// Written in the D programming language

/*
        Copyright (C) 2004-2011 Christopher E. Miller

        Boost Software License - Version 1.0 - August 17th, 2003

        Permission is hereby granted, free of charge, to any person or organization
        obtaining a copy of the software and accompanying documentation covered by
        this license (the "Software") to use, reproduce, display, distribute,
        execute, and transmit the Software, and to prepare derivative works of the
        Software, and to permit third-parties to whom the Software is furnished to
        do so, all subject to the following:

        The copyright notices in the Software and this entire statement, including
        the above license grant, this restriction and the following disclaimer,
        must be included in all copies of the Software, in whole or in part, and
        all derivative works of the Software, unless such copies or derivative
        works are solely in the form of machine-executable object code generated by
        a source language processor.

        THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
        IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
        FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
        SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
        FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
        ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
        DEALINGS IN THE SOFTWARE.

        socket.d 1.4
        Jan 2011

        Thanks to Benjamin Herr for his assistance.
 */

/**
 * Socket primitives.
 * Example: See $(SAMPLESRC listener.d) and $(SAMPLESRC htmlget.d)
 * License: $(HTTP www.boost.org/LICENSE_1_0.txt, Boost License 1.0).
 * Authors: Christopher E. Miller, $(HTTP klickverbot.at, David Nadlinger),
 *      $(HTTP thecybershadow.net, Vladimir Panteleev)
 * Source:  $(PHOBOSSRC std/socket.d)
 */

module std.socket;

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

import core.stdc.config;
import core.time : dur, Duration;
import std.exception;

import std.internal.cstring;


@safe:

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

    import core.sys.windows.windows, std.windows.syserror;
    public import core.sys.windows.winsock2;
    private alias _ctimeval = core.sys.windows.winsock2.timeval;
    private alias _clinger = core.sys.windows.winsock2.linger;

    enum socket_t : SOCKET { INVALID_SOCKET }
    private const int _SOCKET_ERROR = SOCKET_ERROR;


    private int _lasterr() nothrow @nogc
    {
        return WSAGetLastError();
    }
}
else version (Posix)
{
    version (linux)
    {
        enum : int
        {
            TCP_KEEPIDLE  = 4,
            TCP_KEEPINTVL = 5
        }
    }

    public import core.sys.posix.netinet.in_;
    import core.sys.posix.arpa.inet;
    import core.sys.posix.fcntl;
    import core.sys.posix.netdb;
    import core.sys.posix.netinet.tcp;
    import core.sys.posix.sys.select;
    import core.sys.posix.sys.socket;
    import core.sys.posix.sys.time;
    import core.sys.posix.sys.un : sockaddr_un;
    import core.sys.posix.unistd;
    private alias _ctimeval = core.sys.posix.sys.time.timeval;
    private alias _clinger = core.sys.posix.sys.socket.linger;

    import core.stdc.errno;

    enum socket_t : int32_t { init = -1 }
    private const int _SOCKET_ERROR = -1;

    private enum : int
    {
        SD_RECEIVE = SHUT_RD,
        SD_SEND    = SHUT_WR,
        SD_BOTH    = SHUT_RDWR
    }

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

version (unittest)
{
    // Print a message on exception instead of failing the unittest.
    private void softUnittest(void delegate() @safe test, int line = __LINE__) @trusted
    {
        import std.stdio : writefln;
        try
            test();
        catch (Throwable e)
        {
            writefln(" --- std.socket(%d) test fails depending on environment ---", line);
            writefln(" (%s)", e);
        }
    }
}

/// Base exception thrown by `std.socket`.
class SocketException: Exception
{
    mixin basicExceptionCtors;
}

version (CRuntime_Glibc) version = GNU_STRERROR;
version (CRuntime_UClibc) version = GNU_STRERROR;

/*
 * Needs to be public so that SocketOSException can be thrown outside of
 * std.socket (since it uses it as a default argument), but it probably doesn't
 * need to actually show up in the docs, since there's not really any public
 * need for it outside of being a default argument.
 */
string formatSocketError(int err) @trusted
{
    version (Posix)
    {
        char[80] buf;
        const(char)* cs;
        version (GNU_STRERROR)
        {
            cs = strerror_r(err, buf.ptr, buf.length);
        }
        else
        {
            auto errs = strerror_r(err, buf.ptr, buf.length);
            if (errs == 0)
                cs = buf.ptr;
            else
                return "Socket error " ~ to!string(err);
        }

        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);
}

/// Retrieve 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,
         string file = __FILE__,
         size_t line = __LINE__,
         Throwable next = null,
         int err = _lasterr(),
         string function(int) @trusted errorFormatter = &formatSocketError)
    {
        errorCode = err;

        if (msg.length)
            super(msg ~ ": " ~ errorFormatter(err), file, line, next);
        else
            super(errorFormatter(err), file, line, next);
    }

    ///
    this(string msg,
         Throwable next,
         string file = __FILE__,
         size_t line = __LINE__,
         int err = _lasterr(),
         string function(int) @trusted errorFormatter = &formatSocketError)
    {
        this(msg, file, line, next, err, errorFormatter);
    }

    ///
    this(string msg,
         int err,
         string function(int) @trusted errorFormatter = &formatSocketError,
         string file = __FILE__,
         size_t line = __LINE__,
         Throwable next = null)
    {
        this(msg, file, line, next, err, errorFormatter);
    }
}

/// Socket exceptions representing invalid parameters specified by user code.
class SocketParameterException: SocketException
{
    mixin basicExceptionCtors;
}

/**
 * Socket exceptions representing attempts to use network capabilities not
 * available on the current system.
 */
class SocketFeatureException: SocketException
{
    mixin basicExceptionCtors;
}


/**
 * Returns:
 * `true` if the last socket operation failed because the socket
 * was in non-blocking mode and the operation would have blocked.
 */
bool wouldHaveBlocked() nothrow @nogc
{
    version (Windows)
        return _lasterr() == WSAEWOULDBLOCK;
    else version (Posix)
        return _lasterr() == EAGAIN;
    else
        static assert(0);
}


private immutable
{
    typeof(&getnameinfo) getnameinfoPointer;
    typeof(&getaddrinfo) getaddrinfoPointer;
    typeof(&freeaddrinfo) freeaddrinfoPointer;
}

shared static this() @system
{
    version (Windows)
    {
        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);

        // These functions may not be present on older Windows versions.
        // See the comment in InternetAddress.toHostNameString() for details.
        auto ws2Lib = GetModuleHandleA("ws2_32.dll");
        if (ws2Lib)
        {
            getnameinfoPointer = cast(typeof(getnameinfoPointer))
                                 GetProcAddress(ws2Lib, "getnameinfo");
            getaddrinfoPointer = cast(typeof(getaddrinfoPointer))
                                 GetProcAddress(ws2Lib, "getaddrinfo");
            freeaddrinfoPointer = cast(typeof(freeaddrinfoPointer))
                                 GetProcAddress(ws2Lib, "freeaddrinfo");
        }
    }
    else version (Posix)
    {
        getnameinfoPointer = &getnameinfo;
        getaddrinfoPointer = &getaddrinfo;
        freeaddrinfoPointer = &freeaddrinfo;
    }
}


shared static ~this() @system nothrow @nogc
{
    version (Windows)
    {
        WSACleanup();
    }
}

/**
 * The communication domain used to resolve an address.
 */
enum AddressFamily: int
{
    UNSPEC =     AF_UNSPEC,     /// Unspecified address family
    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
    RAW =   IPPROTO_RAW,        /// Raw IP packets
    IPV6 =  IPPROTO_IPV6,       /// Internet Protocol version 6
}


/**
 * `Protocol` is a class for retrieving protocol information.
 *
 * Example:
 * ---
 * auto proto = new Protocol;
 * writeln("About protocol TCP:");
 * if (proto.getProtocolByType(ProtocolType.TCP))
 * {
 *     writefln("  Name: %s", proto.name);
 *     foreach (string s; proto.aliases)
 *          writefln("  Alias: %s", s);
 * }
 * else
 *     writeln("  No information found");
 * ---
 */
class Protocol
{
    /// These members are populated when one of the following functions are called successfully:
    ProtocolType type;
    string name;                /// ditto
    string[] aliases;           /// ditto


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

        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]);
            }
        }
        else
        {
            aliases = null;
        }
    }

    /** Returns: false on failure */
    bool getProtocolByName(scope const(char)[] name) @trusted nothrow
    {
        protoent* proto;
        proto = getprotobyname(name.tempCString());
        if (!proto)
            return false;
        populate(proto);
        return true;
    }


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


// Skip this test on Android because getprotobyname/number are
// unimplemented in bionic.
version (CRuntime_Bionic) {} else
@safe unittest
{
    // import std.stdio : writefln;
    softUnittest({
        Protocol proto = new Protocol;
        assert(proto.getProtocolByType(ProtocolType.TCP));
        //writeln("About protocol TCP:");
        //writefln("\tName: %s", proto.name);
        // foreach (string s; proto.aliases)
        // {
        //      writefln("\tAlias: %s", s);
        // }
        assert(proto.name == "tcp");
        assert(proto.aliases.length == 1 && proto.aliases[0] == "TCP");
    });
}


/**
 * `Service` is a class for retrieving service information.
 *
 * Example:
 * ---
 * auto serv = new Service;
 * writeln("About service epmap:");
 * if (serv.getServiceByName("epmap", "tcp"))
 * {
 *     writefln("  Service: %s", serv.name);
 *     writefln("  Port: %d", serv.port);
 *     writefln("  Protocol: %s", serv.protocolName);
 *     foreach (string s; serv.aliases)
 *          writefln("  Alias: %s", s);
 * }
 * else
 *     writefln("  No service for epmap.");
 * ---
 */
class Service
{
    /// These members are populated when one of the following functions are called successfully:
    string name;
    string[] aliases;           /// ditto
    ushort port;                /// ditto
    string protocolName;        /// ditto


    void populate(servent* serv) @system pure nothrow
    {
        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]);
            }
        }
        else
        {
            aliases = null;
        }
    }

