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Source file src/cmd/link/internal/ld/lib.go

Documentation: cmd/link/internal/ld

  // Inferno utils/8l/asm.c
  // https://bitbucket.org/inferno-os/inferno-os/src/default/utils/8l/asm.c
  //
  //	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
  //	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
  //	Portions Copyright © 1997-1999 Vita Nuova Limited
  //	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
  //	Portions Copyright © 2004,2006 Bruce Ellis
  //	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
  //	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
  //	Portions Copyright © 2009 The Go Authors. All rights reserved.
  //
  // Permission is hereby granted, free of charge, to any person obtaining a copy
  // of this software and associated documentation files (the "Software"), to deal
  // in the Software without restriction, including without limitation the rights
  // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  // copies of the Software, and to permit persons to whom the Software is
  // furnished to do so, subject to the following conditions:
  //
  // The above copyright notice and this permission notice shall be included in
  // all copies or substantial portions of the Software.
  //
  // 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 AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
  // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  // THE SOFTWARE.
  
  package ld
  
  import (
  	"bufio"
  	"bytes"
  	"cmd/internal/bio"
  	"cmd/internal/objabi"
  	"cmd/internal/sys"
  	"crypto/sha1"
  	"debug/elf"
  	"encoding/binary"
  	"encoding/hex"
  	"fmt"
  	"io"
  	"io/ioutil"
  	"log"
  	"os"
  	"os/exec"
  	"path/filepath"
  	"runtime"
  	"strings"
  	"sync"
  )
  
  // Data layout and relocation.
  
  // Derived from Inferno utils/6l/l.h
  // https://bitbucket.org/inferno-os/inferno-os/src/default/utils/6l/l.h
  //
  //	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
  //	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
  //	Portions Copyright © 1997-1999 Vita Nuova Limited
  //	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
  //	Portions Copyright © 2004,2006 Bruce Ellis
  //	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
  //	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
  //	Portions Copyright © 2009 The Go Authors. All rights reserved.
  //
  // Permission is hereby granted, free of charge, to any person obtaining a copy
  // of this software and associated documentation files (the "Software"), to deal
  // in the Software without restriction, including without limitation the rights
  // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  // copies of the Software, and to permit persons to whom the Software is
  // furnished to do so, subject to the following conditions:
  //
  // The above copyright notice and this permission notice shall be included in
  // all copies or substantial portions of the Software.
  //
  // 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 AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
  // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  // THE SOFTWARE.
  
  type Arch struct {
  	Funcalign        int
  	Maxalign         int
  	Minalign         int
  	Dwarfregsp       int
  	Dwarfreglr       int
  	Linuxdynld       string
  	Freebsddynld     string
  	Netbsddynld      string
  	Openbsddynld     string
  	Dragonflydynld   string
  	Solarisdynld     string
  	Adddynrel        func(*Link, *Symbol, *Reloc) bool
  	Archinit         func(*Link)
  	Archreloc        func(*Link, *Reloc, *Symbol, *int64) int
  	Archrelocvariant func(*Link, *Reloc, *Symbol, int64) int64
  	Trampoline       func(*Link, *Reloc, *Symbol)
  	Asmb             func(*Link)
  	Elfreloc1        func(*Link, *Reloc, int64) int
  	Elfsetupplt      func(*Link)
  	Gentext          func(*Link)
  	Machoreloc1      func(*Symbol, *Reloc, int64) int
  	PEreloc1         func(*Symbol, *Reloc, int64) bool
  	Wput             func(uint16)
  	Lput             func(uint32)
  	Vput             func(uint64)
  	Append16         func(b []byte, v uint16) []byte
  	Append32         func(b []byte, v uint32) []byte
  	Append64         func(b []byte, v uint64) []byte
  
  	// TLSIEtoLE converts a TLS Initial Executable relocation to
  	// a TLS Local Executable relocation.
  	//
  	// This is possible when a TLS IE relocation refers to a local
  	// symbol in an executable, which is typical when internally
  	// linking PIE binaries.
  	TLSIEtoLE func(s *Symbol, off, size int)
  }
  
  var (
  	Thearch Arch
  	Lcsize  int32
  	rpath   Rpath
  	Spsize  int32
  	Symsize int32
  )
  
