phobos/std/socket.d

// Written in the D programming language

/*
        Copyright (C) 2004-2005 Christopher E. Miller

        This software is provided 'as-is', without any express or implied
        warranty.  In no event will the authors be held liable for any damages
        arising from the use of this software.

        Permission is granted to anyone to use this software for any purpose,
        including commercial applications, and to alter it and redistribute it
        freely, subject to the following restrictions:

        1. The origin of this software must not be misrepresented; you must not
           claim that you wrote the original software. If you use this software
           in a product, an acknowledgment in the product documentation would be
           appreciated but is not required.
        2. Altered source versions must be plainly marked as such, and must not
           be misrepresented as being the original software.
        3. This notice may not be removed or altered from any source
           distribution.

        socket.d 1.3
        Jan 2005

        Thanks to Benjamin Herr for his assistance.
 */

/**
 * Notes: For Win32 systems, link with ws2_32.lib.
 * Example: See /dmd/samples/d/listener.d.
 * Authors: Christopher E. Miller, $(WEB klickverbot.at, David Nadlinger)
 * Source:  $(PHOBOSSRC std/_socket.d)
 * Macros:
 *      WIKI=Phobos/StdSocket
 */

module std.socket;

import core.stdc.stdint, std.string, std.c.string, std.c.stdlib, std.conv,
       std.traits;

import core.stdc.config;
import core.time : dur, Duration;
import std.algorithm : max;
import std.exception : assumeUnique, enforce;

version(unittest)
{
    private import std.c.stdio : printf;
}

version(Posix)
{
    version = BsdSockets;
}

version(Win32)
{
    pragma (lib, "ws2_32.lib");
    pragma (lib, "wsock32.lib");

    private import std.c.windows.windows, std.c.windows.winsock, std.windows.syserror;
    private alias std.c.windows.winsock.timeval _ctimeval;

    typedef SOCKET socket_t = INVALID_SOCKET;
    private const int _SOCKET_ERROR = SOCKET_ERROR;


    private int _lasterr()
    {
        return WSAGetLastError();
    }
}
else version(BsdSockets)
{
    version(Posix)
    {
        version(linux)
            import std.c.linux.socket : AF_IPX, AF_APPLETALK, SOCK_RDM,
                   IPPROTO_IGMP, IPPROTO_GGP, IPPROTO_PUP, IPPROTO_IDP,
                   SD_RECEIVE, SD_SEND, SD_BOTH, MSG_NOSIGNAL, INADDR_NONE;
        else version(OSX)
            private import std.c.osx.socket;
        else version(FreeBSD)
        {
            import core.sys.posix.sys.socket;
            import core.sys.posix.sys.select;
            import std.c.freebsd.socket;
            private enum SD_RECEIVE = SHUT_RD;
            private enum SD_SEND    = SHUT_WR;
            private enum SD_BOTH    = SHUT_RDWR;
        }
        else
            static assert(false);

        import core.sys.posix.netdb;
        private import core.sys.posix.fcntl;
        private import core.sys.posix.unistd;
        private import core.sys.posix.arpa.inet;
        private import core.sys.posix.netinet.tcp;
        private import core.sys.posix.netinet.in_;
        private import core.sys.posix.sys.time;
        //private import core.sys.posix.sys.select;
        private import core.sys.posix.sys.socket;
        private alias core.sys.posix.sys.time.timeval _ctimeval;
    }
    private import core.stdc.errno;

    typedef int32_t socket_t = -1;
    private const int _SOCKET_ERROR = -1;


    private int _lasterr()
    {
        return errno;
    }
}
else
{
    static assert(0);     // No socket support yet.
}

/// Base exception thrown by $(D std.socket).
class SocketException: Exception
{
    /// Deprecated. Provided for compatibility with older code using
    /// $(D SocketException). Use $(D SocketOSException) instead.
    deprecated @property int errorCode()
    {
        auto osException = cast(SocketOSException)this;
        if (osException)
            return osException.errorCode;
        else
            return 0;
    }

    this(string msg)
    {
        super(msg);
    }
}

private string formatSocketError(int err)
{
    version(Posix)
    {
        char[80] buf;
        const(char)* cs;
        version (linux)
        {
            cs = strerror_r(errorCode, buf.ptr, buf.length);
        }
        else version (OSX)
        {
            auto errs = strerror_r(errorCode, buf.ptr, buf.length);
            if (errs == 0)
                cs = buf.ptr;
            else
                return "Socket error " ~ to!string(err);
        }
        else version (FreeBSD)
        {
            auto errs = strerror_r(errorCode, buf.ptr, buf.length);
            if (errs == 0)
                cs = buf.ptr;
            else
                return "Socket error " ~ to!string(err);
        }
        else
            static assert(0);

        auto len = strlen(cs);

        if(cs[len - 1] == '\n')
            len--;
        if(cs[len - 1] == '\r')
            len--;
        return cs[0 .. len].idup;
    }
    else
    version(Windows)
    {
        return sysErrorString(err);
    }
    else
        return "Socket error " ~ to!string(err);
}

/// Retreive the error message for the most recently encountered network error.
@property string lastSocketError()
{
    return formatSocketError(_lasterr());
}

/// Socket exceptions representing network errors reported by the operating
/// system.
class SocketOSException: SocketException
{
    int errorCode;     /// Platform-specific error code.

    this(string msg, int err = _lasterr)
    {
        errorCode = err;

        if (msg.length)
            super(msg ~ ": " ~ formatSocketError(err));
        else
            super(formatSocketError(err));
    }
}

/// Socket exceptions representing invalid parameters specified by user code.
class SocketParameterException: SocketException
{
    this(string msg)
    {
        super(msg);
    }
}

private __gshared typeof(&getnameinfo) getnameinfoPointer;

shared static this()
{
    version(Win32)
    {
        WSADATA wd;

        // Winsock will still load if an older version is present.
        // The version is just a request.
        int val;
        val = WSAStartup(0x2020, &wd);
        if(val)         // Request Winsock 2.2 for IPv6.
            throw new SocketOSException("Unable to initialize socket library", val);

        // See the comment in InternetAddress.toHostNameString() for
        // details on the getnameinfo() issue.
        auto ws2Lib = GetModuleHandleA("ws2_32.dll");
        if (ws2Lib)
        {
            getnameinfoPointer = cast(typeof(getnameinfoPointer))
                                 GetProcAddress(ws2Lib, "getnameinfo");
        }
    }
    else version(Posix)
    {
        getnameinfoPointer = &getnameinfo;
    }
}


shared static ~this()
{
    version(Win32)
    {
        WSACleanup();
    }
}

/**
 * The communication domain used to resolve an address.
 */
enum AddressFamily: int
{
    UNSPEC =     AF_UNSPEC,     ///
    UNIX =       AF_UNIX,       /// local communication
    INET =       AF_INET,       /// internet protocol version 4
    IPX =        AF_IPX,        /// novell IPX
    APPLETALK =  AF_APPLETALK,  /// appletalk
    INET6 =      AF_INET6,      /// internet protocol version 6
}


