lib/Object/ArchiveWriter.cpp

   1 //===- ArchiveWriter.cpp - ar File Format implementation --------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines the writeArchive function.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Object/ArchiveWriter.h"
  14 #include "llvm/ADT/ArrayRef.h"
  15 #include "llvm/ADT/StringRef.h"
  16 #include "llvm/BinaryFormat/Magic.h"
  17 #include "llvm/IR/LLVMContext.h"
  18 #include "llvm/Object/Archive.h"
  19 #include "llvm/Object/Error.h"
  20 #include "llvm/Object/ObjectFile.h"
  21 #include "llvm/Object/SymbolicFile.h"
  22 #include "llvm/Support/Alignment.h"
  23 #include "llvm/Support/EndianStream.h"
  24 #include "llvm/Support/Errc.h"
  25 #include "llvm/Support/ErrorHandling.h"
  26 #include "llvm/Support/Format.h"
  27 #include "llvm/Support/Path.h"
  28 #include "llvm/Support/ToolOutputFile.h"
  29 #include "llvm/Support/raw_ostream.h"
  30
  31 #include <map>
  32
  33 #if !defined(_MSC_VER) && !defined(__MINGW32__)
  34 #include <unistd.h>
  35 #else
  36 #include <io.h>
  37 #endif
  38
  39 using namespace llvm;
  40
  41 NewArchiveMember::NewArchiveMember(MemoryBufferRef BufRef)
  42     : Buf(MemoryBuffer::getMemBuffer(BufRef, false)),
  43       MemberName(BufRef.getBufferIdentifier()) {}
  44
  45 Expected<NewArchiveMember>
  46 NewArchiveMember::getOldMember(const object::Archive::Child &OldMember,
  47                                bool Deterministic) {
  48   Expected<llvm::MemoryBufferRef> BufOrErr = OldMember.getMemoryBufferRef();
  49   if (!BufOrErr)
  50     return BufOrErr.takeError();
  51
  52   NewArchiveMember M;
  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   auto FDOrErr = sys::fs::openNativeFileForRead(FileName);
  80   if (!FDOrErr)
  81     return FDOrErr.takeError();
  82   sys::fs::file_t FD = *FDOrErr;
  83   assert(FD != sys::fs::kInvalidFile);
  84
  85   if (auto EC = sys::fs::status(FD, Status))
  86     return errorCodeToError(EC);
  87
  88   // Opening a directory doesn't make sense. Let it fail.
  89   // Linux cannot open directories with open(2), although
  90   // cygwin and *bsd can.
  91   if (Status.type() == sys::fs::file_type::directory_file)
  92     return errorCodeToError(make_error_code(errc::is_a_directory));
  93
  94   ErrorOr<std::unique_ptr<MemoryBuffer>> MemberBufferOrErr =
  95       MemoryBuffer::getOpenFile(FD, FileName, Status.getSize(), false);
  96   if (!MemberBufferOrErr)
  97     return errorCodeToError(MemberBufferOrErr.getError());
  98
  99   if (auto EC = sys::fs::closeFile(FD))
 100     return errorCodeToError(EC);
 101
 102   NewArchiveMember M;
 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, uint64_t 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                           uint64_t 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, uint64_t Size) {
 178   uint64_t PosAfterHeader = Pos + 60 + Name.size();
 179   // Pad so that even 64 bit object files are aligned.
