TUUID.cxx

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// @(#)root/base:$Id$
// Author: Fons Rademakers   30/9/2001
 
/*************************************************************************
 * Copyright (C) 1995-2001, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
/** \class TUUID
\ingroup Base
 
This class defines a UUID (Universally Unique IDentifier), also
known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits
long, and if generated according to this algorithm, is either
guaranteed to be different from all other UUIDs/GUIDs generated
until 3400 A.D. or extremely likely to be different. UUIDs were
originally used in the Network Computing System (NCS) and
later in the Open Software Foundation's (OSF) Distributed Computing
Environment (DCE).
 
\note In the way this UUID is constructed, when used outside of
their original concept (NCS), they are actually not Globally unique
and indeed multiple distinct concurrent processes are actually likely
to generate the same UUID.  Technically this is because the UUID is
constructed only from the node information and time information.
To make a globally unique number, this needs to be combined with
TProcessUUID.
 
Structure of universal unique IDs (UUIDs).
 
Depending on the network data representation, the multi-
octet unsigned integer fields are subject to byte swapping
when communicated between dissimilar endian machines.
~~~ {.cpp}
+-----------------------------------+
|     low 32 bits of time           |  0-3   .fTimeLow
+-------------------------------+----
|     mid 16 bits of time       |      4-5   .fTimeMid
+-------+-----------------------+
| vers. |   hi 12 bits of time  |      6-7   .fTimeHiAndVersion
+-------+-------+---------------+
|Res | clkSeqHi |                      8     .fClockSeqHiAndReserved
+---------------+
|   clkSeqLow   |                      9     .fClockSeqLow
+---------------+------------------+
|            node ID               |   10-15 .fNode
+----------------------------------+
~~~
 
The adjusted time stamp is split into three fields, and the
clockSeq is split into two fields.
 
The timestamp is a 60-bit value. For UUID version 1, this
is represented by Coordinated Universal Time (UTC/GMT) as
a count of 100-nanosecond intervals since 00:00:00.00,
15 October 1582 (the date of Gregorian reform to the
Christian calendar).
 
The version number is multiplexed in the 4 most significant
bits of the 'fTimeHiAndVersion' field. There are two defined
versions:
~~~ {.cpp}
              MSB <---
Version      4-Bit Code      Description
------------------------------------------------------------
|  1           0 0 0 1     DCE version, as specified herein.
|  2           0 0 1 0     DCE Security version, with
|                          embedded POSIX UIDs.
|  3           0 0 1 1     node id is a random value
------------------------------------------------------------
~~~
 
## Clock Sequence
 
The clock sequence value must be changed whenever:
 
   The UUID generator detects that the local value of UTC
   has gone backward; this may be due to re-syncing of the system
   clock.
 
While a node is operational, the UUID service always saves
the last UTC used to create a UUID. Each time a new UUID
is created, the current UTC is compared to the saved value
and if either the current value is less or the saved value
was lost, then the clock sequence is incremented modulo
16,384, thus avoiding production of duplicated UUIDs.
 
The clock sequence must be initialized to a random number
to minimize the correlation across system. This provides
maximum protection against node identifiers that may move
or switch from system to system rapidly.
 
## Clock Adjustment
 
UUIDs may be created at a rate greater than the system clock
resolution. Therefore, the system must also maintain an
adjustment value to be added to the lower-order bits of the
time. Logically, each time the system clock ticks, the
adjustment value is cleared. Every time a UUID is generated,
the current adjustment value is read and incremented, and
then added to the UTC time field of the UUID.
 