/**
 * Communication semantics
 */
enum SocketType: int
{
    STREAM =     SOCK_STREAM,           /// sequenced, reliable, two-way communication-based byte streams
    DGRAM =      SOCK_DGRAM,            /// connectionless, unreliable datagrams with a fixed maximum length; data may be lost or arrive out of order
    RAW =        SOCK_RAW,              /// raw protocol access
    RDM =        SOCK_RDM,              /// reliably-delivered message datagrams
    SEQPACKET =  SOCK_SEQPACKET,        /// sequenced, reliable, two-way connection-based datagrams with a fixed maximum length
}


/**
 * Protocol
 */
enum ProtocolType: int
{
    IP =    IPPROTO_IP,         /// internet protocol version 4
    ICMP =  IPPROTO_ICMP,       /// internet control message protocol
    IGMP =  IPPROTO_IGMP,       /// internet group management protocol
    GGP =   IPPROTO_GGP,        /// gateway to gateway protocol
    TCP =   IPPROTO_TCP,        /// transmission control protocol
    PUP =   IPPROTO_PUP,        /// PARC universal packet protocol
    UDP =   IPPROTO_UDP,        /// user datagram protocol
    IDP =   IPPROTO_IDP,        /// Xerox NS protocol
    IPV6 =  IPPROTO_IPV6,       /// internet protocol version 6
}


/**
 * $(D Protocol) is a class for retrieving protocol information.
 */
class Protocol
{
    ProtocolType type;          /// These members are populated when one of the following functions are called without failure:
    string name;                /// ditto
    string[] aliases;           /// ditto


    void populate(protoent* proto)
    {
        type = cast(ProtocolType)proto.p_proto;
        name = to!string(proto.p_name).idup;

        int i;
        for(i = 0;; i++)
        {
            if(!proto.p_aliases[i])
                break;
        }

        if(i)
        {
            aliases = new string[i];
            for(i = 0; i != aliases.length; i++)
            {
                aliases[i] =
                    to!string(proto.p_aliases[i]).idup;
            }
        }
        else
        {
            aliases = null;
        }
    }

    /** Returns: false on failure */
    bool getProtocolByName(string name)
    {
        protoent* proto;
        proto = getprotobyname(toStringz(name));
        if(!proto)
            return false;
        populate(proto);
        return true;
    }


    /** Returns: false on failure */
    // Same as getprotobynumber().
    bool getProtocolByType(ProtocolType type)
    {
        protoent* proto;
        proto = getprotobynumber(type);
        if(!proto)
            return false;
        populate(proto);
        return true;
    }
}


unittest
{
    Protocol proto = new Protocol;
    version (Windows)
    {
        // These fail, don't know why
        pragma(msg, " --- std.socket(" ~ __LINE__.stringof ~ ") broken test ---");
    }
    else
    {
        assert(proto.getProtocolByType(ProtocolType.TCP));
        //printf("About protocol TCP:\n\tName: %.*s\n", proto.name);
        // foreach(string s; proto.aliases)
        // {
        //      printf("\tAlias: %.*s\n", s);
        // }
        assert(proto.name == "tcp");
        assert(proto.aliases.length == 1 && proto.aliases[0] == "TCP");
    }
}


/**
 * $(D Service) is a class for retrieving service information.
 */
class Service
{
    /** These members are populated when one of the following functions are called without failure: */
    string name;
    string[] aliases;           /// ditto
    ushort port;                /// ditto
    string protocolName;        /// ditto


    void populate(servent* serv)
    {
        name = to!string(serv.s_name);
        port = ntohs(cast(ushort)serv.s_port);
        protocolName = to!string(serv.s_proto);

        int i;
        for(i = 0;; i++)
        {
            if(!serv.s_aliases[i])
                break;
        }

        if(i)
        {
            aliases = new string[i];
            for(i = 0; i != aliases.length; i++)
            {
                aliases[i] =
                    to!string(serv.s_aliases[i]).idup;
            }
        }
        else
        {
            aliases = null;
        }
    }

    /**
     * If a protocol name is omitted, any protocol will be matched.
     * Returns: false on failure.
     */
    bool getServiceByName(string name, string protocolName)
    {
        servent* serv;
        serv = getservbyname(toStringz(name), toStringz(protocolName));
        if(!serv)
            return false;
        populate(serv);
        return true;
    }


    // Any protocol name will be matched.
    /// ditto
    bool getServiceByName(string name)
    {
        servent* serv;
        serv = getservbyname(toStringz(name), null);
        if(!serv)
            return false;
        populate(serv);
        return true;
    }


    /// ditto
    bool getServiceByPort(ushort port, string protocolName)
    {
        servent* serv;
        serv = getservbyport(port, toStringz(protocolName));
        if(!serv)
            return false;
        populate(serv);
        return true;
    }


    // Any protocol name will be matched.
    /// ditto
    bool getServiceByPort(ushort port)
    {
        servent* serv;
        serv = getservbyport(port, null);
        if(!serv)
            return false;
        populate(serv);
        return true;
    }
}


unittest
{
    Service serv = new Service;
    if(serv.getServiceByName("epmap", "tcp"))
    {
        // printf("About service epmap:\n\tService: %.*s\n"
        //         "\tPort: %d\n\tProtocol: %.*s\n",
        //         serv.name, serv.port, serv.protocolName);
        // foreach(string s; serv.aliases)
        // {
        //      printf("\tAlias: %.*s\n", s);
        // }
        // For reasons unknown this is loc-srv on Wine and epmap on Windows
        assert(serv.name == "loc-srv" || serv.name == "epmap", serv.name);
        assert(serv.port == 135);
        assert(serv.protocolName == "tcp");
        // This assert used to pass, don't know why it fails now
        //assert(serv.aliases.length == 1 && serv.aliases[0] == "epmap");
    }
    else
    {
        printf("No service for epmap.\n");
    }
}


/**
 * Base exception thrown from an $(D InternetHost).
 */
class HostException: SocketOSException
{
    this(string msg, int err = _lasterr())
    {
        super(msg, err);
    }
}

/**
 * $(D InternetHost) is a class for resolving IPv4 addresses.
 */
class InternetHost
{
    /** These members are populated when one of the following functions are called without failure: */
    string name;
    string[] aliases;           /// ditto
    uint32_t[] addrList;        /// ditto


    void validHostent(hostent* he)
    {
        if(he.h_addrtype != cast(int)AddressFamily.INET || he.h_length != 4)
            throw new HostException("Address family mismatch");
    }


    void populate(hostent* he)
    {
        int i;
        char* p;

        name = to!string(he.h_name).idup;

        for(i = 0;; i++)
        {
            p = he.h_aliases[i];
            if(!p)
                break;
        }

        if(i)
        {
            aliases = new string[i];
            for(i = 0; i != aliases.length; i++)
            {
                aliases[i] =
                    to!string(he.h_aliases[i]).idup;
            }
        }
        else
        {
            aliases = null;
        }

        for(i = 0;; i++)
        {
            p = he.h_addr_list[i];
            if(!p)
                break;
        }

        if(i)
        {
            addrList = new uint32_t[i];
            for(i = 0; i != addrList.length; i++)
            {
                addrList[i] = ntohl(*(cast(uint32_t*)he.h_addr_list[i]));
            }
        }
        else
        {
            addrList = null;
        }
    }

