lib/Object/ArchiveWriter.cpp

   1 //===- ArchiveWriter.cpp - ar File Format implementation --------*- C++ -*-===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file defines the writeArchive function.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Object/ArchiveWriter.h"
  15 #include "llvm/ADT/ArrayRef.h"
  16 #include "llvm/ADT/StringRef.h"
  17 #include "llvm/BinaryFormat/Magic.h"
  18 #include "llvm/IR/LLVMContext.h"
  19 #include "llvm/Object/Archive.h"
  20 #include "llvm/Object/ObjectFile.h"
  21 #include "llvm/Object/SymbolicFile.h"
  22 #include "llvm/Support/EndianStream.h"
  23 #include "llvm/Support/Errc.h"
  24 #include "llvm/Support/ErrorHandling.h"
  25 #include "llvm/Support/Format.h"
  26 #include "llvm/Support/Path.h"
  27 #include "llvm/Support/ToolOutputFile.h"
  28 #include "llvm/Support/raw_ostream.h"
  29
  30 #include <map>
  31
  32 #if !defined(_MSC_VER) && !defined(__MINGW32__)
  33 #include <unistd.h>
  34 #else
  35 #include <io.h>
  36 #endif
  37
  38 using namespace llvm;
  39
  40 NewArchiveMember::NewArchiveMember(MemoryBufferRef BufRef)
  41     : Buf(MemoryBuffer::getMemBuffer(BufRef, false)),
  42       MemberName(BufRef.getBufferIdentifier()) {}
  43
  44 Expected<NewArchiveMember>
  45 NewArchiveMember::getOldMember(const object::Archive::Child &OldMember,
  46                                bool Deterministic) {
  47   Expected<llvm::MemoryBufferRef> BufOrErr = OldMember.getMemoryBufferRef();
  48   if (!BufOrErr)
  49     return BufOrErr.takeError();
  50
  51   NewArchiveMember M;
  52   assert(M.IsNew == false);
  53   M.Buf = MemoryBuffer::getMemBuffer(*BufOrErr, false);
  54   M.MemberName = M.Buf->getBufferIdentifier();
  55   if (!Deterministic) {
  56     auto ModTimeOrErr = OldMember.getLastModified();
  57     if (!ModTimeOrErr)
  58       return ModTimeOrErr.takeError();
  59     M.ModTime = ModTimeOrErr.get();
  60     Expected<unsigned> UIDOrErr = OldMember.getUID();
  61     if (!UIDOrErr)
  62       return UIDOrErr.takeError();
  63     M.UID = UIDOrErr.get();
  64     Expected<unsigned> GIDOrErr = OldMember.getGID();
  65     if (!GIDOrErr)
  66       return GIDOrErr.takeError();
  67     M.GID = GIDOrErr.get();
  68     Expected<sys::fs::perms> AccessModeOrErr = OldMember.getAccessMode();
  69     if (!AccessModeOrErr)
  70       return AccessModeOrErr.takeError();
  71     M.Perms = AccessModeOrErr.get();
  72   }
  73   return std::move(M);
  74 }
  75
  76 Expected<NewArchiveMember> NewArchiveMember::getFile(StringRef FileName,
  77                                                      bool Deterministic) {
  78   sys::fs::file_status Status;
  79   int FD;
  80   if (auto EC = sys::fs::openFileForRead(FileName, FD))
  81     return errorCodeToError(EC);
  82   assert(FD != -1);
  83
  84   if (auto EC = sys::fs::status(FD, Status))
  85     return errorCodeToError(EC);
  86
  87   // Opening a directory doesn't make sense. Let it fail.
  88   // Linux cannot open directories with open(2), although
  89   // cygwin and *bsd can.