  // Terrible but standard terminology.
  // A segment describes a block of file to load into memory.
  // A section further describes the pieces of that block for
  // use in debuggers and such.
  
  const (
  	MINFUNC = 16 // minimum size for a function
  )
  
  type Segment struct {
  	Rwx      uint8  // permission as usual unix bits (5 = r-x etc)
  	Vaddr    uint64 // virtual address
  	Length   uint64 // length in memory
  	Fileoff  uint64 // file offset
  	Filelen  uint64 // length on disk
  	Sections []*Section
  }
  
  type Section struct {
  	Rwx     uint8
  	Extnum  int16
  	Align   int32
  	Name    string
  	Vaddr   uint64
  	Length  uint64
  	Seg     *Segment
  	Elfsect *ElfShdr
  	Reloff  uint64
  	Rellen  uint64
  }
  
  // DynlinkingGo returns whether we are producing Go code that can live
  // in separate shared libraries linked together at runtime.
  func (ctxt *Link) DynlinkingGo() bool {
  	if !ctxt.Loaded {
  		panic("DynlinkingGo called before all symbols loaded")
  	}
  	canUsePlugins := ctxt.Syms.ROLookup("plugin.Open", 0) != nil
  	return Buildmode == BuildmodeShared || *FlagLinkshared || Buildmode == BuildmodePlugin || canUsePlugins
  }
  
  // UseRelro returns whether to make use of "read only relocations" aka
  // relro.
  func UseRelro() bool {
  	switch Buildmode {
  	case BuildmodeCArchive, BuildmodeCShared, BuildmodeShared, BuildmodePIE, BuildmodePlugin:
  		return Iself
  	default:
  		return *FlagLinkshared
  	}
  }
  
  var (
  	SysArch         *sys.Arch
  	dynexp          []*Symbol
  	dynlib          []string
  	ldflag          []string
  	havedynamic     int
  	Funcalign       int
  	iscgo           bool
  	elfglobalsymndx int
  	interpreter     string
  
  	debug_s  bool // backup old value of debug['s']
  	HEADR    int32
  	Headtype objabi.HeadType
  
  	nerrors  int
  	liveness int64
  )
  
  var (
  	Segtext      Segment
  	Segrodata    Segment
  	Segrelrodata Segment
  	Segdata      Segment
  	Segdwarf     Segment
  )
  
  /* whence for ldpkg */
  const (
  	FileObj = 0 + iota
  	ArchiveObj
  	Pkgdef
  )
  
  // TODO(dfc) outBuf duplicates bio.Writer
  type outBuf struct {
  	w   *bufio.Writer
  	f   *os.File
  	off int64
  }
  
  func (w *outBuf) Write(p []byte) (n int, err error) {
  	n, err = w.w.Write(p)
  	w.off += int64(n)
  	return n, err
  }
  
  func (w *outBuf) WriteString(s string) (n int, err error) {
  	n, err = coutbuf.w.WriteString(s)
  	w.off += int64(n)
  	return n, err
  }
  
  func (w *outBuf) Offset() int64 {
  	return w.off
  }
  
  var coutbuf outBuf
  
  const pkgname = "__.PKGDEF"
  
  var (
  	// Set if we see an object compiled by the host compiler that is not
  	// from a package that is known to support internal linking mode.
  	externalobj = false
  	theline     string
  )
  
  func Lflag(ctxt *Link, arg string) {
  	ctxt.Libdir = append(ctxt.Libdir, arg)
  }
  
  /*
   * Unix doesn't like it when we write to a running (or, sometimes,
   * recently run) binary, so remove the output file before writing it.
   * On Windows 7, remove() can force a subsequent create() to fail.
   * S_ISREG() does not exist on Plan 9.
   */
  func mayberemoveoutfile() {
  	if fi, err := os.Lstat(*flagOutfile); err == nil && !fi.Mode().IsRegular() {
  		return
  	}
  	os.Remove(*flagOutfile)
  }
  
  func libinit(ctxt *Link) {
  	Funcalign = Thearch.Funcalign
  
  	// add goroot to the end of the libdir list.
  	suffix := ""
  
  	suffixsep := ""
  	if *flagInstallSuffix != "" {
  		suffixsep = "_"
  		suffix = *flagInstallSuffix
  	} else if *flagRace {
  		suffixsep = "_"
  		suffix = "race"
  	} else if *flagMsan {
  		suffixsep = "_"
  		suffix = "msan"
  	}
  