    /**
     * Resolve host name.
     * Returns: false if unable to resolve.
     */
    bool getHostByName(string name)
    {
        version(Windows)
        {
            // TODO gethostbyname is deprecated in windows, use getaddrinfo
            auto he = gethostbyname(toStringz(name));
            if(!he)
                return false;
            validHostent(he);
            populate(he);
        }
        else
        {
            // posix systems use global state for return value, so we
            // must synchronize across all threads
            synchronized(this.classinfo)
            {
                auto he = gethostbyname(toStringz(name));
                if(!he)
                    return false;
                validHostent(he);
                populate(he);
            }
        }
        return true;
    }


    /**
     * Resolve IPv4 address number.
     *
     * Params:
     *   addr = The IPv4 address to resolve, in network byte order.
     * Returns:
     *   false if unable to resolve.
     */
    bool getHostByAddr(uint addr)
    {
        uint x = htonl(addr);
        version(Windows)
        {
            // TODO gethostbyaddr is deprecated in windows, use getnameinfo
            auto he = gethostbyaddr(&x, 4, cast(int)AddressFamily.INET);
            if(!he)
                return false;
            validHostent(he);
            populate(he);
        }
        else
        {
            // posix systems use global state for return value, so we
            // must synchronize across all threads
            synchronized(this.classinfo)
            {
                auto he = gethostbyaddr(&x, 4, cast(int)AddressFamily.INET);
                if(!he)
                    return false;
                validHostent(he);
                populate(he);
            }
        }
        return true;
    }


    /**
     * Same as previous, but addr is an IPv4 address string in the
     * dotted-decimal form $(I a.b.c.d).
     * Returns: false if unable to resolve.
     */
    bool getHostByAddr(string addr)
    {
        uint x = inet_addr(std.string.toStringz(addr));
        version(Windows)
        {
            // TODO gethostbyaddr is deprecated in windows, use getnameinfo
            auto he = gethostbyaddr(&x, 4, cast(int)AddressFamily.INET);
            if(!he)
                return false;
            validHostent(he);
            populate(he);
        }
        else
        {
            // posix systems use global state for return value, so we
            // must synchronize across all threads
            synchronized(this.classinfo)
            {
                auto he = gethostbyaddr(&x, 4, cast(int)AddressFamily.INET);
                if(!he)
                    return false;
                validHostent(he);
                populate(he);
            }
        }
        return true;
    }
}


unittest
{
    try
    {
        InternetHost ih = new InternetHost;
        if (!ih.getHostByName("www.digitalmars.com"))
            return;             // don't fail if not connected to internet
        //printf("addrList.length = %d\n", ih.addrList.length);
        assert(ih.addrList.length);
        InternetAddress ia = new InternetAddress(ih.addrList[0], InternetAddress.PORT_ANY);
        assert(ih.name == "www.digitalmars.com" || ih.name == "digitalmars.com",
               ih.name);
        // printf("IP address = %.*s\nname = %.*s\n", ia.toAddrString(), ih.name);
        // foreach(int i, string s; ih.aliases)
        // {
        //      printf("aliases[%d] = %.*s\n", i, s);
        // }
        // printf("---\n");

        assert(ih.getHostByAddr(ih.addrList[0]));
        // printf("name = %.*s\n", ih.name);
        // foreach(int i, string s; ih.aliases)
        // {
        //      printf("aliases[%d] = %.*s\n", i, s);
        // }
    }
    catch (Throwable e)
    {
        // Test fails or succeeds depending on environment!
        printf(" --- std.socket(%u) broken test ---\n", __LINE__);
        printf(" (%.*s)\n", e.toString());
    }
}


/**
 * Base exception thrown from an $(D Address).
 */
class AddressException: SocketOSException
{
    this(string msg, int err = _lasterr())
    {
        super(msg, err);
    }
}


/**
 * $(D Address) is an abstract class for representing a network addresses.
 */
abstract class Address
{
    protected sockaddr* name();
    protected int nameLen();
    AddressFamily addressFamily();      /// Family of this address.
    override string toString();                 /// Human readable string representing this address.
}

/**
 *
 */
class UnknownAddress: Address
{
protected:
    sockaddr sa;


    override sockaddr* name()
    {
        return &sa;
    }


    override int nameLen()
    {
        return sa.sizeof;
    }


public:
    override AddressFamily addressFamily()
    {
        return cast(AddressFamily)sa.sa_family;
    }


    override string toString()
    {
        return "Unknown";
    }
}


/**
 * $(D InternetAddress) is a class that represents an IPv4 (internet protocol
 * version 4) address and port.
 */
class InternetAddress: Address
{
protected:
    sockaddr_in sin;


    override sockaddr* name()
    {
        return cast(sockaddr*)&sin;
    }


    override int nameLen()
    {
        return sin.sizeof;
    }


    this()
    {
    }


public:
    enum uint ADDR_ANY = INADDR_ANY;           /// Any IPv4 address number.
    enum uint ADDR_NONE = INADDR_NONE;         /// An invalid IPv4 address number.
    enum ushort PORT_ANY = 0;                  /// Any IPv4 port number.

    /// Overridden to return $(D AddressFamily.INET).
    override AddressFamily addressFamily()
    {
        return cast(AddressFamily)AddressFamily.INET;
    }

    /// Returns the IPv4 port number.
    ushort port()
    {
        return ntohs(sin.sin_port);
    }

    /// Returns the IPv4 address number.
    uint addr()
    {
        return ntohl(sin.sin_addr.s_addr);
    }

    /**
     * Params:
     *   addr = an IPv4 address string in the dotted-decimal form a.b.c.d,
     *          or a host name that will be resolved using an $(D InternetHost)
     *          object.
     *   port = may be $(D PORT_ANY) as stated below.
     */
    this(string addr, ushort port)
    {
        uint uiaddr = parse(addr);
        if(ADDR_NONE == uiaddr)
        {
            InternetHost ih = new InternetHost;
            if(!ih.getHostByName(addr))
                //throw new AddressException("Invalid internet address");
                throw new AddressException(
                          "Unable to resolve host '" ~ addr ~ "'");
            uiaddr = ih.addrList[0];
        }
        sin.sin_family = AddressFamily.INET;
        sin.sin_addr.s_addr = htonl(uiaddr);
        sin.sin_port = htons(port);
    }