  90   if (Status.type() == sys::fs::file_type::directory_file)
  91     return errorCodeToError(make_error_code(errc::is_a_directory));
  92
  93   ErrorOr<std::unique_ptr<MemoryBuffer>> MemberBufferOrErr =
  94       MemoryBuffer::getOpenFile(FD, FileName, Status.getSize(), false);
  95   if (!MemberBufferOrErr)
  96     return errorCodeToError(MemberBufferOrErr.getError());
  97
  98   if (close(FD) != 0)
  99     return errorCodeToError(std::error_code(errno, std::generic_category()));
 100
 101   NewArchiveMember M;
 102   M.IsNew = true;
 103   M.Buf = std::move(*MemberBufferOrErr);
 104   M.MemberName = M.Buf->getBufferIdentifier();
 105   if (!Deterministic) {
 106     M.ModTime = std::chrono::time_point_cast<std::chrono::seconds>(
 107         Status.getLastModificationTime());
 108     M.UID = Status.getUser();
 109     M.GID = Status.getGroup();
 110     M.Perms = Status.permissions();
 111   }
 112   return std::move(M);
 113 }
 114
 115 template <typename T>
 116 static void printWithSpacePadding(raw_ostream &OS, T Data, unsigned Size) {
 117   uint64_t OldPos = OS.tell();
 118   OS << Data;
 119   unsigned SizeSoFar = OS.tell() - OldPos;
 120   assert(SizeSoFar <= Size && "Data doesn't fit in Size");
 121   OS.indent(Size - SizeSoFar);
 122 }
 123
 124 static bool isDarwin(object::Archive::Kind Kind) {
 125   return Kind == object::Archive::K_DARWIN ||
 126          Kind == object::Archive::K_DARWIN64;
 127 }
 128
 129 static bool isBSDLike(object::Archive::Kind Kind) {
 130   switch (Kind) {
 131   case object::Archive::K_GNU:
 132   case object::Archive::K_GNU64:
 133     return false;
 134   case object::Archive::K_BSD:
 135   case object::Archive::K_DARWIN:
 136   case object::Archive::K_DARWIN64:
 137     return true;
 138   case object::Archive::K_COFF:
 139     break;
 140   }
 141   llvm_unreachable("not supported for writting");
 142 }
 143
 144 template <class T>
 145 static void print(raw_ostream &Out, object::Archive::Kind Kind, T Val) {
 146   support::endian::write(Out, Val,
 147                          isBSDLike(Kind) ? support::little : support::big);
 148 }
 149
 150 static void printRestOfMemberHeader(
 151     raw_ostream &Out, const sys::TimePoint<std::chrono::seconds> &ModTime,
 152     unsigned UID, unsigned GID, unsigned Perms, unsigned Size) {
 153   printWithSpacePadding(Out, sys::toTimeT(ModTime), 12);
 154
 155   // The format has only 6 chars for uid and gid. Truncate if the provided
 156   // values don't fit.
 157   printWithSpacePadding(Out, UID % 1000000, 6);
 158   printWithSpacePadding(Out, GID % 1000000, 6);
 159
 160   printWithSpacePadding(Out, format("%o", Perms), 8);
 161   printWithSpacePadding(Out, Size, 10);
 162   Out << "`\n";
 163 }
 164
 165 static void
 166 printGNUSmallMemberHeader(raw_ostream &Out, StringRef Name,
 167                           const sys::TimePoint<std::chrono::seconds> &ModTime,
 168                           unsigned UID, unsigned GID, unsigned Perms,
 169                           unsigned Size) {
 170   printWithSpacePadding(Out, Twine(Name) + "/", 16);
 171   printRestOfMemberHeader(Out, ModTime, UID, GID, Perms, Size);
 172 }
 173
 174 static void
 175 printBSDMemberHeader(raw_ostream &Out, uint64_t Pos, StringRef Name,
 176                      const sys::TimePoint<std::chrono::seconds> &ModTime,
 177                      unsigned UID, unsigned GID, unsigned Perms,
 178                      unsigned Size) {
 179   uint64_t PosAfterHeader = Pos + 60 + Name.size();
 180   // Pad so that even 64 bit object files are aligned.