    /**
     * If a protocol name is omitted, any protocol will be matched.
     * Returns: false on failure.
     */
    bool getServiceByName(scope const(char)[] name, scope const(char)[] protocolName = null) @trusted nothrow
    {
        servent* serv;
        serv = getservbyname(name.tempCString(), protocolName.tempCString());
        if (!serv)
            return false;
        populate(serv);
        return true;
    }


    /// ditto
    bool getServiceByPort(ushort port, scope const(char)[] protocolName = null) @trusted nothrow
    {
        servent* serv;
        serv = getservbyport(port, protocolName.tempCString());
        if (!serv)
            return false;
        populate(serv);
        return true;
    }
}


@safe unittest
{
    import std.stdio : writefln;
    softUnittest({
        Service serv = new Service;
        if (serv.getServiceByName("epmap", "tcp"))
        {
            // writefln("About service epmap:");
            // writefln("\tService: %s", serv.name);
            // writefln("\tPort: %d", serv.port);
            // writefln("\tProtocol: %s", serv.protocolName);
            // foreach (string s; serv.aliases)
            // {
            //      writefln("\tAlias: %s", 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");
        }
        else
        {
            writefln("No service for epmap.");
        }
    });
}


private mixin template socketOSExceptionCtors()
{
    ///
    this(string msg, string file = __FILE__, size_t line = __LINE__,
         Throwable next = null, int err = _lasterr())
    {
        super(msg, file, line, next, err);
    }

    ///
    this(string msg, Throwable next, string file = __FILE__,
         size_t line = __LINE__, int err = _lasterr())
    {
        super(msg, next, file, line, err);
    }

    ///
    this(string msg, int err, string file = __FILE__, size_t line = __LINE__,
         Throwable next = null)
    {
        super(msg, next, file, line, err);
    }
}


/**
 * Class for exceptions thrown from an `InternetHost`.
 */
class HostException: SocketOSException
{
    mixin socketOSExceptionCtors;
}

/**
 * `InternetHost` is a class for resolving IPv4 addresses.
 *
 * Consider using `getAddress`, `parseAddress` and `Address` methods
 * instead of using this class directly.
 */
class InternetHost
{
    /// These members are populated when one of the following functions are called successfully:
    string name;
    string[] aliases;           /// ditto
    uint[] addrList;            /// ditto


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


    void populate(hostent* he) @system pure nothrow
    {
        int i;
        char* p;

        name = to!string(he.h_name);

        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]);
            }
        }
        else
        {
            aliases = null;
        }

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

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

    private bool getHostNoSync(string opMixin, T)(T param) @system
    {
        mixin(opMixin);
        if (!he)
            return false;
        validHostent(he);
        populate(he);
        return true;
    }

    version (Windows)
        alias getHost = getHostNoSync;
    else
    {
        // posix systems use global state for return value, so we
        // must synchronize across all threads
        private bool getHost(string opMixin, T)(T param) @system
        {
            synchronized(this.classinfo)
                return getHostNoSync!(opMixin, T)(param);
        }
    }

    /**
     * Resolve host name.
     * Returns: false if unable to resolve.
     */
    bool getHostByName(scope const(char)[] name) @trusted
    {
        static if (is(typeof(gethostbyname_r)))
        {
            return getHostNoSync!q{
                hostent he_v;
                hostent* he;
                ubyte[256] buffer_v = void;
                auto buffer = buffer_v[];
                auto param_zTmp = param.tempCString();
                while (true)
                {
                    he = &he_v;
                    int errno;
                    if (gethostbyname_r(param_zTmp, he, buffer.ptr, buffer.length, &he, &errno) == ERANGE)
                        buffer.length = buffer.length * 2;
                    else
                        break;
                }
            }(name);
        }
        else
        {
            return getHost!q{
                auto he = gethostbyname(param.tempCString());
            }(name);
        }
    }

    /**
     * Resolve IPv4 address number.
     *
     * Params:
     *   addr = The IPv4 address to resolve, in host byte order.
     * Returns:
     *   false if unable to resolve.
     */
    bool getHostByAddr(uint addr) @trusted
    {
        return getHost!q{
            auto x = htonl(param);
            auto he = gethostbyaddr(&x, 4, cast(int) AddressFamily.INET);
        }(addr);
    }

    /**
     * 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(scope const(char)[] addr) @trusted
    {
        return getHost!q{
            auto x = inet_addr(param.tempCString());
            enforce(x != INADDR_NONE,
                new SocketParameterException("Invalid IPv4 address"));
            auto he = gethostbyaddr(&x, 4, cast(int) AddressFamily.INET);
        }(addr);
    }
}

///
@safe unittest
{
    InternetHost ih = new InternetHost;

    ih.getHostByAddr(0x7F_00_00_01);
    assert(ih.addrList[0] == 0x7F_00_00_01);
    ih.getHostByAddr("127.0.0.1");
    assert(ih.addrList[0] == 0x7F_00_00_01);

    if (!ih.getHostByName("www.digitalmars.com"))
        return;             // don't fail if not connected to internet

    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);

    assert(ih.getHostByAddr(ih.addrList[0]));
    string getHostNameFromInt = ih.name.dup;

    assert(ih.getHostByAddr(ia.toAddrString()));
    string getHostNameFromStr = ih.name.dup;

    assert(getHostNameFromInt == getHostNameFromStr);
}


/// Holds information about a socket _address retrieved by `getAddressInfo`.
struct AddressInfo
{
    AddressFamily family;   /// Address _family
    SocketType type;        /// Socket _type
    ProtocolType protocol;  /// Protocol
    Address address;        /// Socket _address
    string canonicalName;   /// Canonical name, when `AddressInfoFlags.CANONNAME` is used.
}

/**
 * A subset of flags supported on all platforms with getaddrinfo.
 * Specifies option flags for `getAddressInfo`.
 */
enum AddressInfoFlags: int
{
    /// The resulting addresses will be used in a call to `Socket.bind`.
    PASSIVE = AI_PASSIVE,

    /// The canonical name is returned in `canonicalName` member in the first `AddressInfo`.
    CANONNAME = AI_CANONNAME,

    /**
     * The `node` parameter passed to `getAddressInfo` must be a numeric string.
     * This will suppress any potentially lengthy network host address lookups.
     */
    NUMERICHOST = AI_NUMERICHOST,
}


/**
 * On POSIX, getaddrinfo uses its own error codes, and thus has its own
 * formatting function.
 */
private string formatGaiError(int err) @trusted
{
    version (Windows)
    {
        return sysErrorString(err);
    }
    else
    {
        synchronized
            return to!string(gai_strerror(err));
    }
}

/**
 * Provides _protocol-independent translation from host names to socket
 * addresses. If advanced functionality is not required, consider using
 * `getAddress` for compatibility with older systems.
 *
 * Returns: Array with one `AddressInfo` per socket address.
 *
 * Throws: `SocketOSException` on failure, or `SocketFeatureException`
 * if this functionality is not available on the current system.
 *
 * Params:
 *  node     = string containing host name or numeric address
 *  options  = optional additional parameters, identified by type:
 *             $(UL $(LI `string` - service name or port number)
 *                  $(LI `AddressInfoFlags` - option flags)
 *                  $(LI `AddressFamily` - address family to filter by)
 *                  $(LI `SocketType` - socket type to filter by)
 *                  $(LI `ProtocolType` - protocol to filter by))
 *
 * Example:
 * ---
 * // Roundtrip DNS resolution
 * auto results = getAddressInfo("www.digitalmars.com");
 * assert(results[0].address.toHostNameString() ==
 *     "digitalmars.com");
 *
 * // Canonical name
 * results = getAddressInfo("www.digitalmars.com",
 *     AddressInfoFlags.CANONNAME);
 * assert(results[0].canonicalName == "digitalmars.com");
 *
 * // IPv6 resolution
 * results = getAddressInfo("ipv6.google.com");
 * assert(results[0].family == AddressFamily.INET6);
 *
 * // Multihomed resolution
 * results = getAddressInfo("google.com");
 * assert(results.length > 1);
 *
 * // Parsing IPv4
 * results = getAddressInfo("127.0.0.1",
 *     AddressInfoFlags.NUMERICHOST);
 * assert(results.length && results[0].family ==
 *     AddressFamily.INET);
 *
 * // Parsing IPv6
 * results = getAddressInfo("::1",
 *     AddressInfoFlags.NUMERICHOST);
 * assert(results.length && results[0].family ==
 *     AddressFamily.INET6);
 * ---
 */
AddressInfo[] getAddressInfo(T...)(scope const(char)[] node, scope T options)
{
    const(char)[] service = null;
    addrinfo hints;
    hints.ai_family = AF_UNSPEC;

    foreach (i, option; options)
    {
        static if (is(typeof(option) : const(char)[]))
            service = options[i];
        else
        static if (is(typeof(option) == AddressInfoFlags))
            hints.ai_flags |= option;
        else
        static if (is(typeof(option) == AddressFamily))
            hints.ai_family = option;
        else
        static if (is(typeof(option) == SocketType))
            hints.ai_socktype = option;
        else
        static if (is(typeof(option) == ProtocolType))
            hints.ai_protocol = option;
        else
            static assert(0, "Unknown getAddressInfo option type: " ~ typeof(option).stringof);
    }

    return () @trusted { return getAddressInfoImpl(node, service, &hints); }();
}

@system unittest
{
    struct Oops
    {
        const(char[]) breakSafety()
        {
            *cast(int*) 0xcafebabe = 0xdeadbeef;
            return null;
        }
        alias breakSafety this;
    }
    assert(!__traits(compiles, () {
        getAddressInfo("", Oops.init);
    }), "getAddressInfo breaks @safe");
}

private AddressInfo[] getAddressInfoImpl(scope const(char)[] node, scope const(char)[] service, addrinfo* hints) @system
{
        import std.array : appender;

    if (getaddrinfoPointer && freeaddrinfoPointer)
    {
        addrinfo* ai_res;

        int ret = getaddrinfoPointer(
            node.tempCString(),
            service.tempCString(),
            hints, &ai_res);
        enforce(ret == 0, new SocketOSException("getaddrinfo error", ret, &formatGaiError));
        scope(exit) freeaddrinfoPointer(ai_res);

        auto result = appender!(AddressInfo[])();