 180   unsigned Pad = offsetToAlignment(PosAfterHeader, llvm::Align(8));
 181   unsigned NameWithPadding = Name.size() + Pad;
 182   printWithSpacePadding(Out, Twine("#1/") + Twine(NameWithPadding), 16);
 183   printRestOfMemberHeader(Out, ModTime, UID, GID, Perms,
 184                           NameWithPadding + Size);
 185   Out << Name;
 186   while (Pad--)
 187     Out.write(uint8_t(0));
 188 }
 189
 190 static bool useStringTable(bool Thin, StringRef Name) {
 191   return Thin || Name.size() >= 16 || Name.contains('/');
 192 }
 193
 194 static bool is64BitKind(object::Archive::Kind Kind) {
 195   switch (Kind) {
 196   case object::Archive::K_GNU:
 197   case object::Archive::K_BSD:
 198   case object::Archive::K_DARWIN:
 199   case object::Archive::K_COFF:
 200     return false;
 201   case object::Archive::K_DARWIN64:
 202   case object::Archive::K_GNU64:
 203     return true;
 204   }
 205   llvm_unreachable("not supported for writting");
 206 }
 207
 208 static void
 209 printMemberHeader(raw_ostream &Out, uint64_t Pos, raw_ostream &StringTable,
 210                   StringMap<uint64_t> &MemberNames, object::Archive::Kind Kind,
 211                   bool Thin, const NewArchiveMember &M,
 212                   sys::TimePoint<std::chrono::seconds> ModTime, uint64_t Size) {
 213   if (isBSDLike(Kind))
 214     return printBSDMemberHeader(Out, Pos, M.MemberName, ModTime, M.UID, M.GID,
 215                                 M.Perms, Size);
 216   if (!useStringTable(Thin, M.MemberName))
 217     return printGNUSmallMemberHeader(Out, M.MemberName, ModTime, M.UID, M.GID,
 218                                      M.Perms, Size);
 219   Out << '/';
 220   uint64_t NamePos;
 221   if (Thin) {
 222     NamePos = StringTable.tell();
 223     StringTable << M.MemberName << "/\n";
 224   } else {
 225     auto Insertion = MemberNames.insert({M.MemberName, uint64_t(0)});
 226     if (Insertion.second) {
 227       Insertion.first->second = StringTable.tell();
 228       StringTable << M.MemberName << "/\n";
 229     }
 230     NamePos = Insertion.first->second;
 231   }
 232   printWithSpacePadding(Out, NamePos, 15);
 233   printRestOfMemberHeader(Out, ModTime, M.UID, M.GID, M.Perms, Size);
 234 }
 235
 236 namespace {
 237 struct MemberData {
 238   std::vector<unsigned> Symbols;
 239   std::string Header;
 240   StringRef Data;
 241   StringRef Padding;
 242 };
 243 } // namespace
 244
 245 static MemberData computeStringTable(StringRef Names) {
 246   unsigned Size = Names.size();
 247   unsigned Pad = offsetToAlignment(Size, llvm::Align(2));
 248   std::string Header;
 249   raw_string_ostream Out(Header);
 250   printWithSpacePadding(Out, "//", 48);
 251   printWithSpacePadding(Out, Size + Pad, 10);
 252   Out << "`\n";
 253   Out.flush();
 254   return {{}, std::move(Header), Names, Pad ? "\n" : ""};
 255 }
 256
 257 static sys::TimePoint<std::chrono::seconds> now(bool Deterministic) {
 258   using namespace std::chrono;
 259
 260   if (!Deterministic)
 261     return time_point_cast<seconds>(system_clock::now());
 262   return sys::TimePoint<seconds>();
 263 }
 264
 265 static bool isArchiveSymbol(const object::BasicSymbolRef &S) {
 266   uint32_t Symflags = S.getFlags();
 267   if (Symflags & object::SymbolRef::SF_FormatSpecific)
 268     return false;
 269   if (!(Symflags & object::SymbolRef::SF_Global))
 270     return false;
 271   if (Symflags & object::SymbolRef::SF_Undefined)
 272     return false;
 273   return true;
 274 }
 275
 276 static void printNBits(raw_ostream &Out, object::Archive::Kind Kind,
 277                        uint64_t Val) {
 278   if (is64BitKind(Kind))
 279     print<uint64_t>(Out, Kind, Val);
 280   else
 281     print<uint32_t>(Out, Kind, Val);
 282 }
 283
 284 static void writeSymbolTable(raw_ostream &Out, object::Archive::Kind Kind,
 285                              bool Deterministic, ArrayRef<MemberData> Members,
 286                              StringRef StringTable) {
 287   // We don't write a symbol table on an archive with no members -- except on
 288   // Darwin, where the linker will abort unless the archive has a symbol table.