 181   unsigned Pad = OffsetToAlignment(PosAfterHeader, 8);
 182   unsigned NameWithPadding = Name.size() + Pad;
 183   printWithSpacePadding(Out, Twine("#1/") + Twine(NameWithPadding), 16);
 184   printRestOfMemberHeader(Out, ModTime, UID, GID, Perms,
 185                           NameWithPadding + Size);
 186   Out << Name;
 187   while (Pad--)
 188     Out.write(uint8_t(0));
 189 }
 190
 191 static bool useStringTable(bool Thin, StringRef Name) {
 192   return Thin || Name.size() >= 16 || Name.contains('/');
 193 }
 194
 195 // Compute the relative path from From to To.
 196 static std::string computeRelativePath(StringRef From, StringRef To) {
 197   if (sys::path::is_absolute(From) || sys::path::is_absolute(To))
 198     return To;
 199
 200   StringRef DirFrom = sys::path::parent_path(From);
 201   auto FromI = sys::path::begin(DirFrom);
 202   auto ToI = sys::path::begin(To);
 203   while (*FromI == *ToI) {
 204     ++FromI;
 205     ++ToI;
 206   }
 207
 208   SmallString<128> Relative;
 209   for (auto FromE = sys::path::end(DirFrom); FromI != FromE; ++FromI)
 210     sys::path::append(Relative, "..");
 211
 212   for (auto ToE = sys::path::end(To); ToI != ToE; ++ToI)
 213     sys::path::append(Relative, *ToI);
 214
 215 #ifdef _WIN32
 216   // Replace backslashes with slashes so that the path is portable between *nix
 217   // and Windows.
 218   std::replace(Relative.begin(), Relative.end(), '\\', '/');
 219 #endif
 220
 221   return Relative.str();
 222 }
 223
 224 static bool is64BitKind(object::Archive::Kind Kind) {
 225   switch (Kind) {
 226   case object::Archive::K_GNU:
 227   case object::Archive::K_BSD:
 228   case object::Archive::K_DARWIN:
 229   case object::Archive::K_COFF:
 230     return false;
 231   case object::Archive::K_DARWIN64:
 232   case object::Archive::K_GNU64:
 233     return true;
 234   }
 235   llvm_unreachable("not supported for writting");
 236 }
 237
 238 static void addToStringTable(raw_ostream &Out, StringRef ArcName,
 239                              const NewArchiveMember &M, bool Thin) {
 240   StringRef ID = M.Buf->getBufferIdentifier();
 241   if (Thin) {
 242     if (M.IsNew)
 243       Out << computeRelativePath(ArcName, ID);
 244     else
 245       Out << ID;
 246   } else
 247     Out << M.MemberName;
 248   Out << "/\n";
 249 }
 250
 251 static void printMemberHeader(raw_ostream &Out, uint64_t Pos,
 252                               raw_ostream &StringTable,
 253                               object::Archive::Kind Kind, bool Thin,
 254                               StringRef ArcName, const NewArchiveMember &M,
 255                               sys::TimePoint<std::chrono::seconds> ModTime,
 256                               unsigned Size) {
 257
 258   if (isBSDLike(Kind))
 259     return printBSDMemberHeader(Out, Pos, M.MemberName, ModTime, M.UID, M.GID,
 260                                 M.Perms, Size);
 261   if (!useStringTable(Thin, M.MemberName))
 262     return printGNUSmallMemberHeader(Out, M.MemberName, ModTime, M.UID, M.GID,
 263                                      M.Perms, Size);
 264   Out << '/';
 265   uint64_t NamePos = StringTable.tell();
 266   addToStringTable(StringTable, ArcName, M, Thin);
 267   printWithSpacePadding(Out, NamePos, 15);
 268   printRestOfMemberHeader(Out, ModTime, M.UID, M.GID, M.Perms, Size);
 269 }
 270
 271 namespace {
 272 struct MemberData {
 273   std::vector<unsigned> Symbols;
 274   std::string Header;
 275   StringRef Data;
 276   StringRef Padding;
 277 };
 278 } // namespace
 279
 280 static MemberData computeStringTable(StringRef Names) {
 281   unsigned Size = Names.size();
 282   unsigned Pad = OffsetToAlignment(Size, 2);
 283   std::string Header;
 284   raw_string_ostream Out(Header);
 285   printWithSpacePadding(Out, "//", 48);
 286   printWithSpacePadding(Out, Size + Pad, 10);