    /**
     * Construct a new $(D Address). $(D addr) may be $(D ADDR_ANY) (default)
     * and $(D port) may be $(D PORT_ANY), and the actual numbers may not be
     * known until a connection is made.
     */
    this(uint addr, ushort port)
    {
        sin.sin_family = AddressFamily.INET;
        sin.sin_addr.s_addr = htonl(addr);
        sin.sin_port = htons(port);
    }

    /// ditto
    this(ushort port)
    {
        sin.sin_family = AddressFamily.INET;
        sin.sin_addr.s_addr = 0;         //any, "0.0.0.0"
        sin.sin_port = htons(port);
    }

    /// Human readable string representing the IPv4 address in dotted-decimal form.
    string toAddrString()
    {
        return to!string(inet_ntoa(sin.sin_addr)).idup;
    }

    /// Human readable string representing the IPv4 port.
    string toPortString()
    {
        return std.conv.to!string(port());
    }

    /**
     * Returns the host name as a fully qualified domain name, if
     * available, or the IP address in dotted-decimal notation otherwise.
     */
    string toHostNameString()
    {
        // getnameinfo() is the recommended way to perform a reverse (name)
        // lookup on both Posix and Windows. However, it is only available
        // on Windows XP and above, and not included with the WinSock import
        // libraries shipped with DMD. Thus, we check for getnameinfo at
        // runtime in the shared module constructor, and fall back to the
        // deprecated getHostByAddr() if it could not be found. See also:
        // http://technet.microsoft.com/en-us/library/aa450403.aspx
        if (getnameinfoPointer is null)
        {
            auto host = new InternetHost();
            enforce(host.getHostByAddr(sin.sin_addr.s_addr),
                    new SocketOSException("Could not get host name."));
            return host.name;
        }

        auto buf = new char[NI_MAXHOST];
        auto rc = getnameinfoPointer(cast(sockaddr*)&sin, sin.sizeof,
                                     buf.ptr, cast(uint)buf.length, null, 0, 0);
        enforce(rc == 0, new SocketOSException(
                    "Could not get host name"));
        return assumeUnique(buf[0 .. strlen(buf.ptr)]);
    }

    /// Human readable string representing the IPv4 address and port in the form $(I a.b.c.d:e).
    override string toString()
    {
        return toAddrString() ~ ":" ~ toPortString();
    }

    /**
     * Parse an IPv4 address string in the dotted-decimal form $(I a.b.c.d)
     * and return the number.
     * Returns: If the string is not a legitimate IPv4 address,
     * $(D ADDR_NONE) is returned.
     */
    static uint parse(string addr)
    {
        return ntohl(inet_addr(std.string.toStringz(addr)));
    }
}


unittest
{
    try
    {
        InternetAddress ia = new InternetAddress("63.105.9.61", 80);
        assert(ia.toString() == "63.105.9.61:80");
    }
    catch (Throwable e)
    {
        printf(" --- std.socket(%u) broken test ---\n", __LINE__);
        printf(" (%.*s)\n", e.toString());
    }
}


/** */
class SocketAcceptException: SocketOSException
{
    this(string msg, int err = _lasterr())
    {
        super(msg, err);
    }
}

/// How a socket is shutdown:
enum SocketShutdown: int
{
    RECEIVE =  SD_RECEIVE,      /// socket receives are disallowed
    SEND =     SD_SEND,         /// socket sends are disallowed
    BOTH =     SD_BOTH,         /// both RECEIVE and SEND
}


/// Flags may be OR'ed together:
enum SocketFlags: int
{
    NONE =       0,                 /// no flags specified

    OOB =        MSG_OOB,           /// out-of-band stream data
    PEEK =       MSG_PEEK,          /// peek at incoming data without removing it from the queue, only for receiving
    DONTROUTE =  MSG_DONTROUTE,     /// data should not be subject to routing; this flag may be ignored. Only for sending
}


/// Duration timeout value.
extern(C) struct timeval
{
    // D interface
    c_long seconds;                /// Number of seconds.
    c_long microseconds;           /// Number of additional microseconds.

    // C interface
    deprecated
    {
        alias seconds tv_sec;
        alias microseconds tv_usec;
    }
}


/// A collection of sockets for use with $(D Socket.select).
class SocketSet
{
private:
    uint maxsockets;     /// max desired sockets, the $(D fd_set) might be capable of holding more
    fd_set set;


    version(Win32)
    {
        uint count()
        {
            return set.fd_count;
        }
    }
    else version(BsdSockets)
    {
        int maxfd;
        uint count;
    }


public:

    /// Set the maximum amount of sockets that may be added.
    this(uint max)
    {
        maxsockets = max;
        reset();
    }

    /// Uses the default maximum for the system.
    this()
    {
        this(FD_SETSIZE);
    }

    /// Reset the $(D SocketSet) so that there are 0 $(D Socket)s in the collection.
    void reset()
    {
        FD_ZERO(&set);

        version(BsdSockets)
        {
            maxfd = -1;
            count = 0;
        }
    }


    void add(socket_t s)
    in
    {
        // Make sure too many sockets don't get added.
        assert(count < maxsockets);
    }
    body
    {
        FD_SET(s, &set);

        version(BsdSockets)
        {
            ++count;
            if(s > maxfd)
                maxfd = s;
        }
    }

    /// Add a $(D Socket) to the collection. Adding more than the maximum has dangerous side affects.
    void add(Socket s)
    {
        add(s.sock);
    }

    void remove(socket_t s)
    {
        FD_CLR(s, &set);
        version(BsdSockets)
        {
            --count;
            // note: adjusting maxfd would require scanning the set, not worth it
        }
    }


    /// Remove this $(D Socket) from the collection.
    void remove(Socket s)
    {
        remove(s.sock);
    }

    int isSet(socket_t s)
    {
        return FD_ISSET(s, &set);
    }


    /// Returns nonzero if this $(D Socket) is in the collection.
    int isSet(Socket s)
    {
        return isSet(s.sock);
    }


    /// Return maximum amount of sockets that can be added, like $(D FD_SETSIZE).
    uint max()
    {
        return maxsockets;
    }


    fd_set* toFd_set()
    {
        return &set;
    }


    int selectn()
    {
        version(Win32)
        {
            return count;
        }
        else version(BsdSockets)
        {
            return maxfd + 1;
        }
    }
}


/// The level at which a socket option is defined:
enum SocketOptionLevel: int
{
    SOCKET =  SOL_SOCKET,               /// socket level
    IP =      ProtocolType.IP,          /// internet protocol version 4 level
    ICMP =    ProtocolType.ICMP,        ///
    IGMP =    ProtocolType.IGMP,        ///
    GGP =     ProtocolType.GGP,         ///
    TCP =     ProtocolType.TCP,         /// transmission control protocol level
    PUP =     ProtocolType.PUP,         ///
    UDP =     ProtocolType.UDP,         /// user datagram protocol level
    IDP =     ProtocolType.IDP,         ///
    IPV6 =    ProtocolType.IPV6,        /// internet protocol version 6 level
}