        // Use const to force UnknownAddressReference to copy the sockaddr.
        for (const(addrinfo)* ai = ai_res; ai; ai = ai.ai_next)
            result ~= AddressInfo(
                cast(AddressFamily) ai.ai_family,
                cast(SocketType   ) ai.ai_socktype,
                cast(ProtocolType ) ai.ai_protocol,
                new UnknownAddressReference(ai.ai_addr, cast(socklen_t) ai.ai_addrlen),
                ai.ai_canonname ? to!string(ai.ai_canonname) : null);

        assert(result.data.length > 0);
        return result.data;
    }

    throw new SocketFeatureException("Address info lookup is not available " ~
        "on this system.");
}


@safe unittest
{
    softUnittest({
        if (getaddrinfoPointer)
        {
            // Roundtrip DNS resolution
            auto results = getAddressInfo("www.digitalmars.com");
            assert(results[0].address.toHostNameString() == "digitalmars.com");

            // Canonical name
            results = getAddressInfo("www.digitalmars.com",
                AddressInfoFlags.CANONNAME);
            assert(results[0].canonicalName == "digitalmars.com");

            // IPv6 resolution
            //results = getAddressInfo("ipv6.google.com");
            //assert(results[0].family == AddressFamily.INET6);

            // Multihomed resolution
            //results = getAddressInfo("google.com");
            //assert(results.length > 1);

            // Parsing IPv4
            results = getAddressInfo("127.0.0.1", AddressInfoFlags.NUMERICHOST);
            assert(results.length && results[0].family == AddressFamily.INET);

            // Parsing IPv6
            results = getAddressInfo("::1", AddressInfoFlags.NUMERICHOST);
            assert(results.length && results[0].family == AddressFamily.INET6);
        }
    });

    if (getaddrinfoPointer)
    {
        auto results = getAddressInfo(null, "1234", AddressInfoFlags.PASSIVE,
                                      SocketType.STREAM, ProtocolType.TCP, AddressFamily.INET);
        assert(results.length == 1 && results[0].address.toString() == "0.0.0.0:1234");
    }
}


private ushort serviceToPort(scope const(char)[] service)
{
    if (service == "")
        return InternetAddress.PORT_ANY;
    else
    if (isNumeric(service))
        return to!ushort(service);
    else
    {
        auto s = new Service();
        s.getServiceByName(service);
        return s.port;
    }
}

/**
 * Provides _protocol-independent translation from host names to socket
 * addresses. Uses `getAddressInfo` if the current system supports it,
 * and `InternetHost` otherwise.
 *
 * Returns: Array with one `Address` instance per socket address.
 *
 * Throws: `SocketOSException` on failure.
 *
 * Example:
 * ---
 * writeln("Resolving www.digitalmars.com:");
 * try
 * {
 *     auto addresses = getAddress("www.digitalmars.com");
 *     foreach (address; addresses)
 *         writefln("  IP: %s", address.toAddrString());
 * }
 * catch (SocketException e)
 *     writefln("  Lookup failed: %s", e.msg);
 * ---
 */
Address[] getAddress(scope const(char)[] hostname, scope const(char)[] service = null)
{
    if (getaddrinfoPointer && freeaddrinfoPointer)
    {
        // use getAddressInfo
        auto infos = getAddressInfo(hostname, service);
        Address[] results;
        results.length = infos.length;
        foreach (i, ref result; results)
            result = infos[i].address;
        return results;
    }
    else
        return getAddress(hostname, serviceToPort(service));
}

/// ditto
Address[] getAddress(scope const(char)[] hostname, ushort port)
{
    if (getaddrinfoPointer && freeaddrinfoPointer)
        return getAddress(hostname, to!string(port));
    else
    {
        // use getHostByName
        auto ih = new InternetHost;
        if (!ih.getHostByName(hostname))
            throw new AddressException(
                        text("Unable to resolve host '", hostname, "'"));

        Address[] results;
        foreach (uint addr; ih.addrList)
            results ~= new InternetAddress(addr, port);
        return results;
    }
}


@safe unittest
{
    softUnittest({
        auto addresses = getAddress("63.105.9.61");
        assert(addresses.length && addresses[0].toAddrString() == "63.105.9.61");

        if (getaddrinfoPointer)
        {
            // test via gethostbyname
            auto getaddrinfoPointerBackup = getaddrinfoPointer;
            cast() getaddrinfoPointer = null;
            scope(exit) cast() getaddrinfoPointer = getaddrinfoPointerBackup;

            addresses = getAddress("63.105.9.61");
            assert(addresses.length && addresses[0].toAddrString() == "63.105.9.61");
        }
    });
}


/**
 * Provides _protocol-independent parsing of network addresses. Does not
 * attempt name resolution. Uses `getAddressInfo` with
 * `AddressInfoFlags.NUMERICHOST` if the current system supports it, and
 * `InternetAddress` otherwise.
 *
 * Returns: An `Address` instance representing specified address.
 *
 * Throws: `SocketException` on failure.
 *
 * Example:
 * ---
 * writeln("Enter IP address:");
 * string ip = readln().chomp();
 * try
 * {
 *     Address address = parseAddress(ip);
 *     writefln("Looking up reverse of %s:",
 *         address.toAddrString());
 *     try
 *     {
 *         string reverse = address.toHostNameString();
 *         if (reverse)
 *             writefln("  Reverse name: %s", reverse);
 *         else
 *             writeln("  Reverse hostname not found.");
 *     }
 *     catch (SocketException e)
 *         writefln("  Lookup error: %s", e.msg);
 * }
 * catch (SocketException e)
 * {
 *     writefln("  %s is not a valid IP address: %s",
 *         ip, e.msg);
 * }
 * ---
 */
Address parseAddress(scope const(char)[] hostaddr, scope const(char)[] service = null)
{
    if (getaddrinfoPointer && freeaddrinfoPointer)
        return getAddressInfo(hostaddr, service, AddressInfoFlags.NUMERICHOST)[0].address;
    else
        return parseAddress(hostaddr, serviceToPort(service));
}

/// ditto
Address parseAddress(scope const(char)[] hostaddr, ushort port)
{
    if (getaddrinfoPointer && freeaddrinfoPointer)
        return parseAddress(hostaddr, to!string(port));
    else
    {
        auto in4_addr = InternetAddress.parse(hostaddr);
        enforce(in4_addr != InternetAddress.ADDR_NONE,
            new SocketParameterException("Invalid IP address"));
        return new InternetAddress(in4_addr, port);
    }
}


@safe unittest
{
    softUnittest({
        auto address = parseAddress("63.105.9.61");
        assert(address.toAddrString() == "63.105.9.61");

        if (getaddrinfoPointer)
        {
            // test via inet_addr
            auto getaddrinfoPointerBackup = getaddrinfoPointer;
            cast() getaddrinfoPointer = null;
            scope(exit) cast() getaddrinfoPointer = getaddrinfoPointerBackup;

            address = parseAddress("63.105.9.61");
            assert(address.toAddrString() == "63.105.9.61");
        }

        assert(collectException!SocketException(parseAddress("Invalid IP address")));
    });
}


/**
 * Class for exceptions thrown from an `Address`.
 */
class AddressException: SocketOSException
{
    mixin socketOSExceptionCtors;
}


/**
 * `Address` is an abstract class for representing a socket addresses.
 *
 * Example:
 * ---
 * writeln("About www.google.com port 80:");
 * try
 * {
 *     Address[] addresses = getAddress("www.google.com", 80);
 *     writefln("  %d addresses found.", addresses.length);
 *     foreach (int i, Address a; addresses)
 *     {
 *         writefln("  Address %d:", i+1);
 *         writefln("    IP address: %s", a.toAddrString());
 *         writefln("    Hostname: %s", a.toHostNameString());
 *         writefln("    Port: %s", a.toPortString());
 *         writefln("    Service name: %s",
 *             a.toServiceNameString());
 *     }
 * }
 * catch (SocketException e)
 *     writefln("  Lookup error: %s", e.msg);
 * ---
 */
abstract class Address
{
    /// Returns pointer to underlying `sockaddr` structure.
    abstract @property sockaddr* name() pure nothrow @nogc;
    abstract @property const(sockaddr)* name() const pure nothrow @nogc; /// ditto

    /// Returns actual size of underlying `sockaddr` structure.
    abstract @property socklen_t nameLen() const pure nothrow @nogc;

    // Socket.remoteAddress, Socket.localAddress, and Socket.receiveFrom
    // use setNameLen to set the actual size of the address as returned by
    // getsockname, getpeername, and recvfrom, respectively.
    // The following implementation is sufficient for fixed-length addresses,
    // and ensures that the length is not changed.
    // Must be overridden for variable-length addresses.
    protected void setNameLen(socklen_t len)
    {
        if (len != this.nameLen)
            throw new AddressException(
                format("%s expects address of length %d, not %d", typeid(this),
                    this.nameLen, len), 0);
    }

    /// Family of this address.
    @property AddressFamily addressFamily() const pure nothrow @nogc
    {
        return cast(AddressFamily) name.sa_family;
    }

    // Common code for toAddrString and toHostNameString
    private string toHostString(bool numeric) @trusted const
    {
        // 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 use it if it's
        // available in the base class method. Classes for specific network
        // families (e.g. InternetHost) override this method and use a
        // deprecated, albeit commonly-available method when getnameinfo()
        // is not available.
        // http://technet.microsoft.com/en-us/library/aa450403.aspx
        if (getnameinfoPointer)
        {
            auto buf = new char[NI_MAXHOST];
            auto ret = getnameinfoPointer(
                        name, nameLen,
                        buf.ptr, cast(uint) buf.length,
                        null, 0,
                        numeric ? NI_NUMERICHOST : NI_NAMEREQD);

            if (!numeric)
            {
                if (ret == EAI_NONAME)
                    return null;
                version (Windows)
                    if (ret == WSANO_DATA)
                        return null;
            }

            enforce(ret == 0, new AddressException("Could not get " ~
                        (numeric ? "host address" : "host name")));
            return assumeUnique(buf[0 .. strlen(buf.ptr)]);
        }

        throw new SocketFeatureException((numeric ? "Host address" : "Host name") ~
            " lookup for this address family is not available on this system.");
    }

