag);
   --  Transmit a message over a socket. Upon return, Last is set to the index
   --  within Item of the last element transmitted. Flags allows control of
   --  the transmission. Raises Socket_Error on any detected error condition.

   procedure Send_Socket
     (Socket : Socket_Type;
      Item   : Ada.Streams.Stream_Element_Array;
      Last   : out Ada.Streams.Stream_Element_Offset;
      To     : Sock_Addr_Type;
      Flags  : Request_Flag_Type := No_Request_Flag);
   --  Transmit a message over a datagram socket. The destination address is
   --  To. Flags allows control of the transmission. Raises Socket_Error on
   --  error.

   procedure Send_Vector
     (Socket : Socket_Type;
      Vector : Vector_Type;
      Count  : out Ada.Streams.Stream_Element_Count;
      Flags  : Request_Flag_Type := No_Request_Flag);
   --  Transmit data gathered from the set of vector elements Vector to a
   --  socket. Count is set to the count of transmitted stream elements. Flags
   --  allow control over transmission.

   procedure Set_Close_On_Exec
     (Socket        : Socket_Type;
      Close_On_Exec : Boolean;
      Status        : out Boolean);
   --  When Close_On_Exec is True, mark Socket to be closed automatically when
   --  a new program is executed by the calling process (i.e. prevent Socket
   --  from being inherited by child processes). When Close_On_Exec is False,
   --  mark Socket to not be closed on exec (i.e. allow it to be inherited).
   --  Status is False if the operation could not be performed, or is not
   --  supported on the target platform.

   procedure Set_Socket_Option
     (Socket : Socket_Type;
      Level  : Level_Type;
      Option : Option_Type);
   --  Manipulate socket options. Raises Socket_Error on error

   procedure Shutdown_Socket
     (Socket : Socket_Type;
      How    : Shutmode_Type := Shut_Read_Write);
   --  Shutdown a connected socket. If How is Shut_Read further receives will
   --  be disallowed. If How is Shut_Write further sends will be disallowed.
   --  If How is Shut_Read_Write further sends and receives will be disallowed.

   type Stream_Access is access all Ada.Streams.Root_Stream_Type'Class;
   --  Same interface as Ada.Streams.Stream_IO

   function Stream (Socket : Socket_Type) return Stream_Access;
   --  Create a stream associated with a connected stream-based socket.
   --  Note: keep in mind that the default stream attributes for composite
   --  types perform separate Read/Write operations for each component,
   --  recursively. If performance is an issue, you may want to consider
   --  introducing a buffering stage.

   function Stream
     (Socket  : Socket_Type;
      Send_To : Sock_Addr_Type) return Stream_Access;
   --  Create a stream associated with an already bound datagram-based socket.
   --  Send_To is the destination address to which messages are being sent.

   function Get_Address
     (Stream : not null Stream_Access) return Sock_Addr_Type;
   --  Return the socket address from which the last message was received

   procedure Free is new Ada.Unchecked_Deallocation
     (Ada.Streams.Root_Stream_Type'Class, Stream_Access);
   --  Destroy a stream created by one of the Stream functions above, releasing
   --  the corresponding resources. The user is responsible for calling this
   --  subprogram when the stream is not needed anymore.

   type Socket_Set_Type is limited private;
   --  This type allows manipulation of sets of sockets. It allows waiting
   --  for events on multiple endpoints at one time. This type has default
   --  initialization, and the default value is the empty set.
   --
   --  Note: This type used to contain a pointer to dynamically allocated
   --  storage, but this is not the case anymore, and no special precautions
   --  are required to avoid memory leaks.

   procedure Clear (Item : in out Socket_Set_Type; Socket : Socket_Type);
   --  Remove Socket from Item

   procedure Copy (Source : Socket_Set_Type; Target : out Socket_Set_Type);
   --  Copy Source into Target as Socket_Set_Type is limited private

   procedure Empty (Item : out Socket_Set_Type);
   --  Remove all Sockets from Item

   procedure Get (Item : in out Socket_Set_Type; Socket : out Socket_Type);
   --  Extract a Socket from socket set Item. Socket is set to
   --  No_Socket when the set is empty.

   function Is_Empty (Item : Socket_Set_Type) return Boolean;
   --  Return True iff Item is empty

   function Is_Set
     (Item   : Socket_Set_Type;
      Socket : Socket_Type) return Boolean;
   --  Return True iff Socket is present in Item

   procedure Set (Item : in out Socket_Set_Type; Socket : Socket_Type);
   --  Insert Socket into Item

   function Image (Item : Socket_Set_Type) return String;
   --  Return a printable image of Item, for debugging purposes