  	Lflag(ctxt, filepath.Join(objabi.GOROOT, "pkg", fmt.Sprintf("%s_%s%s%s", objabi.GOOS, objabi.GOARCH, suffixsep, suffix)))
  
  	mayberemoveoutfile()
  	f, err := os.OpenFile(*flagOutfile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
  	if err != nil {
  		Exitf("cannot create %s: %v", *flagOutfile, err)
  	}
  
  	coutbuf.w = bufio.NewWriter(f)
  	coutbuf.f = f
  
  	if *flagEntrySymbol == "" {
  		switch Buildmode {
  		case BuildmodeCShared, BuildmodeCArchive:
  			*flagEntrySymbol = fmt.Sprintf("_rt0_%s_%s_lib", objabi.GOARCH, objabi.GOOS)
  		case BuildmodeExe, BuildmodePIE:
  			*flagEntrySymbol = fmt.Sprintf("_rt0_%s_%s", objabi.GOARCH, objabi.GOOS)
  		case BuildmodeShared, BuildmodePlugin:
  			// No *flagEntrySymbol for -buildmode=shared and plugin
  		default:
  			Errorf(nil, "unknown *flagEntrySymbol for buildmode %v", Buildmode)
  		}
  	}
  }
  
  func errorexit() {
  	if coutbuf.f != nil {
  		if nerrors != 0 {
  			Cflush()
  		}
  		// For rmtemp run at atexit time on Windows.
  		if err := coutbuf.f.Close(); err != nil {
  			Exitf("close: %v", err)
  		}
  	}
  
  	if nerrors != 0 {
  		if coutbuf.f != nil {
  			mayberemoveoutfile()
  		}
  		Exit(2)
  	}
  
  	Exit(0)
  }
  
  func loadinternal(ctxt *Link, name string) *Library {
  	if *FlagLinkshared && ctxt.PackageShlib != nil {
  		if shlibname := ctxt.PackageShlib[name]; shlibname != "" {
  			return addlibpath(ctxt, "internal", "internal", "", name, shlibname)
  		}
  	}
  	if ctxt.PackageFile != nil {
  		if pname := ctxt.PackageFile[name]; pname != "" {
  			return addlibpath(ctxt, "internal", "internal", pname, name, "")
  		}
  		ctxt.Logf("loadinternal: cannot find %s\n", name)
  		return nil
  	}
  
  	for i := 0; i < len(ctxt.Libdir); i++ {
  		if *FlagLinkshared {
  			shlibname := filepath.Join(ctxt.Libdir[i], name+".shlibname")
  			if ctxt.Debugvlog != 0 {
  				ctxt.Logf("searching for %s.a in %s\n", name, shlibname)
  			}
  			if _, err := os.Stat(shlibname); err == nil {
  				return addlibpath(ctxt, "internal", "internal", "", name, shlibname)
  			}
  		}
  		pname := filepath.Join(ctxt.Libdir[i], name+".a")
  		if ctxt.Debugvlog != 0 {
  			ctxt.Logf("searching for %s.a in %s\n", name, pname)
  		}
  		if _, err := os.Stat(pname); err == nil {
  			return addlibpath(ctxt, "internal", "internal", pname, name, "")
  		}
  	}
  
  	ctxt.Logf("warning: unable to find %s.a\n", name)
  	return nil
  }
  
  // findLibPathCmd uses cmd command to find gcc library libname.
  // It returns library full path if found, or "none" if not found.
  func (ctxt *Link) findLibPathCmd(cmd, libname string) string {
  	if *flagExtld == "" {
  		*flagExtld = "gcc"
  	}
  	args := hostlinkArchArgs()
  	args = append(args, cmd)
  	if ctxt.Debugvlog != 0 {
  		ctxt.Logf("%s %v\n", *flagExtld, args)
  	}
  	out, err := exec.Command(*flagExtld, args...).Output()
  	if err != nil {
  		if ctxt.Debugvlog != 0 {
  			ctxt.Logf("not using a %s file because compiler failed\n%v\n%s\n", libname, err, out)
  		}
  		return "none"
  	}
  	return strings.TrimSpace(string(out))
  }
  