## Clock Overrun
 
The 100-nanosecond granularity of time should prove sufficient
even for bursts of UUID production in the next generation of
high-performance multiprocessors. If a system overruns the
clock adjustment by requesting too many UUIDs within a single
system clock tick, the UUID generator will stall until the
system clock catches up.
*/
 
#include <ROOT/RCryptoRandom.hxx>
 
#include "TROOT.h"
#include "TDatime.h"
#include "TUUID.h"
#include "TBuffer.h"
#include "TError.h"
#include "TSystem.h"
#include "TInetAddress.h"
#include "TMD5.h"
#include "Bytes.h"
#include "TVirtualMutex.h"
#include "ThreadLocalStorage.h"
#include <string.h>
#include <stdlib.h>
#include <cassert>
#include <cstring>
#include <cstdlib>
#ifdef R__WIN32
#include "Windows4Root.h"
#include <Iphlpapi.h>
#include <process.h>
#define getpid() _getpid()
#define srandom(seed) srand(seed)
#define random() rand()
#else
#include <unistd.h>
#include <sys/socket.h>
#include <sys/time.h>
#if defined(R__LINUX) && !defined(R__WINGCC)
#include <sys/sysinfo.h>
#endif
#include <ifaddrs.h>
#include <netinet/in.h>
#endif
#include <chrono>
 
////////////////////////////////////////////////////////////////////////////////
/// Create a UUID version 4 (variant 1) UUID according to RFC 4122.
/// The UUIDv4 also has 16 octets but all but the version and variant information is random.
/// This leaves 122 random bits, which are filled by the system's cryptographic random number generator.
/// For all intents and purposes, the resulting UUIDs are actually globally unique.
 
TUUID TUUID::UUIDv4()
{
   return TUUID{TV4Marker()};
}
 
////////////////////////////////////////////////////////////////////////////////
/// Create a version 4 UUID.
 
TUUID::TUUID(TV4Marker)
{
   // Ensure we can treat the memory starting at uuid.fTimeLow as an array of 16 octets
   assert(&fNode[5] - reinterpret_cast<unsigned char *>(&fTimeLow) + 1 == 16);
 
   const auto rv = ROOT::Internal::GetCryptoRandom(&fTimeLow, 16);
   R__ASSERT(rv);
   // Fix up variant
   fClockSeqHiAndReserved = (fClockSeqHiAndReserved & 0x3F) | (2 << 6);
   // Fix up version
   fTimeHiAndVersion = (fTimeHiAndVersion & 0x0FFF) | (4 << 12);
 
   // TODO(jblomer): we do what the default constructor does but is this still used? Can it be deprecated?
   fUUIDIndex = 1 << 30;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Create a UUID.
 
TUUID::TUUID()
{
   TTHREAD_TLS(uuid_time_t) time_last;
   TTHREAD_TLS(UShort_t) clockseq(0);
   TTHREAD_TLS(Bool_t) firstTime(kTRUE);
   uuid_time_t *time_last_ptr = TTHREAD_TLS_PTR(time_last);
 
   if (firstTime) {
      R__LOCKGUARD(gROOTMutex); // rand and random are not thread safe.
 
      UInt_t seed;
      if (gSystem) {
         // try to get a unique seed per process
         seed = (UInt_t)(Long64_t(gSystem->Now()) + gSystem->GetPid());
      } else {
         using namespace std::chrono;
         system_clock::time_point today = system_clock::now();
         seed = (UInt_t)(system_clock::to_time_t ( today )) + ::getpid();
      }
      srandom(seed);
      GetCurrentTime(time_last_ptr);
      clockseq = 1+(UShort_t)(65536*random()/(RAND_MAX+1.0));
      firstTime = kFALSE;
   }
 
   uuid_time_t timestamp;
 
   // get current time
   GetCurrentTime(&timestamp);
 
   // if clock went backward change clockseq
   if (CmpTime(&timestamp, time_last_ptr) == -1) {
      clockseq = (clockseq + 1) & 0x3FFF;
      if (clockseq == 0) clockseq++;
   }
 
   Format(clockseq, timestamp);
 
   time_last = timestamp;
   fUUIDIndex = 1<<30;
}
 
////////////////////////////////////////////////////////////////////////////////
/// delete this TUUID
 
TUUID::~TUUID()
{
   //gROOT->GetUUIDs()->RemoveUUID(fUUIDIndex);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Compare two time values.
 