   --  The select(2) system call waits for events to occur on any of a set of
   --  file descriptors. Usually, three independent sets of descriptors are
   --  watched (read, write  and exception). A timeout gives an upper bound
   --  on the amount of time elapsed before select returns. This function
   --  blocks until an event occurs. On some platforms, the select(2) system
   --  can block the full process (not just the calling thread).
   --
   --  Check_Selector provides the very same behavior. The only difference is
   --  that it does not watch for exception events. Note that on some platforms
   --  it is kept process blocking on purpose. The timeout parameter allows the
   --  user to have the behavior he wants. Abort_Selector allows the safe
   --  abort of a blocked Check_Selector call. A special socket is opened by
   --  Create_Selector and included in each call to Check_Selector.
   --
   --  Abort_Selector causes an event to occur on this descriptor in order to
   --  unblock Check_Selector. Note that each call to Abort_Selector will cause
   --  exactly one call to Check_Selector to return with Aborted status. The
   --  special socket created by Create_Selector is closed when Close_Selector
   --  is called.
   --
   --  A typical case where it is useful to abort a Check_Selector operation is
   --  the situation where a change to the monitored sockets set must be made.

   procedure Create_Selector (Selector : out Selector_Type);
   --  Initialize (open) a new selector

   procedure Close_Selector (Selector : in out Selector_Type);
   --  Close Selector and all internal descriptors associated; deallocate any
   --  associated resources. This subprogram may be called only when there is
   --  no other task still using Selector (i.e. still executing Check_Selector
   --  or Abort_Selector on this Selector). Has no effect if Selector is
   --  already closed.

   procedure Check_Selector
     (Selector     : Selector_Type;
      R_Socket_Set : in out Socket_Set_Type;
      W_Socket_Set : in out Socket_Set_Type;
      Status       : out Selector_Status;
      Timeout      : Selector_Duration := Forever);
   --  Return when one Socket in R_Socket_Set has some data to be read or if
   --  one Socket in W_Socket_Set is ready to transmit some data. In these
   --  cases Status is set to Completed and sockets that are ready are set in
   --  R_Socket_Set or W_Socket_Set. Status is set to Expired if no socket was
   --  ready after a Timeout expiration. Status is set to Aborted if an abort
   --  signal has been received while checking socket status.
   --
   --  Note that two different Socket_Set_Type objects must be passed as
   --  R_Socket_Set and W_Socket_Set (even if they denote the same set of
   --  Sockets), or some event may be lost. Also keep in mind that this
   --  procedure modifies the passed socket sets to indicate which sockets
   --  actually had events upon return. The socket set therefore has to
   --  be reset by the caller for further calls.
   --
   --  Socket_Error is raised when the select(2) system call returns an error
   --  condition, or when a read error occurs on the signalling socket used for
   --  the implementation of Abort_Selector.

   procedure Check_Selector
     (Selector     : Selector_Type;
      R_Socket_Set : in out Socket_Set_Type;
      W_Socket_Set : in out Socket_Set_Type;
      E_Socket_Set : in out Socket_Set_Type;
      Status       : out Selector_Status;
      Timeout      : Selector_Duration := Forever);
   --  This refined version of Check_Selector allows watching for exception
   --  events (i.e. notifications of out-of-band transmission and reception).
   --  As above, all of R_Socket_Set, W_Socket_Set and E_Socket_Set must be
   --  different objects.

   procedure Abort_Selector (Selector : Selector_Type);
   --  Send an abort signal to the selector. The Selector may not be the
   --  Null_Selector.

   type Fd_Set is private;
   --  ??? This type must not be used directly, it needs to be visible because
   --  it is used in the visible part of GNAT.Sockets.Thin_Common. This is
   --  really an inversion of abstraction. The private part of GNAT.Sockets
   --  needs to have visibility on this type, but since Thin_Common is a child
   --  of Sockets, the type can't be declared there. The correct fix would
   --  be to move the thin sockets binding outside of GNAT.Sockets altogether,
   --  e.g. by renaming it to GNAT.Sockets_Thin.

private

   package ASU renames Ada.Strings.Unbounded;

   type Socket_Type is new Integer;
   No_Socket : constant Socket_Type := -1;

   --  A selector is either a null selector, which is always "open" and can
   --  never be aborted, or a regular selector, which is created "closed",
   --  becomes "open" when Create_Selector is called, and "closed" again when
   --  Close_Selector is called.