/// Linger information for use with SocketOption.LINGER.
extern(C) struct linger
{
    // D interface
    version(Win32)
    {
        uint16_t on;            /// Nonzero for on.
        uint16_t time;          /// Linger time.
    }
    else version(BsdSockets)
    {
        int32_t on;
        int32_t time;
    }

    // C interface
    deprecated
    {
        alias on l_onoff;
        alias time l_linger;
    }
}


/// Specifies a socket option:
enum SocketOption: int
{
    DEBUG =                SO_DEBUG,            /// record debugging information
    BROADCAST =            SO_BROADCAST,        /// allow transmission of broadcast messages
    REUSEADDR =            SO_REUSEADDR,        /// allow local reuse of address
    LINGER =               SO_LINGER,           /// linger on close if unsent data is present
    OOBINLINE =            SO_OOBINLINE,        /// receive out-of-band data in band
    SNDBUF =               SO_SNDBUF,           /// send buffer size
    RCVBUF =               SO_RCVBUF,           /// receive buffer size
    DONTROUTE =            SO_DONTROUTE,        /// do not route
    SNDTIMEO =             SO_SNDTIMEO,         /// send timeout
    RCVTIMEO =             SO_RCVTIMEO,         /// receive timeout

    // SocketOptionLevel.TCP:
    TCP_NODELAY =          .TCP_NODELAY,        /// disable the Nagle algorithm for send coalescing

    // SocketOptionLevel.IPV6:
    IPV6_UNICAST_HOPS =    .IPV6_UNICAST_HOPS,          ///
    IPV6_MULTICAST_IF =    .IPV6_MULTICAST_IF,          ///
    IPV6_MULTICAST_LOOP =  .IPV6_MULTICAST_LOOP,        ///
    IPV6_JOIN_GROUP =      .IPV6_JOIN_GROUP,            ///
    IPV6_LEAVE_GROUP =     .IPV6_LEAVE_GROUP,           ///
}


/**
 * $(D Socket) is a class that creates a network communication endpoint using
 * the Berkeley sockets interface.
 */
class Socket
{
private:
    socket_t sock;
    AddressFamily _family;

    version(Win32)
        bool _blocking = false;         /// Property to get or set whether the socket is blocking or nonblocking.

    // The WinSock timeouts seem to be effectively skewed by a constant
    // offset of about half a second (value in milliseconds). This has
    // been confirmed on updated (as of Jun 2011) Windows XP, Windows 7
    // and Windows Server 2008 R2 boxes.
    enum WINSOCK_TIMEOUT_SKEW = 500;

    void setSock(socket_t handle)
    {
        assert(handle != socket_t.init);
        sock = handle;

        // Set the option to disable SIGPIPE on send() if the platform
        // has it (e.g. on OS X).
        static if (is(typeof(SO_NOSIGPIPE)))
        {
            setOption(SocketOptionLevel.SOCKET, cast(SocketOption)SO_NOSIGPIPE, true);
        }
    }


    // For use with accepting().
    protected this()
    {
    }


public:

    /**
     * Create a blocking socket. If a single protocol type exists to support
     * this socket type within the address family, the $(D ProtocolType) may be
     * omitted.
     */
    this(AddressFamily af, SocketType type, ProtocolType protocol)
    {
        _family = af;
        auto handle = cast(socket_t) socket(af, type, protocol);
        if(handle == socket_t.init)
            throw new SocketOSException("Unable to create socket");
        setSock(handle);
    }


    // A single protocol exists to support this socket type within the
    // protocol family, so the ProtocolType is assumed.
    /// ditto
    this(AddressFamily af, SocketType type)
    {
        this(af, type, cast(ProtocolType)0);         // Pseudo protocol number.
    }


    /// ditto
    this(AddressFamily af, SocketType type, string protocolName)
    {
        protoent* proto;
        proto = getprotobyname(toStringz(protocolName));
        if(!proto)
            throw new SocketOSException("Unable to find the protocol");
        this(af, type, cast(ProtocolType)proto.p_proto);
    }


    ~this()
    {
        close();
    }


    /// Get underlying socket handle.
    socket_t handle()
    {
        return sock;
    }

    /**
     * Get/set socket's blocking flag.
     *
     * When a socket is blocking, calls to receive(), accept(), and send()
     * will block and wait for data/action.
     * A non-blocking socket will immediately return instead of blocking.
     */
    bool blocking()
    {
        version(Win32)
        {
            return _blocking;
        }
        else version(BsdSockets)
        {
            return !(fcntl(handle, F_GETFL, 0) & O_NONBLOCK);
        }
    }

    /// ditto
    void blocking(bool byes)
    {
        version(Win32)
        {
            uint num = !byes;
            if(_SOCKET_ERROR == ioctlsocket(sock, FIONBIO, &num))
                goto err;
            _blocking = byes;
        }
        else version(BsdSockets)
        {
            int x = fcntl(sock, F_GETFL, 0);
            if(-1 == x)
                goto err;
            if(byes)
                x &= ~O_NONBLOCK;
            else
                x |= O_NONBLOCK;
            if(-1 == fcntl(sock, F_SETFL, x))
                goto err;
        }
        return;         // Success.

 err:
        throw new SocketOSException("Unable to set socket blocking");
    }


    /// Get the socket's address family.
    AddressFamily addressFamily()     // getter
    {
        return _family;
    }

    /// Property that indicates if this is a valid, alive socket.
    bool isAlive()     // getter
    {
        int type;
        socklen_t typesize = cast(socklen_t) type.sizeof;
        return !getsockopt(sock, SOL_SOCKET, SO_TYPE, cast(char*)&type, &typesize);
    }

    /// Associate a local address with this socket.
    void bind(Address addr)
    {
        if(_SOCKET_ERROR == .bind(sock, addr.name(), addr.nameLen()))
            throw new SocketOSException("Unable to bind socket");
    }

    /**
     * Establish a connection. If the socket is blocking, connect waits for
     * the connection to be made. If the socket is nonblocking, connect
     * returns immediately and the connection attempt is still in progress.
     */
    void connect(Address to)
    {
        if(_SOCKET_ERROR == .connect(sock, to.name(), to.nameLen()))
        {
            int err;
            err = _lasterr();

            if(!blocking)
            {
                version(Win32)
                {
                    if(WSAEWOULDBLOCK == err)
                        return;
                }
                else version(Posix)
                {
                    if(EINPROGRESS == err)
                        return;
                }
                else
                {
                    static assert(0);
                }
            }
            throw new SocketOSException("Unable to connect socket", err);
        }
    }

    /**
     * Listen for an incoming connection. $(D bind) must be called before you
     * can $(D listen). The $(D backlog) is a request of how many pending
     * incoming connections are queued until $(D accept)'ed.
     */
    void listen(int backlog)
    {
        if(_SOCKET_ERROR == .listen(sock, backlog))
            throw new SocketOSException("Unable to listen on socket");
    }