Int_t TUUID::CmpTime(uuid_time_t *t1, uuid_time_t *t2)
{
   if (t1->high < t2->high) return -1;
   if (t1->high > t2->high) return 1;
   if (t1->low  < t2->low)  return -1;
   if (t1->low  > t2->low)  return 1;
   return 0;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Set this UUID to the value specified in uuid ((which must be in
/// TUUID::AsString() format).
 
void TUUID::SetFromString(const char *uuid)
{
   // Format is tttttttt-tttt-cccc-cccc-nnnnnnnnnnnn.
   Long_t  timeLo;
   int     timeMid;
   int     timeHiAndVersion;
   int     clockSeqHiAndRes;
   int     clockSeqLo;
   int     node[6];
 
   sscanf(uuid, "%8lx-%4x-%4x-%2x%2x-%2x%2x%2x%2x%2x%2x",
          &timeLo,
          &timeMid,
          &timeHiAndVersion,
          &clockSeqHiAndRes,
          &clockSeqLo,
          &node[0], &node[1], &node[2], &node[3], &node[4], &node[5]);
 
   // Note that we're going through this agony because scanf is
   // defined to know only to scan into "int"s or "long"s.
   fTimeLow               = (UInt_t) timeLo;
   fTimeMid               = (UShort_t) timeMid;
   fTimeHiAndVersion      = (UShort_t) timeHiAndVersion;
   fClockSeqHiAndReserved = (UChar_t) clockSeqHiAndRes;
   fClockSeqLow           = (UChar_t) clockSeqLo;
   fNode[0]               = (UChar_t) node[0];
   fNode[1]               = (UChar_t) node[1];
   fNode[2]               = (UChar_t) node[2];
   fNode[3]               = (UChar_t) node[3];
   fNode[4]               = (UChar_t) node[4];
   fNode[5]               = (UChar_t) node[5];
   fUUIDIndex             = 1<<30;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Initialize a TUUID with uuid (which must be in TUUID::AsString() format).
 
TUUID::TUUID(const char *uuid)
{
   fTimeLow               = 0;
   fTimeMid               = 0;
   fTimeHiAndVersion      = 0;
   fClockSeqHiAndReserved = 0;
   fClockSeqLow           = 0;
   fNode[0]               = 0;
   fUUIDIndex             = 0;
 
   if (!uuid || !*uuid)
      Error("TUUID", "null string not allowed");
   else
      SetFromString(uuid);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Stream UUID into output buffer.
 
void TUUID::FillBuffer(char *&buffer)
{
   Version_t version = TUUID::Class_Version();
   tobuf(buffer, version);
   tobuf(buffer, fTimeLow);
   tobuf(buffer, fTimeMid);
   tobuf(buffer, fTimeHiAndVersion);
   tobuf(buffer, fClockSeqHiAndReserved);
   tobuf(buffer, fClockSeqLow);
   for (Int_t i = 0; i < 6; i++)
      tobuf(buffer, fNode[i]);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Stream UUID from input buffer.
 
void TUUID::ReadBuffer(char *&buffer)
{
   Version_t version;
   frombuf(buffer, &version);
   frombuf(buffer, &fTimeLow);
   frombuf(buffer, &fTimeMid);
   frombuf(buffer, &fTimeHiAndVersion);
   frombuf(buffer, &fClockSeqHiAndReserved);
   frombuf(buffer, &fClockSeqLow);
   for (Int_t i = 0; i < 6; i++)
      frombuf(buffer, &fNode[i]);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Stream UUID from input buffer.
/// This function is for the exclusive use of TDirectory::Streamer() to
/// read a non-versioned version of TUUID.
 
void TUUID::StreamerV1(TBuffer &b)
{
   b >> fTimeLow;
   b >> fTimeMid;
   b >> fTimeHiAndVersion;
   b >> fClockSeqHiAndReserved;
   b >> fClockSeqLow;
   for (UInt_t i = 0; i < 6; i++) {
      b >> fNode[i];
   }
}
 
////////////////////////////////////////////////////////////////////////////////
/// Make a UUID from timestamp, clockseq and node id.
 
void TUUID::Format(UShort_t clockseq, uuid_time_t ts)
{
   fTimeLow = ts.low;
   fTimeMid = (UShort_t)(ts.high & 0xFFFF);
   fTimeHiAndVersion = (UShort_t)((ts.high >> 16) & 0x0FFF);
   fTimeHiAndVersion |= (1 << 12);
   fClockSeqLow = clockseq & 0xFF;
   fClockSeqHiAndReserved = (clockseq & 0x3F00) >> 8;
   fClockSeqHiAndReserved |= 0x80;
   GetNodeIdentifier();
}
 