   type Selector_Type (Is_Null : Boolean := False) is limited record
      case Is_Null is
         when True =>
            null;

         when False =>
            R_Sig_Socket : Socket_Type := No_Socket;
            W_Sig_Socket : Socket_Type := No_Socket;
            --  Signalling sockets used to abort a select operation
      end case;
   end record;

   pragma Volatile (Selector_Type);

   Null_Selector : constant Selector_Type := (Is_Null => True);

   type Fd_Set is
     new System.Storage_Elements.Storage_Array (1 .. SOSC.SIZEOF_fd_set);
   for Fd_Set'Alignment use Interfaces.C.long'Alignment;
   --  Set conservative alignment so that our Fd_Sets are always adequately
   --  aligned for the underlying data type (which is implementation defined
   --  and may be an array of C long integers).

   type Fd_Set_Access is access all Fd_Set;
   pragma Convention (C, Fd_Set_Access);
   No_Fd_Set_Access : constant Fd_Set_Access := null;

   type Socket_Set_Type is record
      Last : Socket_Type := No_Socket;
      --  Highest socket in set. Last = No_Socket denotes an empty set (which
      --  is the default initial value).

      Set : aliased Fd_Set;
      --  Underlying socket set. Note that the contents of this component is
      --  undefined if Last = No_Socket.
   end record;

   Any_Port : constant Port_Type := 0;
   No_Port  : constant Port_Type := 0;

   Any_Inet_Addr       : constant Inet_Addr_Type :=
                           (Family_Inet, (others => 0));
   Any_Inet6_Addr      : constant Inet_Addr_Type :=
                           (Family_Inet6, (others => 0));
   No_Inet_Addr        : constant Inet_Addr_Type :=
                           (Family_Inet, (others => 0));
   Broadcast_Inet_Addr : constant Inet_Addr_Type :=
                           (Family_Inet, (others => 255));
   Loopback_Inet_Addr  : constant Inet_Addr_Type :=
                           (Family_Inet, (127, 0, 0, 1));
   Loopback_Inet6_Addr : constant Inet_Addr_Type :=
                           (Family_Inet6,
                            (0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1));

   Unspecified_Group_Inet_Addr : constant Inet_Addr_Type :=
                                   (Family_Inet, (224, 0, 0, 0));
   All_Hosts_Group_Inet_Addr   : constant Inet_Addr_Type :=
                                   (Family_Inet, (224, 0, 0, 1));
   All_Routers_Group_Inet_Addr : constant Inet_Addr_Type :=
                                   (Family_Inet, (224, 0, 0, 2));

   Unspecified_Group_Inet6_Addr : constant Inet_Addr_Type :=
     (Family_Inet6, (255, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0));
   All_Hosts_Group_Inet6_Addr   : constant Inet_Addr_Type :=
     (Family_Inet6, (255, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1));
   All_Routers_Group_Inet6_Addr : constant Inet_Addr_Type :=
     (Family_Inet6, (255, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2));

   No_Sock_Addr : constant Sock_Addr_Type := (Family_Inet, No_Inet_Addr, 0);

   Max_Name_Length : constant := 64;
   --  The constant MAXHOSTNAMELEN is usually set to 64

   subtype Name_Index is Natural range 1 .. Max_Name_Length;

   type Name_Type (Length : Name_Index := Max_Name_Length) is record
      Name : String (1 .. Length);
   end record;
   --  We need fixed strings to avoid access types in host entry type

   type Name_Array is array (Positive range <>) of Name_Type;
   type Inet_Addr_Array is array (Positive range <>) of Inet_Addr_Type;

   type Host_Entry_Type (Aliases_Length, Addresses_Length : Natural) is record
      Official  : Name_Type;
      Aliases   : Name_Array (1 .. Aliases_Length);
      Addresses : Inet_Addr_Array (1 .. Addresses_Length);
   end record;

   type Service_Entry_Type (Aliases_Length : Natural) is record
      Official : Name_Type;
      Port     : Port_Type;
      Protocol : Name_Type;
      Aliases  : Name_Array (1 .. Aliases_Length);
   end record;

   type Request_Flag_Type is mod 2 ** 8;
   No_Request_Flag           : constant Request_Flag_Type := 0;
   Process_Out_Of_Band_Data  : constant Request_Flag_Type := 1;
   Peek_At_Incoming_Data     : constant Request_Flag_Type := 2;
   Wait_For_A_Full_Reception : constant Request_Flag_Type := 4;
   Send_End_Of_Record        : constant Request_Flag_Type := 8;

end GNAT.Sockets;