 289   if (StringTable.empty() && !isDarwin(Kind))
 290     return;
 291
 292   unsigned NumSyms = 0;
 293   for (const MemberData &M : Members)
 294     NumSyms += M.Symbols.size();
 295
 296   unsigned Size = 0;
 297   unsigned OffsetSize = is64BitKind(Kind) ? sizeof(uint64_t) : sizeof(uint32_t);
 298
 299   Size += OffsetSize; // Number of entries
 300   if (isBSDLike(Kind))
 301     Size += NumSyms * OffsetSize * 2; // Table
 302   else
 303     Size += NumSyms * OffsetSize; // Table
 304   if (isBSDLike(Kind))
 305     Size += OffsetSize; // byte count
 306   Size += StringTable.size();
 307   // ld64 expects the members to be 8-byte aligned for 64-bit content and at
 308   // least 4-byte aligned for 32-bit content.  Opt for the larger encoding
 309   // uniformly.
 310   // We do this for all bsd formats because it simplifies aligning members.
 311   const llvm::Align Alignment(isBSDLike(Kind) ? 8 : 2);
 312   unsigned Pad = offsetToAlignment(Size, Alignment);
 313   Size += Pad;
 314
 315   if (isBSDLike(Kind)) {
 316     const char *Name = is64BitKind(Kind) ? "__.SYMDEF_64" : "__.SYMDEF";
 317     printBSDMemberHeader(Out, Out.tell(), Name, now(Deterministic), 0, 0, 0,
 318                          Size);
 319   } else {
 320     const char *Name = is64BitKind(Kind) ? "/SYM64" : "";
 321     printGNUSmallMemberHeader(Out, Name, now(Deterministic), 0, 0, 0, Size);
 322   }
 323
 324   uint64_t Pos = Out.tell() + Size;
 325
 326   if (isBSDLike(Kind))
 327     printNBits(Out, Kind, NumSyms * 2 * OffsetSize);
 328   else
 329     printNBits(Out, Kind, NumSyms);
 330
 331   for (const MemberData &M : Members) {
 332     for (unsigned StringOffset : M.Symbols) {
 333       if (isBSDLike(Kind))
 334         printNBits(Out, Kind, StringOffset);
 335       printNBits(Out, Kind, Pos); // member offset
 336     }
 337     Pos += M.Header.size() + M.Data.size() + M.Padding.size();
 338   }
 339
 340   if (isBSDLike(Kind))
 341     // byte count of the string table
 342     printNBits(Out, Kind, StringTable.size());
 343   Out << StringTable;
 344
 345   while (Pad--)
 346     Out.write(uint8_t(0));
 347 }
 348
 349 static Expected<std::vector<unsigned>>
 350 getSymbols(MemoryBufferRef Buf, raw_ostream &SymNames, bool &HasObject) {
 351   std::vector<unsigned> Ret;
 352
 353   // In the scenario when LLVMContext is populated SymbolicFile will contain a
 354   // reference to it, thus SymbolicFile should be destroyed first.
 355   LLVMContext Context;
 356   std::unique_ptr<object::SymbolicFile> Obj;
 357   if (identify_magic(Buf.getBuffer()) == file_magic::bitcode) {
 358     auto ObjOrErr = object::SymbolicFile::createSymbolicFile(
 359         Buf, file_magic::bitcode, &Context);
 360     if (!ObjOrErr) {
 361       // FIXME: check only for "not an object file" errors.
 362       consumeError(ObjOrErr.takeError());
 363       return Ret;
 364     }
 365     Obj = std::move(*ObjOrErr);
 366   } else {
 367     auto ObjOrErr = object::SymbolicFile::createSymbolicFile(Buf);
 368     if (!ObjOrErr) {
 369       // FIXME: check only for "not an object file" errors.