  // findLibPath searches for library libname.
  // It returns library full path if found, or "none" if not found.
  func (ctxt *Link) findLibPath(libname string) string {
  	return ctxt.findLibPathCmd("--print-file-name="+libname, libname)
  }
  
  func (ctxt *Link) loadlib() {
  	switch Buildmode {
  	case BuildmodeCShared, BuildmodePlugin:
  		s := ctxt.Syms.Lookup("runtime.islibrary", 0)
  		s.Attr |= AttrDuplicateOK
  		Adduint8(ctxt, s, 1)
  	case BuildmodeCArchive:
  		s := ctxt.Syms.Lookup("runtime.isarchive", 0)
  		s.Attr |= AttrDuplicateOK
  		Adduint8(ctxt, s, 1)
  	}
  
  	loadinternal(ctxt, "runtime")
  	if SysArch.Family == sys.ARM {
  		loadinternal(ctxt, "math")
  	}
  	if *flagRace {
  		loadinternal(ctxt, "runtime/race")
  	}
  	if *flagMsan {
  		loadinternal(ctxt, "runtime/msan")
  	}
  
  	var i int
  	for i = 0; i < len(ctxt.Library); i++ {
  		iscgo = iscgo || ctxt.Library[i].Pkg == "runtime/cgo"
  		if ctxt.Library[i].Shlib == "" {
  			if ctxt.Debugvlog > 1 {
  				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), ctxt.Library[i].File, ctxt.Library[i].Objref)
  			}
  			objfile(ctxt, ctxt.Library[i])
  		}
  	}
  
  	for i = 0; i < len(ctxt.Library); i++ {
  		if ctxt.Library[i].Shlib != "" {
  			if ctxt.Debugvlog > 1 {
  				ctxt.Logf("%5.2f autolib: %s (from %s)\n", Cputime(), ctxt.Library[i].Shlib, ctxt.Library[i].Objref)
  			}
  			ldshlibsyms(ctxt, ctxt.Library[i].Shlib)
  		}
  	}
  
  	// We now have enough information to determine the link mode.
  	determineLinkMode(ctxt)
  
  	// Recalculate pe parameters now that we have Linkmode set.
  	if Headtype == objabi.Hwindows {
  		Peinit(ctxt)
  	}
  
  	if Headtype == objabi.Hdarwin && Linkmode == LinkExternal {
  		*FlagTextAddr = 0
  	}
  
  	if Linkmode == LinkExternal && SysArch.Family == sys.PPC64 {
  		toc := ctxt.Syms.Lookup(".TOC.", 0)
  		toc.Type = SDYNIMPORT
  	}
  
  	if Linkmode == LinkExternal && !iscgo {
  		// This indicates a user requested -linkmode=external.
  		// The startup code uses an import of runtime/cgo to decide
  		// whether to initialize the TLS.  So give it one. This could
  		// be handled differently but it's an unusual case.
  		loadinternal(ctxt, "runtime/cgo")
  
  		if i < len(ctxt.Library) {
  			if ctxt.Library[i].Shlib != "" {
  				ldshlibsyms(ctxt, ctxt.Library[i].Shlib)
  			} else {
  				if Buildmode == BuildmodeShared || *FlagLinkshared {
  					Exitf("cannot implicitly include runtime/cgo in a shared library")
  				}
  				objfile(ctxt, ctxt.Library[i])
  			}
  		}
  	}
  
  	if Linkmode == LinkInternal {
  		// Drop all the cgo_import_static declarations.
  		// Turns out we won't be needing them.
  		for _, s := range ctxt.Syms.Allsym {
  			if s.Type == SHOSTOBJ {
  				// If a symbol was marked both
  				// cgo_import_static and cgo_import_dynamic,
  				// then we want to make it cgo_import_dynamic
  				// now.
  				if s.Extname != "" && s.Dynimplib != "" && !s.Attr.CgoExport() {
  					s.Type = SDYNIMPORT
  				} else {
  					s.Type = 0
  				}
  			}
  		}
  	}
  
  	tlsg := ctxt.Syms.Lookup("runtime.tlsg", 0)
  