////////////////////////////////////////////////////////////////////////////////
/// Get current time as 60 bit 100ns ticks since whenever.
/// Compensate for the fact that real clock resolution is less
/// than 100ns.
 
void TUUID::GetCurrentTime(uuid_time_t *timestamp)
{
   const UShort_t uuids_per_tick = 1024;
 
   TTHREAD_TLS(uuid_time_t) time_last;
   TTHREAD_TLS(UShort_t)    uuids_this_tick(0);
   TTHREAD_TLS(Bool_t)      init(kFALSE);
 
   uuid_time_t *time_last_ptr = TTHREAD_TLS_PTR(time_last);
 
   if (!init) {
      GetSystemTime(time_last_ptr);
      uuids_this_tick = uuids_per_tick;
      init = kTRUE;
   }
 
   uuid_time_t time_now;
 
   while (1) {
      GetSystemTime(&time_now);
 
      // if clock reading changed since last UUID generated
      if (CmpTime(time_last_ptr, &time_now))  {
         // reset count of uuid's generated with this clock reading
         uuids_this_tick = 0;
         break;
      }
      if (uuids_this_tick < uuids_per_tick) {
         uuids_this_tick++;
         break;
      }
      // going too fast for our clock; spin
   }
 
   time_last = time_now;
 
   if (uuids_this_tick != 0) {
      if (time_now.low & 0x80000000) {
         time_now.low += uuids_this_tick;
         if (!(time_now.low & 0x80000000))
            time_now.high++;
      } else
         time_now.low += uuids_this_tick;
   }
 
   timestamp->high = time_now.high;
   timestamp->low  = time_now.low;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Get system time with 100ns precision. Time is since Oct 15, 1582.
 
void TUUID::GetSystemTime(uuid_time_t *timestamp)
{
#ifdef R__WIN32
   ULARGE_INTEGER time;
   GetSystemTimeAsFileTime((FILETIME *)&time);
   // NT keeps time in FILETIME format which is 100ns ticks since
   // Jan 1, 1601. UUIDs use time in 100ns ticks since Oct 15, 1582.
   // The difference is 17 Days in Oct + 30 (Nov) + 31 (Dec)
   // + 18 years and 5 leap days.
   time.QuadPart +=
            (unsigned __int64) (1000*1000*10)       // seconds
          * (unsigned __int64) (60 * 60 * 24)       // days
          * (unsigned __int64) (17+30+31+365*18+5); // # of days
 
   timestamp->high = time.HighPart;
   timestamp->low  = time.LowPart;
#else
   struct timeval tp;
   gettimeofday(&tp, nullptr);
   // Offset between UUID formatted times and Unix formatted times.
   // UUID UTC base time is October 15, 1582.
   // Unix base time is January 1, 1970.
   ULong64_t uuid_time = ((ULong64_t)tp.tv_sec * 10000000) + (tp.tv_usec * 10) +
                         0x01B21DD213814000LL;
   timestamp->high = (UInt_t) (uuid_time >> 32);
   timestamp->low  = (UInt_t) (uuid_time & 0xFFFFFFFF);
#endif
}
 
////////////////////////////////////////////////////////////////////////////////
/// Get node identifier. Try first to get network address, if no
/// network interface try random info based on some machine parameters.
 
void TUUID::GetNodeIdentifier()
{
   static UInt_t adr = 0;
 
   if (gSystem) {
#ifndef R__WIN32
      if (!adr) {
         UInt_t addr = 0;
 
         struct ifaddrs *ifAddrStruct = nullptr;
         struct ifaddrs *ifa = nullptr;
 