    /**
     * Called by $(D accept) when a new $(D Socket) must be created for a new
     * connection. To use a derived class, override this method and return an
     * instance of your class. The returned $(D Socket)'s handle must not be
     * set; $(D Socket) has a protected constructor $(D this()) to use in this
     * situation.
     */
    // Override to use a derived class.
    // The returned socket's handle must not be set.
    protected Socket accepting()
    {
        return new Socket;
    }

    /**
     * Accept an incoming connection. If the socket is blocking, $(D accept)
     * waits for a connection request. Throws $(D SocketAcceptException) if
     * unable to _accept. See $(D accepting) for use with derived classes.
     */
    Socket accept()
    {
        auto newsock = cast(socket_t).accept(sock, null, null);
        if(socket_t.init == newsock)
            throw new SocketAcceptException("Unable to accept socket connection");

        Socket newSocket;
        try
        {
            newSocket = accepting();
            assert(newSocket.sock == socket_t.init);

            newSocket.setSock(newsock);
            version(Win32)
                newSocket._blocking = _blocking;                 //inherits blocking mode
            newSocket._family = _family;             //same family
        }
        catch(Throwable o)
        {
            _close(newsock);
            throw o;
        }

        return newSocket;
    }

    /// Disables sends and/or receives.
    void shutdown(SocketShutdown how)
    {
        .shutdown(sock, cast(int)how);
    }


    private static void _close(socket_t sock)
    {
        version(Win32)
        {
            .closesocket(sock);
        }
        else version(BsdSockets)
        {
            .close(sock);
        }
    }


    /**
     * Immediately drop any connections and release socket resources.
     * Calling $(D shutdown) before $(D close) is recommended for
     * connection-oriented sockets. The $(D Socket) object is no longer
     * usable after $(D close).
     */
    //calling shutdown() before this is recommended
    //for connection-oriented sockets
    void close()
    {
        _close(sock);
        sock = socket_t.init;
    }


    private Address newFamilyObject()
    {
        Address result;
        switch(_family)
        {
        case cast(AddressFamily)AddressFamily.INET:
            result = new InternetAddress;
            break;

        default:
            result = new UnknownAddress;
        }
        return result;
    }


    /// Returns the local machine's host name.
    // Idea from mango.
    static string hostName()     // getter
    {
        char[256] result;         // Host names are limited to 255 chars.
        if(_SOCKET_ERROR == .gethostname(result.ptr, result.length))
            throw new SocketOSException("Unable to obtain host name");
        return to!string(cast(char*)result).idup;
    }

    /// Remote endpoint $(D Address).
    Address remoteAddress()
    {
        Address addr = newFamilyObject();
        socklen_t nameLen = cast(socklen_t) addr.nameLen();
        if(_SOCKET_ERROR == .getpeername(sock, addr.name(), &nameLen))
            throw new SocketOSException("Unable to obtain remote socket address");
        assert(addr.addressFamily() == _family);
        return addr;
    }

    /// Local endpoint $(D Address).
    Address localAddress()
    {
        Address addr = newFamilyObject();
        socklen_t nameLen = cast(socklen_t) addr.nameLen();
        if(_SOCKET_ERROR == .getsockname(sock, addr.name(), &nameLen))
            throw new SocketOSException("Unable to obtain local socket address");
        assert(addr.addressFamily() == _family);
        return addr;
    }

    /// Send or receive error code.
    enum int ERROR = _SOCKET_ERROR;

    /**
     * Send data on the connection. If the socket is blocking and there is no
     * buffer space left, $(D send) waits.
     * Returns: The number of bytes actually sent, or $(D ERROR) on failure.
     */
    //returns number of bytes actually sent, or -1 on error
    Select!(size_t.sizeof > 4, long, int)
    send(const(void)[] buf, SocketFlags flags)
    {
        static if (is(typeof(MSG_NOSIGNAL)))
        {
            flags = cast(SocketFlags)(flags | MSG_NOSIGNAL);
        }
        version( Windows )
            auto sent = .send(sock, buf.ptr, to!int(buf.length), cast(int)flags);
        else
            auto sent = .send(sock, buf.ptr, buf.length, cast(int)flags);
        return sent;
    }

    /// ditto
    Select!(size_t.sizeof > 4, long, int) send(const(void)[] buf)
    {
        return send(buf, SocketFlags.NONE);
    }

    /**
     * Send data to a specific destination Address. If the destination address is
     * not specified, a connection must have been made and that address is used.
     * If the socket is blocking and there is no buffer space left, $(D sendTo) waits.
     */
    Select!(size_t.sizeof > 4, long, int)
    sendTo(const(void)[] buf, SocketFlags flags, Address to)
    {
        static if (is(typeof(MSG_NOSIGNAL)))
        {
            flags = cast(SocketFlags)(flags | MSG_NOSIGNAL);
        }
        version( Windows )
            return .sendto(
                       sock, buf.ptr, std.conv.to!int(buf.length),
                       cast(int)flags, to.name(), to.nameLen()
                       );
        else
            return .sendto(sock, buf.ptr, buf.length, cast(int)flags, to.name(), to.nameLen());
    }

    /// ditto
    Select!(size_t.sizeof > 4, long, int) sendTo(const(void)[] buf, Address to)
    {
        return sendTo(buf, SocketFlags.NONE, to);
    }


    //assumes you connect()ed
    /// ditto
    Select!(size_t.sizeof > 4, long, int) sendTo(const(void)[] buf, SocketFlags flags)
    {
        static if (is(typeof(MSG_NOSIGNAL)))
        {
            flags = cast(SocketFlags)(flags | MSG_NOSIGNAL);
        }
        version(Windows)
            return .sendto(sock, buf.ptr, to!int(buf.length), cast(int)flags, null, 0);
        else
            return .sendto(sock, buf.ptr, buf.length, cast(int)flags, null, 0);
    }


    //assumes you connect()ed
    /// ditto
    Select!(size_t.sizeof > 4, long, int) sendTo(const(void)[] buf)
    {
        return sendTo(buf, SocketFlags.NONE);
    }


    /**
     * Receive data on the connection. If the socket is blocking, $(D receive)
     * waits until there is data to be received.
     * Returns: The number of bytes actually received, $(D 0) if the remote side
     * has closed the connection, or $(D ERROR) on failure.
     */
    //returns number of bytes actually received, 0 on connection closure, or -1 on error
    ptrdiff_t receive(void[] buf, SocketFlags flags)
    {
        version(Win32)         // Does not use size_t
        {
            return buf.length
                   ? .recv(sock, buf.ptr, to!int(buf.length), cast(int)flags)
                   : 0;
        } else {
            return buf.length
                   ? .recv(sock, buf.ptr, buf.length, cast(int)flags)
                   : 0;
        }
    }

    /// ditto
    ptrdiff_t receive(void[] buf)
    {
        return receive(buf, SocketFlags.NONE);
    }