  	// runtime.tlsg is used for external linking on platforms that do not define
  	// a variable to hold g in assembly (currently only intel).
  	if tlsg.Type == 0 {
  		tlsg.Type = STLSBSS
  		tlsg.Size = int64(SysArch.PtrSize)
  	} else if tlsg.Type != SDYNIMPORT {
  		Errorf(nil, "runtime declared tlsg variable %v", tlsg.Type)
  	}
  	tlsg.Attr |= AttrReachable
  	ctxt.Tlsg = tlsg
  
  	var moduledata *Symbol
  	if Buildmode == BuildmodePlugin {
  		moduledata = ctxt.Syms.Lookup("local.pluginmoduledata", 0)
  		moduledata.Attr |= AttrLocal
  	} else {
  		moduledata = ctxt.Syms.Lookup("runtime.firstmoduledata", 0)
  	}
  	if moduledata.Type != 0 && moduledata.Type != SDYNIMPORT {
  		// If the module (toolchain-speak for "executable or shared
  		// library") we are linking contains the runtime package, it
  		// will define the runtime.firstmoduledata symbol and we
  		// truncate it back to 0 bytes so we can define its entire
  		// contents in symtab.go:symtab().
  		moduledata.Size = 0
  
  		// In addition, on ARM, the runtime depends on the linker
  		// recording the value of GOARM.
  		if SysArch.Family == sys.ARM {
  			s := ctxt.Syms.Lookup("runtime.goarm", 0)
  			s.Type = SRODATA
  			s.Size = 0
  			Adduint8(ctxt, s, uint8(objabi.GOARM))
  		}
  
  		if objabi.Framepointer_enabled(objabi.GOOS, objabi.GOARCH) {
  			s := ctxt.Syms.Lookup("runtime.framepointer_enabled", 0)
  			s.Type = SRODATA
  			s.Size = 0
  			Adduint8(ctxt, s, 1)
  		}
  	} else {
  		// If OTOH the module does not contain the runtime package,
  		// create a local symbol for the moduledata.
  		moduledata = ctxt.Syms.Lookup("local.moduledata", 0)
  		moduledata.Attr |= AttrLocal
  	}
  	// In all cases way we mark the moduledata as noptrdata to hide it from
  	// the GC.
  	moduledata.Type = SNOPTRDATA
  	moduledata.Attr |= AttrReachable
  	ctxt.Moduledata = moduledata
  
  	// Now that we know the link mode, trim the dynexp list.
  	x := AttrCgoExportDynamic
  
  	if Linkmode == LinkExternal {
  		x = AttrCgoExportStatic
  	}
  	w := 0
  	for i := 0; i < len(dynexp); i++ {
  		if dynexp[i].Attr&x != 0 {
  			dynexp[w] = dynexp[i]
  			w++
  		}
  	}
  	dynexp = dynexp[:w]
  
  	// In internal link mode, read the host object files.
  	if Linkmode == LinkInternal {
  		hostobjs(ctxt)
  
  		// If we have any undefined symbols in external
  		// objects, try to read them from the libgcc file.
  		any := false
  		for _, s := range ctxt.Syms.Allsym {
  			for _, r := range s.R {
  				if r.Sym != nil && r.Sym.Type&SMASK == SXREF && r.Sym.Name != ".got" {
  					any = true
  					break
  				}
  			}
  		}
  		if any {
  			if *flagLibGCC == "" {
  				*flagLibGCC = ctxt.findLibPathCmd("--print-libgcc-file-name", "libgcc")
  			}
  			if *flagLibGCC != "none" {
  				hostArchive(ctxt, *flagLibGCC)
  			}
  			if Headtype == objabi.Hwindows {
  				if p := ctxt.findLibPath("libmingwex.a"); p != "none" {
  					hostArchive(ctxt, p)
  				}
  				if p := ctxt.findLibPath("libmingw32.a"); p != "none" {
  					hostArchive(ctxt, p)
  				}
  				// TODO: maybe do something similar to peimporteddlls to collect all lib names
  				// and try link them all to final exe just like libmingwex.a and libmingw32.a:
  				/*
  					for:
  					#cgo windows LDFLAGS: -lmsvcrt -lm
  					import:
  					libmsvcrt.a libm.a
  				*/
  			}
  		}
  	} else {
  		hostlinksetup()
  	}
  