 287   Out << "`\n";
 288   Out.flush();
 289   return {{}, std::move(Header), Names, Pad ? "\n" : ""};
 290 }
 291
 292 static sys::TimePoint<std::chrono::seconds> now(bool Deterministic) {
 293   using namespace std::chrono;
 294
 295   if (!Deterministic)
 296     return time_point_cast<seconds>(system_clock::now());
 297   return sys::TimePoint<seconds>();
 298 }
 299
 300 static bool isArchiveSymbol(const object::BasicSymbolRef &S) {
 301   uint32_t Symflags = S.getFlags();
 302   if (Symflags & object::SymbolRef::SF_FormatSpecific)
 303     return false;
 304   if (!(Symflags & object::SymbolRef::SF_Global))
 305     return false;
 306   if (Symflags & object::SymbolRef::SF_Undefined)
 307     return false;
 308   return true;
 309 }
 310
 311 static void printNBits(raw_ostream &Out, object::Archive::Kind Kind,
 312                        uint64_t Val) {
 313   if (is64BitKind(Kind))
 314     print<uint64_t>(Out, Kind, Val);
 315   else
 316     print<uint32_t>(Out, Kind, Val);
 317 }
 318
 319 static void writeSymbolTable(raw_ostream &Out, object::Archive::Kind Kind,
 320                              bool Deterministic, ArrayRef<MemberData> Members,
 321                              StringRef StringTable) {
 322   // We don't write a symbol table on an archive with no members -- except on
 323   // Darwin, where the linker will abort unless the archive has a symbol table.
 324   if (StringTable.empty() && !isDarwin(Kind))
 325     return;
 326
 327   unsigned NumSyms = 0;
 328   for (const MemberData &M : Members)
 329     NumSyms += M.Symbols.size();
 330
 331   unsigned Size = 0;
 332   unsigned OffsetSize = is64BitKind(Kind) ? sizeof(uint64_t) : sizeof(uint32_t);
 333
 334   Size += OffsetSize; // Number of entries
 335   if (isBSDLike(Kind))
 336     Size += NumSyms * OffsetSize * 2; // Table
 337   else
 338     Size += NumSyms * OffsetSize; // Table
 339   if (isBSDLike(Kind))
 340     Size += OffsetSize; // byte count
 341   Size += StringTable.size();
 342   // ld64 expects the members to be 8-byte aligned for 64-bit content and at
 343   // least 4-byte aligned for 32-bit content.  Opt for the larger encoding
 344   // uniformly.
 345   // We do this for all bsd formats because it simplifies aligning members.
 346   unsigned Alignment = isBSDLike(Kind) ? 8 : 2;
 347   unsigned Pad = OffsetToAlignment(Size, Alignment);
 348   Size += Pad;
 349
 350   if (isBSDLike(Kind)) {
 351     const char *Name = is64BitKind(Kind) ? "__.SYMDEF_64" : "__.SYMDEF";
 352     printBSDMemberHeader(Out, Out.tell(), Name, now(Deterministic), 0, 0, 0,
 353                          Size);
 354   } else {
 355     const char *Name = is64BitKind(Kind) ? "/SYM64" : "";
 356     printGNUSmallMemberHeader(Out, Name, now(Deterministic), 0, 0, 0, Size);
 357   }
 358
 359   uint64_t Pos = Out.tell() + Size;
 360
 361   if (isBSDLike(Kind))
 362     printNBits(Out, Kind, NumSyms * 2 * OffsetSize);
 363   else
 364     printNBits(Out, Kind, NumSyms);
 365
 366   for (const MemberData &M : Members) {
 367     for (unsigned StringOffset : M.Symbols) {
 368       if (isBSDLike(Kind))
 369         printNBits(Out, Kind, StringOffset);
 370       printNBits(Out, Kind, Pos); // member offset
 371     }
 372     Pos += M.Header.size() + M.Data.size() + M.Padding.size();
 373   }
 374
 375   if (isBSDLike(Kind))
 376     // byte count of the string table
 377     printNBits(Out, Kind, StringTable.size());
 378   Out << StringTable;
 379
 380   while (Pad--)
 381     Out.write(uint8_t(0));
 382 }
 383
 384 static Expected<std::vector<unsigned>>
 385 getSymbols(MemoryBufferRef Buf, raw_ostream &SymNames, bool &HasObject) {
 386   std::vector<unsigned> Ret;
 387
 388   // In the scenario when LLVMContext is populated SymbolicFile will contain a
 389   // reference to it, thus SymbolicFile should be destroyed first.