 370       consumeError(ObjOrErr.takeError());
 371       return Ret;
 372     }
 373     Obj = std::move(*ObjOrErr);
 374   }
 375
 376   HasObject = true;
 377   for (const object::BasicSymbolRef &S : Obj->symbols()) {
 378     if (!isArchiveSymbol(S))
 379       continue;
 380     Ret.push_back(SymNames.tell());
 381     if (Error E = S.printName(SymNames))
 382       return std::move(E);
 383     SymNames << '\0';
 384   }
 385   return Ret;
 386 }
 387
 388 static Expected<std::vector<MemberData>>
 389 computeMemberData(raw_ostream &StringTable, raw_ostream &SymNames,
 390                   object::Archive::Kind Kind, bool Thin, bool Deterministic,
 391                   ArrayRef<NewArchiveMember> NewMembers) {
 392   static char PaddingData[8] = {'\n', '\n', '\n', '\n', '\n', '\n', '\n', '\n'};
 393
 394   // This ignores the symbol table, but we only need the value mod 8 and the
 395   // symbol table is aligned to be a multiple of 8 bytes
 396   uint64_t Pos = 0;
 397
 398   std::vector<MemberData> Ret;
 399   bool HasObject = false;
 400
 401   // Deduplicate long member names in the string table and reuse earlier name
 402   // offsets. This especially saves space for COFF Import libraries where all
 403   // members have the same name.
 404   StringMap<uint64_t> MemberNames;
 405
 406   // UniqueTimestamps is a special case to improve debugging on Darwin:
 407   //
 408   // The Darwin linker does not link debug info into the final
 409   // binary. Instead, it emits entries of type N_OSO in in the output
 410   // binary's symbol table, containing references to the linked-in
 411   // object files. Using that reference, the debugger can read the
 412   // debug data directly from the object files. Alternatively, an
 413   // invocation of 'dsymutil' will link the debug data from the object
 414   // files into a dSYM bundle, which can be loaded by the debugger,
 415   // instead of the object files.
 416   //
 417   // For an object file, the N_OSO entries contain the absolute path
 418   // path to the file, and the file's timestamp. For an object
 419   // included in an archive, the path is formatted like
 420   // "/absolute/path/to/archive.a(member.o)", and the timestamp is the
 421   // archive member's timestamp, rather than the archive's timestamp.
 422   //
 423   // However, this doesn't always uniquely identify an object within
 424   // an archive -- an archive file can have multiple entries with the
 425   // same filename. (This will happen commonly if the original object
 426   // files started in different directories.) The only way they get
 427   // distinguished, then, is via the timestamp. But this process is
 428   // unable to find the correct object file in the archive when there
 429   // are two files of the same name and timestamp.
 430   //
 431   // Additionally, timestamp==0 is treated specially, and causes the
 432   // timestamp to be ignored as a match criteria.
 433   //
 434   // That will "usually" work out okay when creating an archive not in
 435   // deterministic timestamp mode, because the objects will probably
 436   // have been created at different timestamps.
 437   //
 438   // To ameliorate this problem, in deterministic archive mode (which
 439   // is the default), on Darwin we will emit a unique non-zero
 440   // timestamp for each entry with a duplicated name. This is still
 441   // deterministic: the only thing affecting that timestamp is the
 442   // order of the files in the resultant archive.
 443   //
 444   // See also the functions that handle the lookup:
 445   // in lldb: ObjectContainerBSDArchive::Archive::FindObject()
 446   // in llvm/tools/dsymutil: BinaryHolder::GetArchiveMemberBuffers().
 447   bool UniqueTimestamps = Deterministic && isDarwin(Kind);
 448   std::map<StringRef, unsigned> FilenameCount;
 449   if (UniqueTimestamps) {
 450     for (const NewArchiveMember &M : NewMembers)
 451       FilenameCount[M.MemberName]++;
 452     for (auto &Entry : FilenameCount)
 453       Entry.second = Entry.second > 1 ? 1 : 0;
 454   }
 455
 456   for (const NewArchiveMember &M : NewMembers) {
 457     std::string Header;
 458     raw_string_ostream Out(Header);
 459
 460     MemoryBufferRef Buf = M.Buf->getMemBufferRef();
 461     StringRef Data = Thin ? "" : Buf.getBuffer();
 462
 463     // ld64 expects the members to be 8-byte aligned for 64-bit content and at
 464     // least 4-byte aligned for 32-bit content.  Opt for the larger encoding
 465     // uniformly.  This matches the behaviour with cctools and ensures that ld64
 466     // is happy with archives that we generate.