  	// We've loaded all the code now.
  	ctxt.Loaded = true
  
  	// If there are no dynamic libraries needed, gcc disables dynamic linking.
  	// Because of this, glibc's dynamic ELF loader occasionally (like in version 2.13)
  	// assumes that a dynamic binary always refers to at least one dynamic library.
  	// Rather than be a source of test cases for glibc, disable dynamic linking
  	// the same way that gcc would.
  	//
  	// Exception: on OS X, programs such as Shark only work with dynamic
  	// binaries, so leave it enabled on OS X (Mach-O) binaries.
  	// Also leave it enabled on Solaris which doesn't support
  	// statically linked binaries.
  	if Buildmode == BuildmodeExe {
  		if havedynamic == 0 && Headtype != objabi.Hdarwin && Headtype != objabi.Hsolaris {
  			*FlagD = true
  		}
  	}
  
  	// If package versioning is required, generate a hash of the
  	// the packages used in the link.
  	if Buildmode == BuildmodeShared || Buildmode == BuildmodePlugin || ctxt.Syms.ROLookup("plugin.Open", 0) != nil {
  		for i = 0; i < len(ctxt.Library); i++ {
  			if ctxt.Library[i].Shlib == "" {
  				genhash(ctxt, ctxt.Library[i])
  			}
  		}
  	}
  
  	if SysArch == sys.Arch386 {
  		if (Buildmode == BuildmodeCArchive && Iself) || Buildmode == BuildmodeCShared || Buildmode == BuildmodePIE || ctxt.DynlinkingGo() {
  			got := ctxt.Syms.Lookup("_GLOBAL_OFFSET_TABLE_", 0)
  			got.Type = SDYNIMPORT
  			got.Attr |= AttrReachable
  		}
  	}
  
  	importcycles()
  
  	// put symbols into Textp
  	// do it in postorder so that packages are laid down in dependency order
  	// internal first, then everything else
  	ctxt.Library = postorder(ctxt.Library)
  	for _, doInternal := range [2]bool{true, false} {
  		for _, lib := range ctxt.Library {
  			if isRuntimeDepPkg(lib.Pkg) != doInternal {
  				continue
  			}
  			ctxt.Textp = append(ctxt.Textp, lib.textp...)
  			for _, s := range lib.dupTextSyms {
  				if !s.Attr.OnList() {
  					ctxt.Textp = append(ctxt.Textp, s)
  					s.Attr |= AttrOnList
  					// dupok symbols may be defined in multiple packages. its
  					// associated package is chosen sort of arbitrarily (the
  					// first containing package that the linker loads). canonicalize
  					// it here to the package with which it will be laid down
  					// in text.
  					s.File = objabi.PathToPrefix(lib.Pkg)
  				}
  			}
  		}
  	}
  
  	if len(ctxt.Shlibs) > 0 {
  		// We might have overwritten some functions above (this tends to happen for the
  		// autogenerated type equality/hashing functions) and we don't want to generated
  		// pcln table entries for these any more so remove them from Textp.
  		textp := make([]*Symbol, 0, len(ctxt.Textp))
  		for _, s := range ctxt.Textp {
  			if s.Type != SDYNIMPORT {
  				textp = append(textp, s)
  			}
  		}
  		ctxt.Textp = textp
  	}
  }
  
  /*
   * look for the next file in an archive.
   * adapted from libmach.
   */
  func nextar(bp *bio.Reader, off int64, a *ArHdr) int64 {
  	if off&1 != 0 {
  		off++
  	}
  	bp.Seek(off, 0)
  	var buf [SAR_HDR]byte
  	if n, err := io.ReadFull(bp, buf[:]); err != nil {
  		if n == 0 && err != io.EOF {
  			return -1
  		}
  		return 0
  	}
  
  	a.name = artrim(buf[0:16])
  	a.date = artrim(buf[16:28])
  	a.uid = artrim(buf[28:34])
  	a.gid = artrim(buf[34:40])
  	a.mode = artrim(buf[40:48])
  	a.size = artrim(buf[48:58])
  	a.fmag = artrim(buf[58:60])
  
  	arsize := atolwhex(a.size)
  	if arsize&1 != 0 {
  		arsize++
  	}
  	return arsize + SAR_HDR
  }
  
  func genhash(ctxt *Link, lib *Library) {
  	f, err := bio.Open(lib.File)
  	if err != nil {
  		Errorf(nil, "cannot open file %s for hash generation: %v", lib.File, err)
  		return
  	}
  	defer f.Close()
  
  	var arhdr ArHdr
  	l := nextar(f, int64(len(ARMAG)), &arhdr)
  	if l <= 0 {
  		Errorf(nil, "%s: short read on archive file symbol header", lib.File)
  		return
  	}
  
  	h := sha1.New()
  