 390   LLVMContext Context;
 391   std::unique_ptr<object::SymbolicFile> Obj;
 392   if (identify_magic(Buf.getBuffer()) == file_magic::bitcode) {
 393     auto ObjOrErr = object::SymbolicFile::createSymbolicFile(
 394         Buf, file_magic::bitcode, &Context);
 395     if (!ObjOrErr) {
 396       // FIXME: check only for "not an object file" errors.
 397       consumeError(ObjOrErr.takeError());
 398       return Ret;
 399     }
 400     Obj = std::move(*ObjOrErr);
 401   } else {
 402     auto ObjOrErr = object::SymbolicFile::createSymbolicFile(Buf);
 403     if (!ObjOrErr) {
 404       // FIXME: check only for "not an object file" errors.
 405       consumeError(ObjOrErr.takeError());
 406       return Ret;
 407     }
 408     Obj = std::move(*ObjOrErr);
 409   }
 410
 411   HasObject = true;
 412   for (const object::BasicSymbolRef &S : Obj->symbols()) {
 413     if (!isArchiveSymbol(S))
 414       continue;
 415     Ret.push_back(SymNames.tell());
 416     if (auto EC = S.printName(SymNames))
 417       return errorCodeToError(EC);
 418     SymNames << '\0';
 419   }
 420   return Ret;
 421 }
 422
 423 static Expected<std::vector<MemberData>>
 424 computeMemberData(raw_ostream &StringTable, raw_ostream &SymNames,
 425                   object::Archive::Kind Kind, bool Thin, StringRef ArcName,
 426                   bool Deterministic, ArrayRef<NewArchiveMember> NewMembers) {
 427   static char PaddingData[8] = {'\n', '\n', '\n', '\n', '\n', '\n', '\n', '\n'};
 428
 429   // This ignores the symbol table, but we only need the value mod 8 and the
 430   // symbol table is aligned to be a multiple of 8 bytes
 431   uint64_t Pos = 0;
 432
 433   std::vector<MemberData> Ret;
 434   bool HasObject = false;
 435
 436   // UniqueTimestamps is a special case to improve debugging on Darwin:
 437   //
 438   // The Darwin linker does not link debug info into the final
 439   // binary. Instead, it emits entries of type N_OSO in in the output
 440   // binary's symbol table, containing references to the linked-in
 441   // object files. Using that reference, the debugger can read the
 442   // debug data directly from the object files. Alternatively, an
 443   // invocation of 'dsymutil' will link the debug data from the object
 444   // files into a dSYM bundle, which can be loaded by the debugger,
 445   // instead of the object files.
 446   //
 447   // For an object file, the N_OSO entries contain the absolute path
 448   // path to the file, and the file's timestamp. For an object
 449   // included in an archive, the path is formatted like
 450   // "/absolute/path/to/archive.a(member.o)", and the timestamp is the
 451   // archive member's timestamp, rather than the archive's timestamp.