 467     unsigned MemberPadding =
 468         isDarwin(Kind) ? offsetToAlignment(Data.size(), llvm::Align(8)) : 0;
 469     unsigned TailPadding =
 470         offsetToAlignment(Data.size() + MemberPadding, llvm::Align(2));
 471     StringRef Padding = StringRef(PaddingData, MemberPadding + TailPadding);
 472
 473     sys::TimePoint<std::chrono::seconds> ModTime;
 474     if (UniqueTimestamps)
 475       // Increment timestamp for each file of a given name.
 476       ModTime = sys::toTimePoint(FilenameCount[M.MemberName]++);
 477     else
 478       ModTime = M.ModTime;
 479
 480     uint64_t Size = Buf.getBufferSize() + MemberPadding;
 481     if (Size > object::Archive::MaxMemberSize) {
 482       std::string StringMsg =
 483           "File " + M.MemberName.str() + " exceeds size limit";
 484       return make_error<object::GenericBinaryError>(
 485           std::move(StringMsg), object::object_error::parse_failed);
 486     }
 487
 488     printMemberHeader(Out, Pos, StringTable, MemberNames, Kind, Thin, M,
 489                       ModTime, Size);
 490     Out.flush();
 491
 492     Expected<std::vector<unsigned>> Symbols =
 493         getSymbols(Buf, SymNames, HasObject);
 494     if (auto E = Symbols.takeError())
 495       return std::move(E);
 496
 497     Pos += Header.size() + Data.size() + Padding.size();
 498     Ret.push_back({std::move(*Symbols), std::move(Header), Data, Padding});
 499   }
 500   // If there are no symbols, emit an empty symbol table, to satisfy Solaris
 501   // tools, older versions of which expect a symbol table in a non-empty
 502   // archive, regardless of whether there are any symbols in it.
 503   if (HasObject && SymNames.tell() == 0)
 504     SymNames << '\0' << '\0' << '\0';
 505   return Ret;
 506 }
 507
 508 namespace llvm {
 509
 510 static ErrorOr<SmallString<128>> canonicalizePath(StringRef P) {
 511   SmallString<128> Ret = P;
 512   std::error_code Err = sys::fs::make_absolute(Ret);
 513   if (Err)
 514     return Err;
 515   sys::path::remove_dots(Ret, /*removedotdot*/ true);
 516   return Ret;
 517 }
 518
 519 // Compute the relative path from From to To.
 520 Expected<std::string> computeArchiveRelativePath(StringRef From, StringRef To) {
 521   ErrorOr<SmallString<128>> PathToOrErr = canonicalizePath(To);
 522   ErrorOr<SmallString<128>> DirFromOrErr = canonicalizePath(From);
 523   if (!PathToOrErr || !DirFromOrErr)
 524     return errorCodeToError(std::error_code(errno, std::generic_category()));
 525
 526   const SmallString<128> &PathTo = *PathToOrErr;
 527   const SmallString<128> &DirFrom = sys::path::parent_path(*DirFromOrErr);
 528
 529   // Can't construct a relative path between different roots
 530   if (sys::path::root_name(PathTo) != sys::path::root_name(DirFrom))
 531     return sys::path::convert_to_slash(PathTo);
 532
 533   // Skip common prefixes
 534   auto FromTo =
 535       std::mismatch(sys::path::begin(DirFrom), sys::path::end(DirFrom),
 536                     sys::path::begin(PathTo));
 537   auto FromI = FromTo.first;
 538   auto ToI = FromTo.second;
 539
 540   // Construct relative path
 541   SmallString<128> Relative;
 542   for (auto FromE = sys::path::end(DirFrom); FromI != FromE; ++FromI)
 543     sys::path::append(Relative, sys::path::Style::posix, "..");
 544
 545   for (auto ToE = sys::path::end(PathTo); ToI != ToE; ++ToI)
 546     sys::path::append(Relative, sys::path::Style::posix, *ToI);
 547
 548   return Relative.str();
 549 }
 550
 551 Error writeArchive(StringRef ArcName, ArrayRef<NewArchiveMember> NewMembers,
 552                    bool WriteSymtab, object::Archive::Kind Kind,
 553                    bool Deterministic, bool Thin,
 554                    std::unique_ptr<MemoryBuffer> OldArchiveBuf) {
 555   assert((!Thin || !isBSDLike(Kind)) && "Only the gnu format has a thin mode");
 556
 557   SmallString<0> SymNamesBuf;
 558   raw_svector_ostream SymNames(SymNamesBuf);
 559   SmallString<0> StringTableBuf;
 560   raw_svector_ostream StringTable(StringTableBuf);
 561
 562   Expected<std::vector<MemberData>> DataOrErr = computeMemberData(
 563       StringTable, SymNames, Kind, Thin, Deterministic, NewMembers);
 564   if (Error E = DataOrErr.takeError())
 565     return E;
 566   std::vector<MemberData> &Data = *DataOrErr;
 567
 568   if (!StringTableBuf.empty())
 569     Data.insert(Data.begin(), computeStringTable(StringTableBuf));
 570
 571   // We would like to detect if we need to switch to a 64-bit symbol table.