    // Common code for toPortString and toServiceNameString
    private string toServiceString(bool numeric) @trusted const
    {
        // See toHostNameString() for details about getnameinfo().
        if (getnameinfoPointer)
        {
            auto buf = new char[NI_MAXSERV];
            enforce(getnameinfoPointer(
                        name, nameLen,
                        null, 0,
                        buf.ptr, cast(uint) buf.length,
                        numeric ? NI_NUMERICSERV : NI_NAMEREQD
                    ) == 0, new AddressException("Could not get " ~
                        (numeric ? "port number" : "service name")));
            return assumeUnique(buf[0 .. strlen(buf.ptr)]);
        }

        throw new SocketFeatureException((numeric ? "Port number" : "Service name") ~
            " lookup for this address family is not available on this system.");
    }

    /**
     * Attempts to retrieve the host address as a human-readable string.
     *
     * Throws: `AddressException` on failure, or `SocketFeatureException`
     * if address retrieval for this address family is not available on the
     * current system.
     */
    string toAddrString() const
    {
        return toHostString(true);
    }

    /**
     * Attempts to retrieve the host name as a fully qualified domain name.
     *
     * Returns: The FQDN corresponding to this `Address`, or `null` if
     * the host name did not resolve.
     *
     * Throws: `AddressException` on error, or `SocketFeatureException`
     * if host name lookup for this address family is not available on the
     * current system.
     */
    string toHostNameString() const
    {
        return toHostString(false);
    }

    /**
     * Attempts to retrieve the numeric port number as a string.
     *
     * Throws: `AddressException` on failure, or `SocketFeatureException`
     * if port number retrieval for this address family is not available on the
     * current system.
     */
    string toPortString() const
    {
        return toServiceString(true);
    }

    /**
     * Attempts to retrieve the service name as a string.
     *
     * Throws: `AddressException` on failure, or `SocketFeatureException`
     * if service name lookup for this address family is not available on the
     * current system.
     */
    string toServiceNameString() const
    {
        return toServiceString(false);
    }

    /// Human readable string representing this address.
    override string toString() const
    {
        try
        {
            string host = toAddrString();
            string port = toPortString();
            if (host.indexOf(':') >= 0)
                return "[" ~ host ~ "]:" ~ port;
            else
                return host ~ ":" ~ port;
        }
        catch (SocketException)
            return "Unknown";
    }
}

/**
 * `UnknownAddress` encapsulates an unknown socket address.
 */
class UnknownAddress: Address
{
protected:
    sockaddr sa;


public:
    override @property sockaddr* name()
    {
        return &sa;
    }

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


    override @property socklen_t nameLen() const
    {
        return cast(socklen_t) sa.sizeof;
    }

}


/**
 * `UnknownAddressReference` encapsulates a reference to an arbitrary
 * socket address.
 */
class UnknownAddressReference: Address
{
protected:
    sockaddr* sa;
    socklen_t len;

public:
    /// Constructs an `Address` with a reference to the specified `sockaddr`.
    this(sockaddr* sa, socklen_t len) pure nothrow @nogc
    {
        this.sa  = sa;
        this.len = len;
    }

    /// Constructs an `Address` with a copy of the specified `sockaddr`.
    this(const(sockaddr)* sa, socklen_t len) @system pure nothrow
    {
        this.sa = cast(sockaddr*) (cast(ubyte*) sa)[0 .. len].dup.ptr;
        this.len = len;
    }

    override @property sockaddr* name()
    {
        return sa;
    }

    override @property const(sockaddr)* name() const
    {
        return sa;
    }


    override @property socklen_t nameLen() const
    {
        return cast(socklen_t) len;
    }
}


/**
 * `InternetAddress` encapsulates an IPv4 (Internet Protocol version 4)
 * socket address.
 *
 * Consider using `getAddress`, `parseAddress` and `Address` methods
 * instead of using this class directly.
 */
class InternetAddress: Address
{
protected:
    sockaddr_in sin;


    this() pure nothrow @nogc
    {
    }


public:
    override @property sockaddr* name()
    {
        return cast(sockaddr*)&sin;
    }

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


    override @property socklen_t nameLen() const
    {
        return cast(socklen_t) sin.sizeof;
    }


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

    /// Returns the IPv4 _port number (in host byte order).
    @property ushort port() const pure nothrow @nogc
    {
        return ntohs(sin.sin_port);
    }

    /// Returns the IPv4 address number (in host byte order).
    @property uint addr() const pure nothrow @nogc
    {
        return ntohl(sin.sin_addr.s_addr);
    }

    /**
     * Construct a new `InternetAddress`.
     * Params:
     *   addr = an IPv4 address string in the dotted-decimal form a.b.c.d,
     *          or a host name which will be resolved using an `InternetHost`
     *          object.
     *   port = port number, may be `PORT_ANY`.
     */
    this(scope const(char)[] 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(
                          text("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 `InternetAddress`.
     * Params:
     *   addr = (optional) an IPv4 address in host byte order, may be `ADDR_ANY`.
     *   port = port number, may be `PORT_ANY`.
     */
    this(uint addr, ushort port) pure nothrow @nogc
    {
        sin.sin_family = AddressFamily.INET;
        sin.sin_addr.s_addr = htonl(addr);
        sin.sin_port = htons(port);
    }

    /// ditto
    this(ushort port) pure nothrow @nogc
    {
        sin.sin_family = AddressFamily.INET;
        sin.sin_addr.s_addr = ADDR_ANY;
        sin.sin_port = htons(port);
    }

    /**
     * Construct a new `InternetAddress`.
     * Params:
     *   addr = A sockaddr_in as obtained from lower-level API calls such as getifaddrs.
     */
    this(sockaddr_in addr) pure nothrow @nogc
    {
        assert(addr.sin_family == AddressFamily.INET);
        sin = addr;
    }

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

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

    /**
     * Attempts to retrieve the host name as a fully qualified domain name.
     *
     * Returns: The FQDN corresponding to this `InternetAddress`, or
     * `null` if the host name did not resolve.
     *
     * Throws: `AddressException` on error.
     */
    override string toHostNameString() const
    {
        // 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)
            return super.toHostNameString();
        else
        {
            auto host = new InternetHost();
            if (!host.getHostByAddr(ntohl(sin.sin_addr.s_addr)))
                return null;
            return host.name;
        }
    }

    /**
     * Compares with another InternetAddress of same type for equality
     * Returns: true if the InternetAddresses share the same address and
     * port number.
     */
    override bool opEquals(Object o) const
    {
        auto other = cast(InternetAddress) o;
        return other && this.sin.sin_addr.s_addr == other.sin.sin_addr.s_addr &&
            this.sin.sin_port == other.sin.sin_port;
    }

    ///
    @system unittest
    {
        auto addr1 = new InternetAddress("127.0.0.1", 80);
        auto addr2 = new InternetAddress("127.0.0.2", 80);

        assert(addr1 == addr1);
        assert(addr1 != addr2);
    }

    /**
     * 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,
     * `ADDR_NONE` is returned.
     */
    static uint parse(scope const(char)[] addr) @trusted nothrow
    {
        return ntohl(inet_addr(addr.tempCString()));
    }

    /**
     * Convert an IPv4 address number in host byte order to a human readable
     * string representing the IPv4 address in dotted-decimal form.
     */
    static string addrToString(uint addr) @trusted nothrow
    {
        in_addr sin_addr;
        sin_addr.s_addr = htonl(addr);
        return to!string(inet_ntoa(sin_addr));
    }
}


@safe unittest
{
    softUnittest({
        const InternetAddress ia = new InternetAddress("63.105.9.61", 80);
        assert(ia.toString() == "63.105.9.61:80");
    });

    softUnittest({
        // test construction from a sockaddr_in
        sockaddr_in sin;

        sin.sin_addr.s_addr = htonl(0x7F_00_00_01);  // 127.0.0.1
        sin.sin_family = AddressFamily.INET;
        sin.sin_port = htons(80);

        const InternetAddress ia = new InternetAddress(sin);
        assert(ia.toString() == "127.0.0.1:80");
    });

    softUnittest({
        // test reverse lookup
        auto ih = new InternetHost;
        if (ih.getHostByName("digitalmars.com"))
        {
            const ia = new InternetAddress(ih.addrList[0], 80);
            assert(ia.toHostNameString() == "digitalmars.com");

            if (getnameinfoPointer)
            {
                // test reverse lookup, via gethostbyaddr
                auto getnameinfoPointerBackup = getnameinfoPointer;
                cast() getnameinfoPointer = null;
                scope(exit) cast() getnameinfoPointer = getnameinfoPointerBackup;

                assert(ia.toHostNameString() == "digitalmars.com");
            }
        }
    });

    version (SlowTests)
    softUnittest({
        // test failing reverse lookup
        const InternetAddress ia = new InternetAddress("127.114.111.120", 80);
        assert(ia.toHostNameString() is null);

        if (getnameinfoPointer)
        {
            // test failing reverse lookup, via gethostbyaddr
            auto getnameinfoPointerBackup = getnameinfoPointer;
            getnameinfoPointer = null;
            scope(exit) getnameinfoPointer = getnameinfoPointerBackup;

            assert(ia.toHostNameString() is null);
        }
    });
}


/**
 * `Internet6Address` encapsulates an IPv6 (Internet Protocol version 6)
 * socket address.
 *
 * Consider using `getAddress`, `parseAddress` and `Address` methods
 * instead of using this class directly.
 */
class Internet6Address: Address
{
protected:
    sockaddr_in6 sin6;


    this() pure nothrow @nogc
    {
    }


public:
    override @property sockaddr* name()
    {
        return cast(sockaddr*)&sin6;
    }

    override @property const(sockaddr)* name() const
    {
        return cast(const(sockaddr)*)&sin6;
    }


    override @property socklen_t nameLen() const
    {
        return cast(socklen_t) sin6.sizeof;
    }


    /// Any IPv6 host address.
    static @property ref const(ubyte)[16] ADDR_ANY() pure nothrow @nogc
    {
        const(ubyte)[16]* addr;
        static if (is(typeof(IN6ADDR_ANY)))
        {
            addr = &IN6ADDR_ANY.s6_addr;
            return *addr;
        }
        else static if (is(typeof(in6addr_any)))
        {
            addr = &in6addr_any.s6_addr;
            return *addr;
        }
        else
            static assert(0);
    }

    /// Any IPv6 port number.
    enum ushort PORT_ANY = 0;