    /**
     * Receive data and get the remote endpoint $(D Address).
     * If the socket is blocking, $(D receiveFrom) waits until there is data to
     * be received.
     * Returns: The number of bytes actually received, $(D 0) if the remote side
     * has closed the connection, or $(D ERROR) on failure.
     */
    Select!(size_t.sizeof > 4, long, int)
    receiveFrom(void[] buf, SocketFlags flags, out Address from)
    {
        if(!buf.length)         //return 0 and don't think the connection closed
            return 0;
        from = newFamilyObject();
        socklen_t nameLen = cast(socklen_t) from.nameLen();
        version(Win32)
        {
            auto read = .recvfrom(sock, buf.ptr, to!int(buf.length), cast(int)flags, from.name(), &nameLen);
            assert(from.addressFamily() == _family);
            // if(!read) //connection closed
            return read;
        } else {
            auto read = .recvfrom(sock, buf.ptr, buf.length, cast(int)flags, from.name(), &nameLen);
            assert(from.addressFamily() == _family);
            // if(!read) //connection closed
            return read;
        }
    }


    /// ditto
    ptrdiff_t receiveFrom(void[] buf, out Address from)
    {
        return receiveFrom(buf, SocketFlags.NONE, from);
    }


    //assumes you connect()ed
    /// ditto
    Select!(size_t.sizeof > 4, long, int)
    receiveFrom(void[] buf, SocketFlags flags)
    {
        if(!buf.length)         //return 0 and don't think the connection closed
            return 0;
        version(Win32)
        {
            auto read = .recvfrom(sock, buf.ptr, to!int(buf.length), cast(int)flags, null, null);
            // if(!read) //connection closed
            return read;
        } else {
            auto read = .recvfrom(sock, buf.ptr, buf.length, cast(int)flags, null, null);
            // if(!read) //connection closed
            return read;
        }
    }


    //assumes you connect()ed
    /// ditto
    ptrdiff_t receiveFrom(void[] buf)
    {
        return receiveFrom(buf, SocketFlags.NONE);
    }


    /// Get a socket option.
    /// Returns: The number of bytes written to $(D result).
    //returns the length, in bytes, of the actual result - very different from getsockopt()
    int getOption(SocketOptionLevel level, SocketOption option, void[] result)
    {
        socklen_t len = cast(socklen_t) result.length;
        if(_SOCKET_ERROR == .getsockopt(sock, cast(int)level, cast(int)option, result.ptr, &len))
            throw new SocketOSException("Unable to get socket option");
        return len;
    }


    /// Common case of getting integer and boolean options.
    int getOption(SocketOptionLevel level, SocketOption option, out int32_t result)
    {
        return getOption(level, option, (&result)[0 .. 1]);
    }


    /// Get the linger option.
    int getOption(SocketOptionLevel level, SocketOption option, out linger result)
    {
        //return getOption(cast(SocketOptionLevel)SocketOptionLevel.SOCKET, SocketOption.LINGER, (&result)[0 .. 1]);
        return getOption(level, option, (&result)[0 .. 1]);
    }

    /// Get a timeout (duration) option.
    void getOption(SocketOptionLevel level, SocketOption option, out Duration result)
    {
        enforce(option == SocketOption.SNDTIMEO || option == SocketOption.RCVTIMEO,
                new SocketParameterException("Not a valid timeout option: " ~ to!string(option)));
        // WinSock returns the timeout values as a milliseconds DWORD,
        // while Linux and BSD return a timeval struct.
        version (Win32)
        {
            int msecs;
            getOption(level, option, (&msecs)[0 .. 1]);
            if (option == SocketOption.RCVTIMEO)
                msecs += WINSOCK_TIMEOUT_SKEW;
            result = dur!"msecs"(msecs);
        }
        else version (BsdSockets)
        {
            timeval tv;
            getOption(level, option, (&tv)[0..1]);
            result = dur!"seconds"(tv.seconds) + dur!"usecs"(tv.microseconds);
        }
        else static assert(false);
    }

    // Set a socket option.
    void setOption(SocketOptionLevel level, SocketOption option, void[] value)
    {
        if(_SOCKET_ERROR == .setsockopt(sock, cast(int)level,
                                        cast(int)option, value.ptr, cast(uint) value.length))
            throw new SocketOSException("Unable to set socket option");
    }


    /// Common case for setting integer and boolean options.
    void setOption(SocketOptionLevel level, SocketOption option, int32_t value)
    {
        setOption(level, option, (&value)[0 .. 1]);
    }


    /// Set the linger option.
    void setOption(SocketOptionLevel level, SocketOption option, linger value)
    {
        //setOption(cast(SocketOptionLevel)SocketOptionLevel.SOCKET, SocketOption.LINGER, (&value)[0 .. 1]);
        setOption(level, option, (&value)[0 .. 1]);
    }

    /**
     * Sets a timeout (duration) option, i.e. $(D SocketOption.SNDTIMEO) or
     * $(D RCVTIMEO). Zero indicates no timeout.
     *
     * In a typical application, you might also want to consider using
     * a non-blocking socket instead of setting a timeout on a blocking one.
     *
     * Note: While the receive timeout setting is generally quite accurate
     * on *nix systems even for smaller durations, there are two issues to
     * be aware of on Windows: First, although undocumented, the effective
     * timeout duration seems to be the one set on the socket plus half
     * a second. $(D setOption()) tries to compensate for that, but still,
     * timeouts under 500ms are not possible on Windows. Second, be aware
     * that the actual amount of time spent until a blocking call returns
     * randomly varies on the order of 10ms.
     *
     * Params:
     *   value = The timeout duration to set. Must not be negative.
     *
     * Throws: $(D SocketException) if setting the options fails.
     *
     * Example:
     * ---
     * import std.datetime;
     * auto pair = socketPair();
     * scope(exit) foreach (s; pair) s.close();
     *
     * // Set a receive timeout, and then wait at one end of
     * // the socket pair, knowing that no data will arrive.
     * pair[0].setOption(SocketOptionLevel.SOCKET,
     *     SocketOption.RCVTIMEO, dur!"seconds"(1));
     *
     * auto sw = StopWatch(AutoStart.yes);
     * ubyte[1] buffer;
     * pair[0].receive(buffer);
     * writefln("Waited %s ms until the socket timed out.",
     *     sw.peek.msecs);
     * ---
     */
    void setOption(SocketOptionLevel level, SocketOption option, Duration value)
    {
        enforce(option == SocketOption.SNDTIMEO || option == SocketOption.RCVTIMEO,
                new SocketParameterException("Not a valid timeout option: " ~ to!string(option)));

        enforce(value >= dur!"hnsecs"(0), new SocketParameterException(
                    "Timeout duration must not be negative."));

        version (Win32)
        {
            auto msecs = cast(int)value.total!"msecs"();
            if (msecs == 0 || option != SocketOption.RCVTIMEO)
            {
                setOption(level, option, msecs);
            }
            else
            {
                setOption(level, option, cast(int)
                          max(1, msecs - WINSOCK_TIMEOUT_SKEW));
            }
        }
        else version (BsdSockets)
        {
            timeval tv = { seconds: cast(int)value.total!"seconds"(),
                           microseconds: value.fracSec.usecs };
            setOption(level, option, (&tv)[0 .. 1]);
        }
        else static assert(false);
    }