 452   //
 453   // However, this doesn't always uniquely identify an object within
 454   // an archive -- an archive file can have multiple entries with the
 455   // same filename. (This will happen commonly if the original object
 456   // files started in different directories.) The only way they get
 457   // distinguished, then, is via the timestamp. But this process is
 458   // unable to find the correct object file in the archive when there
 459   // are two files of the same name and timestamp.
 460   //
 461   // Additionally, timestamp==0 is treated specially, and causes the
 462   // timestamp to be ignored as a match criteria.
 463   //
 464   // That will "usually" work out okay when creating an archive not in
 465   // deterministic timestamp mode, because the objects will probably
 466   // have been created at different timestamps.
 467   //
 468   // To ameliorate this problem, in deterministic archive mode (which
 469   // is the default), on Darwin we will emit a unique non-zero
 470   // timestamp for each entry with a duplicated name. This is still
 471   // deterministic: the only thing affecting that timestamp is the
 472   // order of the files in the resultant archive.
 473   //
 474   // See also the functions that handle the lookup:
 475   // in lldb: ObjectContainerBSDArchive::Archive::FindObject()
 476   // in llvm/tools/dsymutil: BinaryHolder::GetArchiveMemberBuffers().
 477   bool UniqueTimestamps = Deterministic && isDarwin(Kind);
 478   std::map<StringRef, unsigned> FilenameCount;
 479   if (UniqueTimestamps) {
 480     for (const NewArchiveMember &M : NewMembers)
 481       FilenameCount[M.MemberName]++;
 482     for (auto &Entry : FilenameCount)
 483       Entry.second = Entry.second > 1 ? 1 : 0;
 484   }
 485
 486   for (const NewArchiveMember &M : NewMembers) {
 487     std::string Header;
 488     raw_string_ostream Out(Header);
 489
 490     MemoryBufferRef Buf = M.Buf->getMemBufferRef();
 491     StringRef Data = Thin ? "" : Buf.getBuffer();
 492
 493     // ld64 expects the members to be 8-byte aligned for 64-bit content and at
 494     // least 4-byte aligned for 32-bit content.  Opt for the larger encoding
 495     // uniformly.  This matches the behaviour with cctools and ensures that ld64
 496     // is happy with archives that we generate.
 497     unsigned MemberPadding =
 498         isDarwin(Kind) ? OffsetToAlignment(Data.size(), 8) : 0;
 499     unsigned TailPadding = OffsetToAlignment(Data.size() + MemberPadding, 2);
 500     StringRef Padding = StringRef(PaddingData, MemberPadding + TailPadding);
 501
 502     sys::TimePoint<std::chrono::seconds> ModTime;
 503     if (UniqueTimestamps)
 504       // Increment timestamp for each file of a given name.
 505       ModTime = sys::toTimePoint(FilenameCount[M.MemberName]++);
 506     else
 507       ModTime = M.ModTime;
 508     printMemberHeader(Out, Pos, StringTable, Kind, Thin, ArcName, M, ModTime,
 509                       Buf.getBufferSize() + MemberPadding);
 510     Out.flush();
 511
 512     Expected<std::vector<unsigned>> Symbols =
 513         getSymbols(Buf, SymNames, HasObject);
 514     if (auto E = Symbols.takeError())
 515       return std::move(E);
 516
 517     Pos += Header.size() + Data.size() + Padding.size();
 518     Ret.push_back({std::move(*Symbols), std::move(Header), Data, Padding});
 519   }
 520   // If there are no symbols, emit an empty symbol table, to satisfy Solaris
 521   // tools, older versions of which expect a symbol table in a non-empty
 522   // archive, regardless of whether there are any symbols in it.