    /// Returns the IPv6 port number.
    @property ushort port() const pure nothrow @nogc
    {
        return ntohs(sin6.sin6_port);
    }

    /// Returns the IPv6 address.
    @property ubyte[16] addr() const pure nothrow @nogc
    {
        return sin6.sin6_addr.s6_addr;
    }

    /**
     * Construct a new `Internet6Address`.
     * Params:
     *   addr    = an IPv6 host address string in the form described in RFC 2373,
     *             or a host name which will be resolved using `getAddressInfo`.
     *   service = (optional) service name.
     */
    this(scope const(char)[] addr, scope const(char)[] service = null) @trusted
    {
        auto results = getAddressInfo(addr, service, AddressFamily.INET6);
        assert(results.length && results[0].family == AddressFamily.INET6);
        sin6 = *cast(sockaddr_in6*) results[0].address.name;
    }

    /**
     * Construct a new `Internet6Address`.
     * Params:
     *   addr = an IPv6 host address string in the form described in RFC 2373,
     *          or a host name which will be resolved using `getAddressInfo`.
     *   port = port number, may be `PORT_ANY`.
     */
    this(scope const(char)[] addr, ushort port)
    {
        if (port == PORT_ANY)
            this(addr);
        else
            this(addr, to!string(port));
    }

    /**
     * Construct a new `Internet6Address`.
     * Params:
     *   addr = (optional) an IPv6 host address in host byte order, or
     *          `ADDR_ANY`.
     *   port = port number, may be `PORT_ANY`.
     */
    this(ubyte[16] addr, ushort port) pure nothrow @nogc
    {
        sin6.sin6_family = AddressFamily.INET6;
        sin6.sin6_addr.s6_addr = addr;
        sin6.sin6_port = htons(port);
    }

    /// ditto
    this(ushort port) pure nothrow @nogc
    {
        sin6.sin6_family = AddressFamily.INET6;
        sin6.sin6_addr.s6_addr = ADDR_ANY;
        sin6.sin6_port = htons(port);
    }

     /**
     * Construct a new `Internet6Address`.
     * Params:
     *   addr = A sockaddr_in6 as obtained from lower-level API calls such as getifaddrs.
     */
    this(sockaddr_in6 addr) pure nothrow @nogc
    {
        assert(addr.sin6_family == AddressFamily.INET6);
        sin6 = addr;
    }

   /**
     * Parse an IPv6 host address string as described in RFC 2373, and return the
     * address.
     * Throws: `SocketException` on error.
     */
    static ubyte[16] parse(scope const(char)[] addr) @trusted
    {
        // Although we could use inet_pton here, it's only available on Windows
        // versions starting with Vista, so use getAddressInfo with NUMERICHOST
        // instead.
        auto results = getAddressInfo(addr, AddressInfoFlags.NUMERICHOST);
        if (results.length && results[0].family == AddressFamily.INET6)
            return (cast(sockaddr_in6*) results[0].address.name).sin6_addr.s6_addr;
        throw new AddressException("Not an IPv6 address", 0);
    }
}


@safe unittest
{
    softUnittest({
        const Internet6Address ia = new Internet6Address("::1", 80);
        assert(ia.toString() == "[::1]:80");
    });

    softUnittest({
        // test construction from a sockaddr_in6
        sockaddr_in6 sin;

        sin.sin6_addr.s6_addr = [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];  // [::1]
        sin.sin6_family = AddressFamily.INET6;
        sin.sin6_port = htons(80);

        const Internet6Address ia = new Internet6Address(sin);
        assert(ia.toString() == "[::1]:80");
    });
}


version (StdDdoc)
{
    static if (!is(sockaddr_un))
    {
        // This exists only to allow the constructor taking
        // a sockaddr_un to be compilable for documentation
        // on platforms that don't supply a sockaddr_un.
        struct sockaddr_un
        {
        }
    }

    /**
     * `UnixAddress` encapsulates an address for a Unix domain socket
     * (`AF_UNIX`), i.e. a socket bound to a path name in the file system.
     * Available only on supported systems.
     *
     * Linux also supports an abstract address namespace, in which addresses
     * are independent of the file system. A socket address is abstract
     * iff `path` starts with a _null byte (`'\0'`). Null bytes in other
     * positions of an abstract address are allowed and have no special
     * meaning.
     *
     * Example:
     * ---
     * auto addr = new UnixAddress("/var/run/dbus/system_bus_socket");
     * auto abstractAddr = new UnixAddress("\0/tmp/dbus-OtHLWmCLPR");
     * ---
     *
     * See_Also: $(HTTP http://man7.org/linux/man-pages/man7/unix.7.html, UNIX(7))
     */
    class UnixAddress: Address
    {
        private this() pure nothrow @nogc {}

        /// Construct a new `UnixAddress` from the specified path.
        this(scope const(char)[] path) { }

        /**
         * Construct a new `UnixAddress`.
         * Params:
         *   addr = A sockaddr_un as obtained from lower-level API calls.
         */
        this(sockaddr_un addr) pure nothrow @nogc { }

        /// Get the underlying _path.
        @property string path() const { return null; }

        /// ditto
        override string toString() const { return null; }

        override @property sockaddr* name() { return null; }
        override @property const(sockaddr)* name() const { return null; }
        override @property socklen_t nameLen() const { return 0; }
    }
}
else
static if (is(sockaddr_un))
{
    class UnixAddress: Address
    {
    protected:
        socklen_t _nameLen;

        struct
        {
        align (1):
            sockaddr_un sun;
            char unused = '\0'; // placeholder for a terminating '\0'
        }

        this() pure nothrow @nogc
        {
            sun.sun_family = AddressFamily.UNIX;
            sun.sun_path = '?';
            _nameLen = sun.sizeof;
        }

        override void setNameLen(socklen_t len) @trusted
        {
            if (len > sun.sizeof)
                throw new SocketParameterException("Not enough socket address storage");
            _nameLen = len;
        }

    public:
        override @property sockaddr* name()
        {
            return cast(sockaddr*)&sun;
        }

        override @property const(sockaddr)* name() const
        {
            return cast(const(sockaddr)*)&sun;
        }

        override @property socklen_t nameLen() @trusted const
        {
            return _nameLen;
        }

        this(scope const(char)[] path) @trusted pure
        {
            enforce(path.length <= sun.sun_path.sizeof, new SocketParameterException("Path too long"));
            sun.sun_family = AddressFamily.UNIX;
            sun.sun_path.ptr[0 .. path.length] = (cast(byte[]) path)[];
            _nameLen = cast(socklen_t)
                {
                    auto len = sockaddr_un.init.sun_path.offsetof + path.length;
                    // Pathname socket address must be terminated with '\0'
                    // which must be included in the address length.
                    if (sun.sun_path.ptr[0])
                    {
                        sun.sun_path.ptr[path.length] = 0;
                        ++len;
                    }
                    return len;
                }();
        }

        this(sockaddr_un addr) pure nothrow @nogc
        {
            assert(addr.sun_family == AddressFamily.UNIX);
            sun = addr;
        }

        @property string path() @trusted const pure
        {
            auto len = _nameLen - sockaddr_un.init.sun_path.offsetof;
            // For pathname socket address we need to strip off the terminating '\0'
            if (sun.sun_path.ptr[0])
                --len;
            return (cast(const(char)*) sun.sun_path.ptr)[0 .. len].idup;
        }

        override string toString() const pure
        {
            return path;
        }
    }

    @safe unittest
    {
        import core.stdc.stdio : remove;
        import std.file : deleteme;

        immutable ubyte[] data = [1, 2, 3, 4];
        Socket[2] pair;

        auto names = [ deleteme ~ "-unix-socket" ];
        version (linux)
            names ~= "\0" ~ deleteme ~ "-abstract\0unix\0socket";
        foreach (name; names)
        {
            auto address = new UnixAddress(name);

            auto listener = new Socket(AddressFamily.UNIX, SocketType.STREAM);
            scope(exit) listener.close();
            listener.bind(address);
            scope(exit) () @trusted { if (name[0]) remove(name.tempCString()); } ();
            assert(listener.localAddress.toString == name);

            listener.listen(1);

            pair[0] = new Socket(AddressFamily.UNIX, SocketType.STREAM);
            scope(exit) listener.close();

            pair[0].connect(address);
            scope(exit) pair[0].close();

            pair[1] = listener.accept();
            scope(exit) pair[1].close();

            pair[0].send(data);

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


/**
 * Class for exceptions thrown by `Socket.accept`.
 */
class SocketAcceptException: SocketOSException
{
    mixin socketOSExceptionCtors;
}

/// 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
}


private mixin template FieldProxy(string target, string field)
{
    mixin(`
        @property typeof(`~target~`) `~field~`() const pure nothrow @nogc
        {
            return `~target~`;
        }

        /// ditto
        @property typeof(`~target~`) `~field~`(typeof(`~target~`) value) pure nothrow @nogc
        {
            return `~target~` = value;
        }
    `);
}


/// Duration timeout value.
struct TimeVal
{
    _ctimeval ctimeval;
    alias tv_sec_t = typeof(ctimeval.tv_sec);
    alias tv_usec_t = typeof(ctimeval.tv_usec);

    version (StdDdoc) // no DDoc for string mixins, can't forward individual fields
    {
        tv_sec_t seconds;           /// Number of _seconds.
        tv_usec_t microseconds;     /// Number of additional _microseconds.
    }
    else
    {
        // D interface
        mixin FieldProxy!(`ctimeval.tv_sec`, `seconds`);
        mixin FieldProxy!(`ctimeval.tv_usec`, `microseconds`);
    }
}


/**
 * A collection of sockets for use with `Socket.select`.
 *
 * `SocketSet` wraps the platform `fd_set` type. However, unlike
 * `fd_set`, `SocketSet` is not statically limited to `FD_SETSIZE`
 * or any other limit, and grows as needed.
 */
class SocketSet
{
private:
    version (Windows)
    {
        // On Windows, fd_set is an array of socket handles,
        // following a word containing the fd_set instance size.
        // We use one dynamic array for everything, and use its first
        // element(s) for the count.

        alias fd_set_count_type = typeof(fd_set.init.fd_count);
        alias fd_set_type = typeof(fd_set.init.fd_array[0]);
        static assert(fd_set_type.sizeof == socket_t.sizeof);