    /**
     * Wait for a socket to change status. A wait timeout $(D timeval) or
     * $(D int) microseconds may be specified; if a timeout is not specified
     * or the $(D timeval) is $(D null), the maximum timeout is used. The
     * $(D timeval) timeout has an unspecified value when $(D select) returns.
     * Returns: The number of sockets with status changes, $(D 0) on timeout,
     * or $(D -1) on interruption. If the return value is greater than $(D 0),
     * the $(D SocketSets) are updated to only contain the sockets having status
     * changes. For a connecting socket, a write status change means the
     * connection is established and it's able to send. For a listening socket,
     * a read status change means there is an incoming connection request and
     * it's able to accept.
     */
    //SocketSet's updated to include only those sockets which an event occured
    //returns the number of events, 0 on timeout, or -1 on interruption
    //for a connect()ing socket, writeability means connected
    //for a listen()ing socket, readability means listening
    //Winsock: possibly internally limited to 64 sockets per set
    static int select(SocketSet checkRead, SocketSet checkWrite, SocketSet checkError, timeval* tv)
    in
    {
        //make sure none of the SocketSet's are the same object
        if(checkRead)
        {
            assert(checkRead !is checkWrite);
            assert(checkRead !is checkError);
        }
        if(checkWrite)
        {
            assert(checkWrite !is checkError);
        }
    }
    body
    {
        fd_set* fr, fw, fe;
        int n = 0;

        version(Win32)
        {
            // Windows has a problem with empty fd_set`s that aren't null.
            fr = (checkRead && checkRead.count()) ? checkRead.toFd_set() : null;
            fw = (checkWrite && checkWrite.count()) ? checkWrite.toFd_set() : null;
            fe = (checkError && checkError.count()) ? checkError.toFd_set() : null;
        }
        else
        {
            if(checkRead)
            {
                fr = checkRead.toFd_set();
                n = checkRead.selectn();
            }
            else
            {
                fr = null;
            }

            if(checkWrite)
            {
                fw = checkWrite.toFd_set();
                int _n;
                _n = checkWrite.selectn();
                if(_n > n)
                    n = _n;
            }
            else
            {
                fw = null;
            }

            if(checkError)
            {
                fe = checkError.toFd_set();
                int _n;
                _n = checkError.selectn();
                if(_n > n)
                    n = _n;
            }
            else
            {
                fe = null;
            }
        }

        int result = .select(n, fr, fw, fe, cast(_ctimeval*)tv);

        version(Win32)
        {
            if(_SOCKET_ERROR == result && WSAGetLastError() == WSAEINTR)
                return -1;
        }
        else version(Posix)
        {
            if(_SOCKET_ERROR == result && errno == EINTR)
                return -1;
        }
        else
        {
            static assert(0);
        }

        if(_SOCKET_ERROR == result)
            throw new SocketOSException("Socket select error");

        return result;
    }


    /// ditto
    static int select(SocketSet checkRead, SocketSet checkWrite, SocketSet checkError, int microseconds)
    {
        timeval tv;
        tv.seconds = microseconds / 1_000_000;
        tv.microseconds = microseconds % 1_000_000;
        return select(checkRead, checkWrite, checkError, &tv);
    }


    /// ditto
    //maximum timeout
    static int select(SocketSet checkRead, SocketSet checkWrite, SocketSet checkError)
    {
        return select(checkRead, checkWrite, checkError, null);
    }


    /+
       bool poll(events)
       {
            int WSAEventSelect(socket_t s, WSAEVENT hEventObject, int lNetworkEvents); // Winsock 2 ?
            int poll(pollfd* fds, int nfds, int timeout); // Unix ?
       }
     +/
}


/// $(D TcpSocket) is a shortcut class for a TCP Socket.
class TcpSocket: Socket
{
    /// Constructs a blocking TCP Socket.
    this(AddressFamily family)
    {
        super(family, SocketType.STREAM, ProtocolType.TCP);
    }

    /// Constructs a blocking TCP Socket.
    this()
    {
        this(cast(AddressFamily)AddressFamily.INET);
    }


    //shortcut
    /// Constructs a blocking TCP Socket and connects to an $(D InternetAddress).
    this(Address connectTo)
    {
        this(connectTo.addressFamily());
        connect(connectTo);
    }
}


/// $(D UdpSocket) is a shortcut class for a UDP Socket.
class UdpSocket: Socket
{
    /// Constructs a blocking UDP Socket.
    this(AddressFamily family)
    {
        super(family, SocketType.DGRAM, ProtocolType.UDP);
    }


    /// Constructs a blocking UDP Socket.
    this()
    {
        this(cast(AddressFamily)AddressFamily.INET);
    }
}

/**
 * Creates a pair of connected sockets.
 *
 * The two sockets are indistinguishable.
 *
 * Throws: $(D SocketException) if creation of the sockets fails.
 *
 * Example:
 * ---
 * immutable ubyte[] data = [1, 2, 3, 4];
 * auto pair = socketPair();
 * scope(exit) foreach (s; pair) s.close();
 *
 * pair[0].send(data);
 *
 * auto buf = new ubyte[data.length];
 * pair[1].receive(buf);
 * assert(buf == data);
 * ---
 */
Socket[2] socketPair()
{
    version(BsdSockets)
    {
        int[2] socks;
        if (socketpair(AF_UNIX, SOCK_STREAM, 0, socks) == -1)
            throw new SocketOSException("Unable to create socket pair");

        Socket toSocket(size_t id)
        {
            auto s = new Socket;
            s.setSock(cast(socket_t)socks[id]);
            s._family = AddressFamily.UNIX;
            return s;
        }

        return [toSocket(0), toSocket(1)];
    }
    else version(Win32)
    {
        // We do not have socketpair() on Windows, just manually create a
        // pair of sockets connected over some localhost port.
        Socket[2] result;

        auto listener = new TcpSocket();
        listener.setOption(SocketOptionLevel.SOCKET, SocketOption.REUSEADDR, true);
        listener.bind(new InternetAddress(INADDR_LOOPBACK, InternetAddress.PORT_ANY));
        auto addr = listener.localAddress();
        listener.listen(1);

        result[0] = new TcpSocket(addr);
        result[1] = listener.accept();

        listener.close();
        return result;
    }
    else
        static assert(false);
}

unittest
{
    immutable ubyte[] data = [1, 2, 3, 4];
    auto pair = socketPair();
    scope(exit) foreach (s; pair) s.close();

    pair[0].send(data);

    auto buf = new ubyte[data.length];
    pair[1].receive(buf);
    assert(buf == data);
}