 572   if (WriteSymtab) {
 573     uint64_t MaxOffset = 0;
 574     uint64_t LastOffset = MaxOffset;
 575     for (const auto &M : Data) {
 576       // Record the start of the member's offset
 577       LastOffset = MaxOffset;
 578       // Account for the size of each part associated with the member.
 579       MaxOffset += M.Header.size() + M.Data.size() + M.Padding.size();
 580       // We assume 32-bit symbols to see if 32-bit symbols are possible or not.
 581       MaxOffset += M.Symbols.size() * 4;
 582     }
 583
 584     // The SYM64 format is used when an archive's member offsets are larger than
 585     // 32-bits can hold. The need for this shift in format is detected by
 586     // writeArchive. To test this we need to generate a file with a member that
 587     // has an offset larger than 32-bits but this demands a very slow test. To
 588     // speed the test up we use this environment variable to pretend like the
 589     // cutoff happens before 32-bits and instead happens at some much smaller
 590     // value.
 591     const char *Sym64Env = std::getenv("SYM64_THRESHOLD");
 592     int Sym64Threshold = 32;
 593     if (Sym64Env)
 594       StringRef(Sym64Env).getAsInteger(10, Sym64Threshold);
 595
 596     // If LastOffset isn't going to fit in a 32-bit varible we need to switch
 597     // to 64-bit. Note that the file can be larger than 4GB as long as the last
 598     // member starts before the 4GB offset.
 599     if (LastOffset >= (1ULL << Sym64Threshold)) {
 600       if (Kind == object::Archive::K_DARWIN)
 601         Kind = object::Archive::K_DARWIN64;
 602       else
 603         Kind = object::Archive::K_GNU64;
 604     }
 605   }
 606
 607   Expected<sys::fs::TempFile> Temp =
 608       sys::fs::TempFile::create(ArcName + ".temp-archive-%%%%%%%.a");
 609   if (!Temp)
 610     return Temp.takeError();
 611
 612   raw_fd_ostream Out(Temp->FD, false);
 613   if (Thin)
 614     Out << "!<thin>\n";
 615   else
 616     Out << "!<arch>\n";
 617
 618   if (WriteSymtab)
 619     writeSymbolTable(Out, Kind, Deterministic, Data, SymNamesBuf);
 620
 621   for (const MemberData &M : Data)
 622     Out << M.Header << M.Data << M.Padding;
 623
 624   Out.flush();
 625
 626   // At this point, we no longer need whatever backing memory
 627   // was used to generate the NewMembers. On Windows, this buffer
 628   // could be a mapped view of the file we want to replace (if
 629   // we're updating an existing archive, say). In that case, the
 630   // rename would still succeed, but it would leave behind a
 631   // temporary file (actually the original file renamed) because
 632   // a file cannot be deleted while there's a handle open on it,
 633   // only renamed. So by freeing this buffer, this ensures that
 634   // the last open handle on the destination file, if any, is
 635   // closed before we attempt to rename.
 636   OldArchiveBuf.reset();
 637
 638   return Temp->keep(ArcName);
 639 }
 640
 641 } // namespace llvm