        // Number of fd_set_type elements at the start of our array that are
        // used for the socket count and alignment

        enum FD_SET_OFFSET = fd_set.fd_array.offsetof / fd_set_type.sizeof;
        static assert(FD_SET_OFFSET);
        static assert(fd_set.fd_count.offsetof % fd_set_type.sizeof == 0);

        fd_set_type[] set;

        void resize(size_t size) pure nothrow
        {
            set.length = FD_SET_OFFSET + size;
        }

        ref inout(fd_set_count_type) count() @trusted @property inout pure nothrow @nogc
        {
            assert(set.length);
            return *cast(inout(fd_set_count_type)*)set.ptr;
        }

        size_t capacity() @property const pure nothrow @nogc
        {
            return set.length - FD_SET_OFFSET;
        }

        inout(socket_t)[] fds() @trusted inout @property pure nothrow @nogc
        {
            return cast(inout(socket_t)[])set[FD_SET_OFFSET .. FD_SET_OFFSET+count];
        }
    }
    else
    version (Posix)
    {
        // On Posix, fd_set is a bit array. We assume that the fd_set
        // type (declared in core.sys.posix.sys.select) is a structure
        // containing a single field, a static array.

        static assert(fd_set.tupleof.length == 1);

        // This is the type used in the fd_set array.
        // Using the type of the correct size is important for big-endian
        // architectures.

        alias fd_set_type = typeof(fd_set.init.tupleof[0][0]);

        // Number of file descriptors represented by one fd_set_type

        enum FD_NFDBITS = 8 * fd_set_type.sizeof;

        static fd_set_type mask(uint n) pure nothrow @nogc
        {
            return (cast(fd_set_type) 1) << (n % FD_NFDBITS);
        }

        // Array size to fit that many sockets

        static size_t lengthFor(size_t size) pure nothrow @nogc
        {
            return (size + (FD_NFDBITS-1)) / FD_NFDBITS;
        }

        fd_set_type[] set;

        void resize(size_t size) pure nothrow
        {
            set.length = lengthFor(size);
        }

        // Make sure we can fit that many sockets

        void setMinCapacity(size_t size) pure nothrow
        {
            auto length = lengthFor(size);
            if (set.length < length)
                set.length = length;
        }

        size_t capacity() @property const pure nothrow @nogc
        {
            return set.length * FD_NFDBITS;
        }

        int maxfd;
    }
    else
        static assert(false, "Unknown platform");

public:

    /**
     * Create a SocketSet with a specific initial capacity (defaults to
     * `FD_SETSIZE`, the system's default capacity).
     */
    this(size_t size = FD_SETSIZE) pure nothrow
    {
        resize(size);
        reset();
    }

    /// Reset the `SocketSet` so that there are 0 `Socket`s in the collection.
    void reset() pure nothrow @nogc
    {
        version (Windows)
            count = 0;
        else
        {
            set[] = 0;
            maxfd = -1;
        }
    }


    void add(socket_t s) @trusted pure nothrow
    {
        version (Windows)
        {
            if (count == capacity)
            {
                set.length *= 2;
                set.length = set.capacity;
            }
            ++count;
            fds[$-1] = s;
        }
        else
        {
            auto index = s / FD_NFDBITS;
            auto length = set.length;
            if (index >= length)
            {
                while (index >= length)
                    length *= 2;
                set.length = length;
                set.length = set.capacity;
            }
            set[index] |= mask(s);
            if (maxfd < s)
                maxfd = s;
        }
    }

    /**
     * Add a `Socket` to the collection.
     * The socket must not already be in the collection.
     */
    void add(Socket s) pure nothrow
    {
        add(s.sock);
    }

    void remove(socket_t s) pure nothrow
    {
        version (Windows)
        {
            import std.algorithm.searching : countUntil;
            auto fds = fds;
            auto p = fds.countUntil(s);
            if (p >= 0)
                fds[p] = fds[--count];
        }
        else
        {
            auto index = s / FD_NFDBITS;
            if (index >= set.length)
                return;
            set[index] &= ~mask(s);
            // note: adjusting maxfd would require scanning the set, not worth it
        }
    }


    /**
     * Remove this `Socket` from the collection.
     * Does nothing if the socket is not in the collection already.
     */
    void remove(Socket s) pure nothrow
    {
        remove(s.sock);
    }

    int isSet(socket_t s) const pure nothrow @nogc
    {
        version (Windows)
        {
            import std.algorithm.searching : canFind;
            return fds.canFind(s) ? 1 : 0;
        }
        else
        {
            if (s > maxfd)
                return 0;
            auto index = s / FD_NFDBITS;
            return (set[index] & mask(s)) ? 1 : 0;
        }
    }


    /// Return nonzero if this `Socket` is in the collection.
    int isSet(Socket s) const pure nothrow @nogc
    {
        return isSet(s.sock);
    }


    /**
     * Returns:
     * The current capacity of this `SocketSet`. The exact
     * meaning of the return value varies from platform to platform.
     *
     * Note:
     * Since D 2.065, this value does not indicate a
     * restriction, and `SocketSet` will grow its capacity as
     * needed automatically.
     */
    @property uint max() const pure nothrow @nogc
    {
        return cast(uint) capacity;
    }


    fd_set* toFd_set() @trusted pure nothrow @nogc
    {
        return cast(fd_set*) set.ptr;
    }


    int selectn() const pure nothrow @nogc
    {
        version (Windows)
        {
            return count;
        }
        else version (Posix)
        {
            return maxfd + 1;
        }
    }
}

@safe unittest
{
    auto fds = cast(socket_t[])
        [cast(socket_t) 1, 2, 0, 1024, 17, 42, 1234, 77, 77+32, 77+64];
    auto set = new SocketSet();
    foreach (fd; fds) assert(!set.isSet(fd));
    foreach (fd; fds) set.add(fd);
    foreach (fd; fds) assert(set.isSet(fd));

    // Make sure SocketSet reimplements fd_set correctly
    auto fdset = set.toFd_set();
    foreach (fd; fds[0]..cast(socket_t)(fds[$-1]+1))
        assert(cast(bool) set.isSet(fd) == cast(bool)(() @trusted => FD_ISSET(fd, fdset))());

    foreach (fd; fds)
    {
        assert(set.isSet(fd));
        set.remove(fd);
        assert(!set.isSet(fd));
    }
}

@safe unittest
{
    softUnittest({
        enum PAIRS = 768;
        version (Posix)
        () @trusted
        {
            enum LIMIT = 2048;
            static assert(LIMIT > PAIRS*2);
            import core.sys.posix.sys.resource;
            rlimit fileLimit;
            getrlimit(RLIMIT_NOFILE, &fileLimit);
            assert(fileLimit.rlim_max > LIMIT, "Open file hard limit too low");
            fileLimit.rlim_cur = LIMIT;
            setrlimit(RLIMIT_NOFILE, &fileLimit);
        } ();

        Socket[2][PAIRS] pairs;
        foreach (ref pair; pairs)
            pair = socketPair();
        scope(exit)
        {
            foreach (pair; pairs)
            {
                pair[0].close();
                pair[1].close();
            }
        }

        import std.random;
        auto rng = Xorshift(42);
        pairs[].randomShuffle(rng);

        auto readSet = new SocketSet();
        auto writeSet = new SocketSet();
        auto errorSet = new SocketSet();

        foreach (testPair; pairs)
        {
            void fillSets()
            {
                readSet.reset();
                writeSet.reset();
                errorSet.reset();
                foreach (ref pair; pairs)
                    foreach (s; pair[])
                    {
                        readSet.add(s);
                        writeSet.add(s);
                        errorSet.add(s);
                    }
            }

            fillSets();
            auto n = Socket.select(readSet, writeSet, errorSet);
            assert(n == PAIRS*2); // All in writeSet
            assert(writeSet.isSet(testPair[0]));
            assert(writeSet.isSet(testPair[1]));
            assert(!readSet.isSet(testPair[0]));
            assert(!readSet.isSet(testPair[1]));
            assert(!errorSet.isSet(testPair[0]));
            assert(!errorSet.isSet(testPair[1]));

            ubyte[1] b;
            // Socket.send can't be marked with `scope`
            // -> @safe DIP1000 code can't use it - see https://github.com/dlang/phobos/pull/6204
            () @trusted {
                testPair[0].send(b[]);
            }();
            fillSets();
            n = Socket.select(readSet, null, null);
            assert(n == 1); // testPair[1]
            assert(readSet.isSet(testPair[1]));
            assert(!readSet.isSet(testPair[0]));
            // Socket.receive can't be marked with `scope`
            // -> @safe DIP1000 code can't use it - see https://github.com/dlang/phobos/pull/6204
            () @trusted {
                testPair[1].receive(b[]);
            }();
        }
    });
}

@safe unittest // Issue 14012, 14013
{
    auto set = new SocketSet(1);
    assert(set.max >= 0);

    enum LIMIT = 4096;
    foreach (n; 0 .. LIMIT)
        set.add(cast(socket_t) n);
    assert(set.max >= LIMIT);
}

/// 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,        /// Internet Control Message Protocol level
    IGMP =    ProtocolType.IGMP,        /// Internet Group Management Protocol level
    GGP =     ProtocolType.GGP,         /// Gateway to Gateway Protocol level
    TCP =     ProtocolType.TCP,         /// Transmission Control Protocol level
    PUP =     ProtocolType.PUP,         /// PARC Universal Packet Protocol level
    UDP =     ProtocolType.UDP,         /// User Datagram Protocol level
    IDP =     ProtocolType.IDP,         /// Xerox NS protocol level
    RAW =     ProtocolType.RAW,         /// Raw IP packet level
    IPV6 =    ProtocolType.IPV6,        /// Internet Protocol version 6 level
}

/// _Linger information for use with SocketOption.LINGER.
struct Linger
{
    _clinger clinger;

    version (StdDdoc) // no DDoc for string mixins, can't forward individual fields
    {
        private alias l_onoff_t = typeof(_clinger.init.l_onoff );
        private alias l_linger_t = typeof(_clinger.init.l_linger);
        l_onoff_t  on;   /// Nonzero for _on.
        l_linger_t time; /// Linger _time.
    }
    else
    {
        // D interface
        mixin FieldProxy!(`clinger.l_onoff`, `on`);
        mixin FieldProxy!(`clinger.l_linger`, `time`);
    }
}