         if (getifaddrs(&ifAddrStruct) != 0) {
            adr = 1;
         } else {
            for (ifa = ifAddrStruct; ifa != nullptr; ifa = ifa->ifa_next) {
               if (!ifa->ifa_addr) {
                  continue;
               }
               if (ifa->ifa_addr->sa_family != AF_INET) { // check only IP4
                  continue;
               }
               if (strncmp(ifa->ifa_name,"lo",2) == 0) { // skip loop back.
                  continue;
               }
               addr = ntohl(((struct sockaddr_in *)ifa->ifa_addr)->sin_addr.s_addr);
               break;
            }
         }
 
         if (ifAddrStruct != nullptr)
            freeifaddrs(ifAddrStruct);
 
         if (addr)
            adr = addr;
         else
            adr = 1;  // illegal address
      }
#else
      // this way to get the machine's IP address is needed because
      // GetHostByName() on Win32 contacts the DNS which we don't want
      // as firewall tools like ZoneAlarm are likely to catch it and
      // alarm the user
      if (!adr) {
         PIP_ADAPTER_INFO ainfo = (PIP_ADAPTER_INFO) malloc(sizeof(IP_ADAPTER_INFO));
         ULONG buflen = sizeof(IP_ADAPTER_INFO);
         DWORD stat = GetAdaptersInfo(ainfo, &buflen);
         if (stat == ERROR_BUFFER_OVERFLOW) {
            free(ainfo);
            ainfo = (PIP_ADAPTER_INFO) malloc(buflen);
            stat = GetAdaptersInfo(ainfo, &buflen);
         }
         if (stat != ERROR_SUCCESS)
            adr = 1;  // illegal address
         else {
            // take address of first adapter
            PIP_ADAPTER_INFO adapter = ainfo;
            int a, b, c, d;
            sscanf(adapter->IpAddressList.IpAddress.String, "%d.%d.%d.%d",
                   &a, &b, &c, &d);
            adr = (a << 24) | (b << 16) | (c << 8) | d;
         }
         free(ainfo);
      }
#endif
      if (adr > 2) {
         memcpy(fNode, &adr, 4);
         fNode[4] = 0xbe;
         fNode[5] = 0xef;
         return;
      }
   }
   static UChar_t seed[16];
   if (adr < 2) {
      GetRandomInfo(seed);
      seed[0] |= 0x80;
      if (gSystem) adr = 2;  // illegal address
   }
   memcpy(fNode, seed, sizeof(fNode));
   fTimeHiAndVersion |= (3 << 12);    // version == 3: random node info
}
 
////////////////////////////////////////////////////////////////////////////////
/// Get random info based on some machine parameters.
 
void TUUID::GetRandomInfo(UChar_t seed[16])
{
#ifdef R__WIN32
   struct randomness {
      MEMORYSTATUS  m;
      SYSTEM_INFO   s;
      FILETIME      t;
      LARGE_INTEGER pc;
      DWORD         tc;
      DWORD         l;
      char          hostname[MAX_COMPUTERNAME_LENGTH + 1];
   };
   randomness r;
   memset(&r, 0, sizeof(r));  // zero also padding bytes
 
   // memory usage stats
   GlobalMemoryStatus(&r.m);
   // random system stats
   GetSystemInfo(&r.s);
   // 100ns resolution time of day
   GetSystemTimeAsFileTime(&r.t);
   // high resolution performance counter
   QueryPerformanceCounter(&r.pc);
   // milliseconds since last boot
   r.tc = GetTickCount();
   r.l = MAX_COMPUTERNAME_LENGTH + 1;
   GetComputerName(r.hostname, &r.l);
#else
   struct randomness {
#if defined(R__LINUX) && !defined(R__WINGCC)
      struct sysinfo   s;
#endif
      struct timeval   t;
      char             hostname[257];
   };
   randomness r;
   memset(&r, 0, sizeof(r));  // zero also padding bytes
 
#if defined(R__LINUX) && !defined(R__WINGCC)
   sysinfo(&r.s);
#endif
   gettimeofday(&r.t, nullptr);
   gethostname(r.hostname, 256);
#endif
   TMD5 md5;
   md5.Update((UChar_t *)&r, sizeof(randomness));
   md5.Final(seed);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Print UUID.
 
void TUUID::Print() const
{
   printf("%s\n", AsString());
}
 
////////////////////////////////////////////////////////////////////////////////
/// Return UUID as string. Copy string immediately since it will be reused.
 
const char *TUUID::AsString() const
{
   TTHREAD_TLS(char) uuid[40];
 
   snprintf(uuid,40, "%08x-%04x-%04x-%02x%02x-%02x%02x%02x%02x%02x%02x",
           fTimeLow, fTimeMid, fTimeHiAndVersion, fClockSeqHiAndReserved,
           fClockSeqLow, fNode[0], fNode[1], fNode[2], fNode[3], fNode[4],
           fNode[5]);
 
   return uuid;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Compute 16-bit hash value of the UUID.
 