 523   if (HasObject && SymNames.tell() == 0)
 524     SymNames << '\0' << '\0' << '\0';
 525   return Ret;
 526 }
 527
 528 Error llvm::writeArchive(StringRef ArcName,
 529                          ArrayRef<NewArchiveMember> NewMembers,
 530                          bool WriteSymtab, object::Archive::Kind Kind,
 531                          bool Deterministic, bool Thin,
 532                          std::unique_ptr<MemoryBuffer> OldArchiveBuf) {
 533   assert((!Thin || !isBSDLike(Kind)) && "Only the gnu format has a thin mode");
 534
 535   SmallString<0> SymNamesBuf;
 536   raw_svector_ostream SymNames(SymNamesBuf);
 537   SmallString<0> StringTableBuf;
 538   raw_svector_ostream StringTable(StringTableBuf);
 539
 540   Expected<std::vector<MemberData>> DataOrErr = computeMemberData(
 541       StringTable, SymNames, Kind, Thin, ArcName, Deterministic, NewMembers);
 542   if (Error E = DataOrErr.takeError())
 543     return E;
 544   std::vector<MemberData> &Data = *DataOrErr;
 545
 546   if (!StringTableBuf.empty())
 547     Data.insert(Data.begin(), computeStringTable(StringTableBuf));
 548
 549   // We would like to detect if we need to switch to a 64-bit symbol table.
 550   if (WriteSymtab) {
 551     uint64_t MaxOffset = 0;
 552     uint64_t LastOffset = MaxOffset;
 553     for (const auto &M : Data) {
 554       // Record the start of the member's offset
 555       LastOffset = MaxOffset;
 556       // Account for the size of each part associated with the member.
 557       MaxOffset += M.Header.size() + M.Data.size() + M.Padding.size();
 558       // We assume 32-bit symbols to see if 32-bit symbols are possible or not.
 559       MaxOffset += M.Symbols.size() * 4;
 560     }
 561
 562     // The SYM64 format is used when an archive's member offsets are larger than
 563     // 32-bits can hold. The need for this shift in format is detected by
 564     // writeArchive. To test this we need to generate a file with a member that
 565     // has an offset larger than 32-bits but this demands a very slow test. To
 566     // speed the test up we use this environment variable to pretend like the
 567     // cutoff happens before 32-bits and instead happens at some much smaller
 568     // value.
 569     const char *Sym64Env = std::getenv("SYM64_THRESHOLD");
 570     int Sym64Threshold = 32;
 571     if (Sym64Env)
 572       StringRef(Sym64Env).getAsInteger(10, Sym64Threshold);
 573
 574     // If LastOffset isn't going to fit in a 32-bit varible we need to switch
 575     // to 64-bit. Note that the file can be larger than 4GB as long as the last
 576     // member starts before the 4GB offset.
 577     if (LastOffset >= (1ULL << Sym64Threshold)) {
 578       if (Kind == object::Archive::K_DARWIN)
 579         Kind = object::Archive::K_DARWIN64;
 580       else
 581         Kind = object::Archive::K_GNU64;
 582     }
 583   }
 584
 585   Expected<sys::fs::TempFile> Temp =
 586       sys::fs::TempFile::create(ArcName + ".temp-archive-%%%%%%%.a");
 587   if (!Temp)
 588     return Temp.takeError();
 589
 590   raw_fd_ostream Out(Temp->FD, false);
 591   if (Thin)
 592     Out << "!<thin>\n";
 593   else
 594     Out << "!<arch>\n";
 595
 596   if (WriteSymtab)
 597     writeSymbolTable(Out, Kind, Deterministic, Data, SymNamesBuf);
 598
 599   for (const MemberData &M : Data)
 600     Out << M.Header << M.Data << M.Padding;
 601
 602   Out.flush();
 603
 604   // At this point, we no longer need whatever backing memory
 605   // was used to generate the NewMembers. On Windows, this buffer
 606   // could be a mapped view of the file we want to replace (if
 607   // we're updating an existing archive, say). In that case, the
 608   // rename would still succeed, but it would leave behind a
 609   // temporary file (actually the original file renamed) because
 610   // a file cannot be deleted while there's a handle open on it,
 611   // only renamed. So by freeing this buffer, this ensures that
 612   // the last open handle on the destination file, if any, is
 613   // closed before we attempt to rename.
 614   OldArchiveBuf.reset();
 615
 616   return Temp->keep(ArcName);
 617 }