/// 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
    ERROR =                SO_ERROR,            /// Retrieve and clear error status
    KEEPALIVE =            SO_KEEPALIVE,        /// Enable keep-alive packets
    ACCEPTCONN =           SO_ACCEPTCONN,       /// Listen
    RCVLOWAT =             SO_RCVLOWAT,         /// Minimum number of input bytes to process
    SNDLOWAT =             SO_SNDLOWAT,         /// Minimum number of output bytes to process
    TYPE =                 SO_TYPE,             /// Socket type

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

    // SocketOptionLevel.IPV6:
    IPV6_UNICAST_HOPS =    .IPV6_UNICAST_HOPS,          /// IP unicast hop limit
    IPV6_MULTICAST_IF =    .IPV6_MULTICAST_IF,          /// IP multicast interface
    IPV6_MULTICAST_LOOP =  .IPV6_MULTICAST_LOOP,        /// IP multicast loopback
    IPV6_MULTICAST_HOPS =  .IPV6_MULTICAST_HOPS,        /// IP multicast hops
    IPV6_JOIN_GROUP =      .IPV6_JOIN_GROUP,            /// Add an IP group membership
    IPV6_LEAVE_GROUP =     .IPV6_LEAVE_GROUP,           /// Drop an IP group membership
    IPV6_V6ONLY =          .IPV6_V6ONLY,                /// Treat wildcard bind as AF_INET6-only
}


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

    version (Windows)
        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. The unittest below tests this
    // behavior.
    enum WINSOCK_TIMEOUT_SKEW = 500;

    @safe unittest
    {
        version (SlowTests)
        softUnittest({
            import std.datetime.stopwatch;
            import std.typecons;

            enum msecs = 1000;
            auto pair = socketPair();
            auto sock = pair[0];
            sock.setOption(SocketOptionLevel.SOCKET,
                SocketOption.RCVTIMEO, dur!"msecs"(msecs));

            auto sw = StopWatch(Yes.autoStart);
            ubyte[1] buf;
            sock.receive(buf);
            sw.stop();

            Duration readBack = void;
            sock.getOption(SocketOptionLevel.SOCKET, SocketOption.RCVTIMEO, readBack);

            assert(readBack.total!"msecs" == msecs);
            assert(sw.peek().total!"msecs" > msecs - 100 && sw.peek().total!"msecs" < msecs + 100);
        });
    }

    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() pure nothrow @nogc
    {
    }


public:

    /**
     * Create a blocking socket. If a single protocol type exists to support
     * this socket type within the address family, the `ProtocolType` may be
     * omitted.
     */
    this(AddressFamily af, SocketType type, ProtocolType protocol) @trusted
    {
        _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);
    }

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


    /// ditto
    this(AddressFamily af, SocketType type, scope const(char)[] protocolName) @trusted
    {
        protoent* proto;
        proto = getprotobyname(protocolName.tempCString());
        if (!proto)
            throw new SocketOSException("Unable to find the protocol");
        this(af, type, cast(ProtocolType) proto.p_proto);
    }


    /**
     * Create a blocking socket using the parameters from the specified
     * `AddressInfo` structure.
     */
    this(in AddressInfo info)
    {
        this(info.family, info.type, info.protocol);
    }

    /// Use an existing socket handle.
    this(socket_t sock, AddressFamily af) pure nothrow @nogc
    {
        assert(sock != socket_t.init);
        this.sock = sock;
        this._family = af;
    }


    ~this() nothrow @nogc
    {
        close();
    }


    /// Get underlying socket handle.
    @property socket_t handle() const pure nothrow @nogc
    {
        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.
     */
    @property bool blocking() @trusted const nothrow @nogc
    {
        version (Windows)
        {
            return _blocking;
        }
        else version (Posix)
        {
            return !(fcntl(handle, F_GETFL, 0) & O_NONBLOCK);
        }
    }

    /// ditto
    @property void blocking(bool byes) @trusted
    {
        version (Windows)
        {
            uint num = !byes;
            if (_SOCKET_ERROR == ioctlsocket(sock, FIONBIO, &num))
                goto err;
            _blocking = byes;
        }
        else version (Posix)
        {
            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.
    @property AddressFamily addressFamily()
    {
        return _family;
    }

    /// Property that indicates if this is a valid, alive socket.
    @property bool isAlive() @trusted const
    {
        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) @trusted
    {
        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) @trusted
    {
        if (_SOCKET_ERROR == .connect(sock, to.name, to.nameLen))
        {
            int err;
            err = _lasterr();

            if (!blocking)
            {
                version (Windows)
                {
                    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. `bind` must be called before you
     * can `listen`. The `backlog` is a request of how many pending
     * incoming connections are queued until `accept`ed.
     */
    void listen(int backlog) @trusted
    {
        if (_SOCKET_ERROR == .listen(sock, backlog))
            throw new SocketOSException("Unable to listen on socket");
    }

    /**
     * Called by `accept` when a new `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 `Socket`'s handle must not be
     * set; `Socket` has a protected constructor `this()` to use in this
     * situation.
     *
     * Override to use a derived class.
     * The returned socket's handle must not be set.
     */
    protected Socket accepting() pure nothrow
    {
        return new Socket;
    }

    /**
     * Accept an incoming connection. If the socket is blocking, `accept`
     * waits for a connection request. Throws `SocketAcceptException` if
     * unable to _accept. See `accepting` for use with derived classes.
     */
    Socket accept() @trusted
    {
        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 (Windows)
                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) @trusted nothrow @nogc
    {
        .shutdown(sock, cast(int) how);
    }


    private static void _close(socket_t sock) @system nothrow @nogc
    {
        version (Windows)
        {
            .closesocket(sock);
        }
        else version (Posix)
        {
            .close(sock);
        }
    }


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


    /**
     * Returns: the local machine's host name
     */
    static @property string hostName() @trusted     // 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(result.ptr);
    }

    /// Remote endpoint `Address`.
    @property Address remoteAddress() @trusted
    {
        Address addr = createAddress();
        socklen_t nameLen = addr.nameLen;
        if (_SOCKET_ERROR == .getpeername(sock, addr.name, &nameLen))
            throw new SocketOSException("Unable to obtain remote socket address");
        addr.setNameLen(nameLen);
        assert(addr.addressFamily == _family);
        return addr;
    }

    /// Local endpoint `Address`.
    @property Address localAddress() @trusted
    {
        Address addr = createAddress();
        socklen_t nameLen = addr.nameLen;
        if (_SOCKET_ERROR == .getsockname(sock, addr.name, &nameLen))
            throw new SocketOSException("Unable to obtain local socket address");
        addr.setNameLen(nameLen);
        assert(addr.addressFamily == _family);
        return addr;
    }

    /**
     * Send or receive error code. See `wouldHaveBlocked`,
     * `lastSocketError` and `Socket.getErrorText` for obtaining more
     * information about the error.
     */
    enum int ERROR = _SOCKET_ERROR;

    private static int capToInt(size_t size) nothrow @nogc
    {
        // Windows uses int instead of size_t for length arguments.
        // Luckily, the send/recv functions make no guarantee that
        // all the data is sent, so we use that to send at most
        // int.max bytes.
        return size > size_t(int.max) ? int.max : cast(int) size;
    }

    /**
     * Send data on the connection. If the socket is blocking and there is no
     * buffer space left, `send` waits.
     * Returns: The number of bytes actually sent, or `Socket.ERROR` on
     * failure.
     */
    ptrdiff_t send(const(void)[] buf, SocketFlags flags) @trusted
    {
        static if (is(typeof(MSG_NOSIGNAL)))
        {
            flags = cast(SocketFlags)(flags | MSG_NOSIGNAL);
        }
        version (Windows)
            auto sent = .send(sock, buf.ptr, capToInt(buf.length), cast(int) flags);
        else
            auto sent = .send(sock, buf.ptr, buf.length, cast(int) flags);
        return sent;
    }

    /// ditto
    ptrdiff_t 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, `sendTo` waits.
     * Returns: The number of bytes actually sent, or `Socket.ERROR` on
     * failure.
     */
    ptrdiff_t sendTo(const(void)[] buf, SocketFlags flags, Address to) @trusted
    {
        static if (is(typeof(MSG_NOSIGNAL)))
        {
            flags = cast(SocketFlags)(flags | MSG_NOSIGNAL);
        }
        version (Windows)
            return .sendto(
                       sock, buf.ptr, capToInt(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
    ptrdiff_t sendTo(const(void)[] buf, Address to)
    {
        return sendTo(buf, SocketFlags.NONE, to);
    }


    //assumes you connect()ed
    /// ditto
    ptrdiff_t sendTo(const(void)[] buf, SocketFlags flags) @trusted
    {
        static if (is(typeof(MSG_NOSIGNAL)))
        {
            flags = cast(SocketFlags)(flags | MSG_NOSIGNAL);
        }
        version (Windows)
            return .sendto(sock, buf.ptr, capToInt(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
    ptrdiff_t sendTo(const(void)[] buf)
    {
        return sendTo(buf, SocketFlags.NONE);
    }


    /**
     * Receive data on the connection. If the socket is blocking, `receive`
     * waits until there is data to be received.
     * Returns: The number of bytes actually received, `0` if the remote side
     * has closed the connection, or `Socket.ERROR` on failure.
     */
    ptrdiff_t receive(void[] buf, SocketFlags flags) @trusted
    {
        version (Windows)         // Does not use size_t
        {
            return buf.length
                   ? .recv(sock, buf.ptr, capToInt(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 `Address`.
     * If the socket is blocking, `receiveFrom` waits until there is data to
     * be received.
     * Returns: The number of bytes actually received, `0` if the remote side
     * has closed the connection, or `Socket.ERROR` on failure.
     */
    ptrdiff_t receiveFrom(void[] buf, SocketFlags flags, ref Address from) @trusted
    {
        if (!buf.length)         //return 0 and don't think the connection closed
            return 0;
        if (from is null || from.addressFamily != _family)
            from = createAddress();
        socklen_t nameLen = from.nameLen;
        version (Windows)
        {
            auto read = .recvfrom(sock, buf.ptr, capToInt(buf.length), cast(int) flags, from.name, &nameLen);
            from.setNameLen(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);
            from.setNameLen(nameLen);
            assert(from.addressFamily == _family);
            // if (!read) //connection closed
            return read;
        }
    }