UShort_t TUUID::Hash() const
{
   Short_t  c0 = 0, c1 = 0, x, y;
   char    *c = (char *) &fTimeLow;
 
   // For speed lets unroll the following loop:
   //   for (i = 0; i < 16; i++) {
   //      c0 += *c++;
   //      c1 += c0;
   //   }
 
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
 
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
 
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
 
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
   c0 += *c++; c1 += c0;
 
   //  Calculate the value for "First octet" of the hash
   x = -c1 % 255;
   if (x < 0)
      x += 255;
 
   // Calculate the value for "second octet" of the hash
   y = (c1 - c0) % 255;
   if (y < 0)
      y += 255;
 
   return UShort_t((y << 8) + x);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Compare two UUIDs "lexically" and return
///   - -1   this is lexically before u
///   -  0   this is equal to u
///   -  1   this is lexically after u
 
Int_t TUUID::Compare(const TUUID &u) const
{
#define CHECK(f1, f2) if (f1 != f2) return f1 < f2 ? -1 : 1;
   CHECK(fTimeLow, u.fTimeLow)
   CHECK(fTimeMid, u.fTimeMid)
   CHECK(fTimeHiAndVersion, u.fTimeHiAndVersion)
   CHECK(fClockSeqHiAndReserved, u.fClockSeqHiAndReserved)
   CHECK(fClockSeqLow, u.fClockSeqLow)
   for (int i = 0; i < 6; i++) {
      if (fNode[i] < u.fNode[i])
         return -1;
      if (fNode[i] > u.fNode[i])
         return 1;
   }
   return 0;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Get address of host encoded in UUID. If host id is not an ethernet
/// address, but random info, then the returned TInetAddress is not valid.
 
TInetAddress TUUID::GetHostAddress() const
{
   if ((fTimeHiAndVersion >> 12) == 1) {
      UInt_t addr;
      memcpy(&addr, fNode, 4);
      return TInetAddress("????", addr, 0);
   }
   return TInetAddress();
}
 
////////////////////////////////////////////////////////////////////////////////
/// Get time from UUID.
 
TDatime TUUID::GetTime() const
{
   TDatime     dt;
   uuid_time_t ts;
 
   ts.low   = fTimeLow;
   ts.high  = (UInt_t)fTimeMid;
   ts.high |= (UInt_t)((fTimeHiAndVersion & 0x0FFF) << 16);
 
   // Offset between UUID formatted times and Unix formatted times.
   // UUID UTC base time is October 15, 1582.
   // Unix base time is January 1, 1970.
   ULong64_t high = ts.high;
   ULong64_t uuid_time = (high << 32) + ts.low;
   uuid_time -= 0x01B21DD213814000LL;
   uuid_time /= 10000000LL;
   UInt_t tt = (UInt_t) uuid_time;
   dt.Set(tt);
 
   return dt;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Return uuid in specified buffer (16 byte = 128 bits).
 
void TUUID::GetUUID(UChar_t uuid[16]) const
{
   memcpy(uuid, &fTimeLow, 16);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Set this UUID to the value specified in uuid ((which must be in
/// TUUID::AsString() format).
 
void TUUID::SetUUID(const char *uuid)
{
   if (!uuid || !*uuid)
      Error("SetUUID", "null string not allowed");
   else
      SetFromString(uuid);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Input operator.  Delegate to Streamer.
 
TBuffer &operator<<(TBuffer &buf, const TUUID &uuid)
{
   R__ASSERT( buf.IsWriting() );
 
   const_cast<TUUID&>(uuid).Streamer(buf);
   return buf;
}