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


    //assumes you connect()ed
    /// ditto
    ptrdiff_t receiveFrom(void[] buf, SocketFlags flags) @trusted
    {
        if (!buf.length)         //return 0 and don't think the connection closed
            return 0;
        version (Windows)
        {
            auto read = .recvfrom(sock, buf.ptr, capToInt(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 `result`.
     * The length, in bytes, of the actual result - very different from getsockopt()
     */
    int getOption(SocketOptionLevel level, SocketOption option, void[] result) @trusted
    {
        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) @trusted
    {
        return getOption(level, option, (&result)[0 .. 1]);
    }


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

    /// Get a timeout (duration) option.
    void getOption(SocketOptionLevel level, SocketOption option, out Duration result) @trusted
    {
        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 (Windows)
        {
            int msecs;
            getOption(level, option, (&msecs)[0 .. 1]);
            if (option == SocketOption.RCVTIMEO)
                msecs += WINSOCK_TIMEOUT_SKEW;
            result = dur!"msecs"(msecs);
        }
        else version (Posix)
        {
            TimeVal tv;
            getOption(level, option, (&tv.ctimeval)[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) @trusted
    {
        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) @trusted
    {
        setOption(level, option, (&value)[0 .. 1]);
    }


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

    /**
     * Sets a timeout (duration) option, i.e. `SocketOption.SNDTIMEO` or
     * `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. `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:
     *   level  = The level at which a socket option is defined.
     *   option = Either `SocketOption.SNDTIMEO` or `SocketOption.RCVTIMEO`.
     *   value  = The timeout duration to set. Must not be negative.
     *
     * Throws: `SocketException` if setting the options fails.
     *
     * Example:
     * ---
     * import std.datetime;
     * import std.typecons;
     * 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(Yes.autoStart);
     * 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) @trusted
    {
        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 (Windows)
        {
            import std.algorithm.comparison : max;

            auto msecs = to!int(value.total!"msecs");
            if (msecs != 0 && option == SocketOption.RCVTIMEO)
                msecs = max(1, msecs - WINSOCK_TIMEOUT_SKEW);
            setOption(level, option, msecs);
        }
        else version (Posix)
        {
            _ctimeval tv;
            value.split!("seconds", "usecs")(tv.tv_sec, tv.tv_usec);
            setOption(level, option, (&tv)[0 .. 1]);
        }
        else static assert(false);
    }

    /**
     * Get a text description of this socket's error status, and clear the
     * socket's error status.
     */
    string getErrorText()
    {
        int32_t error;
        getOption(SocketOptionLevel.SOCKET, SocketOption.ERROR, error);
        return formatSocketError(error);
    }

    /**
     * Enables TCP keep-alive with the specified parameters.
     *
     * Params:
     *   time     = Number of seconds with no activity until the first
     *              keep-alive packet is sent.
     *   interval = Number of seconds between when successive keep-alive
     *              packets are sent if no acknowledgement is received.
     *
     * Throws: `SocketOSException` if setting the options fails, or
     * `SocketFeatureException` if setting keep-alive parameters is
     * unsupported on the current platform.
     */
    void setKeepAlive(int time, int interval) @trusted
    {
        version (Windows)
        {
            tcp_keepalive options;
            options.onoff = 1;
            options.keepalivetime = time * 1000;
            options.keepaliveinterval = interval * 1000;
            uint cbBytesReturned;
            enforce(WSAIoctl(sock, SIO_KEEPALIVE_VALS,
                             &options, options.sizeof,
                             null, 0,
                             &cbBytesReturned, null, null) == 0,
                    new SocketOSException("Error setting keep-alive"));
        }
        else
        static if (is(typeof(TCP_KEEPIDLE)) && is(typeof(TCP_KEEPINTVL)))
        {
            setOption(SocketOptionLevel.TCP, cast(SocketOption) TCP_KEEPIDLE, time);
            setOption(SocketOptionLevel.TCP, cast(SocketOption) TCP_KEEPINTVL, interval);
            setOption(SocketOptionLevel.SOCKET, SocketOption.KEEPALIVE, true);
        }
        else
            throw new SocketFeatureException("Setting keep-alive options " ~
                "is not supported on this platform");
    }

    /**
     * Wait for a socket to change status. A wait timeout of $(REF Duration, core, time) or
     * `TimeVal`, may be specified; if a timeout is not specified or the
     * `TimeVal` is `null`, the maximum timeout is used. The `TimeVal`
     * timeout has an unspecified value when `select` returns.
     * Returns: The number of sockets with status changes, `0` on timeout,
     * or `-1` on interruption. If the return value is greater than `0`,
     * the `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.
     * 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.
     *
     * Returns:
     * the number of events, 0 on timeout, or -1 on interruption
     */
    static int select(SocketSet checkRead, SocketSet checkWrite, SocketSet checkError, Duration timeout) @trusted
    {
        auto vals = timeout.split!("seconds", "usecs")();
        TimeVal tv;
        tv.seconds      = cast(tv.tv_sec_t ) vals.seconds;
        tv.microseconds = cast(tv.tv_usec_t) vals.usecs;
        return select(checkRead, checkWrite, checkError, &tv);
    }

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

    /// Ditto
    static int select(SocketSet checkRead, SocketSet checkWrite, SocketSet checkError, TimeVal* timeout) @trusted
    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);
        }
    }
    do
    {
        fd_set* fr, fw, fe;
        int n = 0;

        version (Windows)
        {
            // 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;
            }

            // Make sure the sets' capacity matches, to avoid select reading
            // out of bounds just because one set was bigger than another
            if (checkRead ) checkRead .setMinCapacity(n);
            if (checkWrite) checkWrite.setMinCapacity(n);
            if (checkError) checkError.setMinCapacity(n);
        }

        int result = .select(n, fr, fw, fe, &timeout.ctimeval);

        version (Windows)
        {
            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;
    }


    /**
     * Can be overridden to support other addresses.
     * Returns: a new `Address` object for the current address family.
     */
    protected Address createAddress() pure nothrow
    {
        Address result;
        switch (_family)
        {
        static if (is(sockaddr_un))
        {
            case AddressFamily.UNIX:
                result = new UnixAddress;
                break;
        }

        case AddressFamily.INET:
            result = new InternetAddress;
            break;

        case AddressFamily.INET6:
            result = new Internet6Address;
            break;

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

}


/// `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 IPv4 TCP Socket.
    this()
    {
        this(AddressFamily.INET);
    }


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


/// `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 IPv4 UDP Socket.
    this()
    {
        this(AddressFamily.INET);
    }
}

// Issue 16514
@safe unittest
{
    class TestSocket : Socket
    {
        override
        {
            const pure nothrow @nogc @property @safe socket_t handle() { assert(0); }
            const nothrow @nogc @property @trusted bool blocking() { assert(0); }
            @property @trusted void blocking(bool byes) { assert(0); }
            @property @safe AddressFamily addressFamily() { assert(0); }
            const @property @trusted bool isAlive() { assert(0); }
            @trusted void bind(Address addr) { assert(0); }
            @trusted void connect(Address to) { assert(0); }
            @trusted void listen(int backlog) { assert(0); }
            protected pure nothrow @safe Socket accepting() { assert(0); }
            @trusted Socket accept() { assert(0); }
            nothrow @nogc @trusted void shutdown(SocketShutdown how) { assert(0); }
            nothrow @nogc @trusted void close() { assert(0); }
            @property @trusted Address remoteAddress() { assert(0); }
            @property @trusted Address localAddress() { assert(0); }
            @trusted ptrdiff_t send(const(void)[] buf, SocketFlags flags) { assert(0); }
            @safe ptrdiff_t send(const(void)[] buf) { assert(0); }
            @trusted ptrdiff_t sendTo(const(void)[] buf, SocketFlags flags, Address to) { assert(0); }
            @safe ptrdiff_t sendTo(const(void)[] buf, Address to) { assert(0); }
            @trusted ptrdiff_t sendTo(const(void)[] buf, SocketFlags flags) { assert(0); }
            @safe ptrdiff_t sendTo(const(void)[] buf) { assert(0); }
            @trusted ptrdiff_t receive(void[] buf, SocketFlags flags) { assert(0); }
            @safe ptrdiff_t receive(void[] buf) { assert(0); }
            @trusted ptrdiff_t receiveFrom(void[] buf, SocketFlags flags, ref Address from) { assert(0); }
            @safe ptrdiff_t receiveFrom(void[] buf, ref Address from) { assert(0); }
            @trusted ptrdiff_t receiveFrom(void[] buf, SocketFlags flags) { assert(0); }
            @safe ptrdiff_t receiveFrom(void[] buf) { assert(0); }
            @trusted int getOption(SocketOptionLevel level, SocketOption option, void[] result) { assert(0); }
            @trusted int getOption(SocketOptionLevel level, SocketOption option, out int32_t result) { assert(0); }
            @trusted int getOption(SocketOptionLevel level, SocketOption option, out Linger result) { assert(0); }
            @trusted void getOption(SocketOptionLevel level, SocketOption option, out Duration result) { assert(0); }
            @trusted void setOption(SocketOptionLevel level, SocketOption option, void[] value) { assert(0); }
            @trusted void setOption(SocketOptionLevel level, SocketOption option, int32_t value) { assert(0); }
            @trusted void setOption(SocketOptionLevel level, SocketOption option, Linger value) { assert(0); }
            @trusted void setOption(SocketOptionLevel level, SocketOption option, Duration value) { assert(0); }
            @safe string getErrorText() { assert(0); }
            @trusted void setKeepAlive(int time, int interval) { assert(0); }
            protected pure nothrow @safe Address createAddress() { assert(0); }
        }
    }
}

/**
 * Creates a pair of connected sockets.
 *
 * The two sockets are indistinguishable.
 *
 * Throws: `SocketException` if creation of the sockets fails.
 */
Socket[2] socketPair() @trusted
{
    version (Posix)
    {
        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 (Windows)
    {
        // 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);
}

///
@safe 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);
}