  	// To compute the hash of a package, we hash the first line of
  	// __.PKGDEF (which contains the toolchain version and any
  	// GOEXPERIMENT flags) and the export data (which is between
  	// the first two occurences of "\n$$").
  
  	pkgDefBytes := make([]byte, atolwhex(arhdr.size))
  	_, err = io.ReadFull(f, pkgDefBytes)
  	if err != nil {
  		Errorf(nil, "%s: error reading package data: %v", lib.File, err)
  		return
  	}
  	firstEOL := bytes.Index(pkgDefBytes, []byte("\n"))
  	if firstEOL < 0 {
  		Errorf(nil, "cannot parse package data of %s for hash generation, no newline found", lib.File)
  		return
  	}
  	firstDoubleDollar := bytes.Index(pkgDefBytes, []byte("\n$$"))
  	if firstDoubleDollar < 0 {
  		Errorf(nil, "cannot parse package data of %s for hash generation, no \\n$$ found", lib.File)
  		return
  	}
  	secondDoubleDollar := bytes.Index(pkgDefBytes[firstDoubleDollar+1:], []byte("\n$$"))
  	if secondDoubleDollar < 0 {
  		Errorf(nil, "cannot parse package data of %s for hash generation, only one \\n$$ found", lib.File)
  		return
  	}
  	h.Write(pkgDefBytes[0:firstEOL])
  	h.Write(pkgDefBytes[firstDoubleDollar : firstDoubleDollar+secondDoubleDollar])
  	lib.hash = hex.EncodeToString(h.Sum(nil))
  }
  
  func objfile(ctxt *Link, lib *Library) {
  	pkg := objabi.PathToPrefix(lib.Pkg)
  
  	if ctxt.Debugvlog > 1 {
  		ctxt.Logf("%5.2f ldobj: %s (%s)\n", Cputime(), lib.File, pkg)
  	}
  	f, err := bio.Open(lib.File)
  	if err != nil {
  		Exitf("cannot open file %s: %v", lib.File, err)
  	}
  
  	for i := 0; i < len(ARMAG); i++ {
  		if c, err := f.ReadByte(); err == nil && c == ARMAG[i] {
  			continue
  		}
  
  		/* load it as a regular file */
  		l := f.Seek(0, 2)
  
  		f.Seek(0, 0)
  		ldobj(ctxt, f, lib, l, lib.File, lib.File, FileObj)
  		f.Close()
  
  		return
  	}
  
  	/* process __.PKGDEF */
  	off := f.Offset()
  
  	var arhdr ArHdr
  	l := nextar(f, off, &arhdr)
  	var pname string
  	if l <= 0 {
  		Errorf(nil, "%s: short read on archive file symbol header", lib.File)
  		goto out
  	}
  
  	if !strings.HasPrefix(arhdr.name, pkgname) {
  		Errorf(nil, "%s: cannot find package header", lib.File)
  		goto out
  	}
  
  	off += l
  
  	ldpkg(ctxt, f, pkg, atolwhex(arhdr.size), lib.File, Pkgdef)
  
  	/*
  	 * load all the object files from the archive now.
  	 * this gives us sequential file access and keeps us
  	 * from needing to come back later to pick up more
  	 * objects.  it breaks the usual C archive model, but
  	 * this is Go, not C.  the common case in Go is that
  	 * we need to load all the objects, and then we throw away
  	 * the individual symbols that are unused.
  	 *
  	 * loading every object will also make it possible to
  	 * load foreign objects not referenced by __.PKGDEF.
  	 */
  	for {
  		l = nextar(f, off, &arhdr)
  		if l == 0 {
  			break
  		}
  		if l < 0 {
  			Exitf("%s: malformed archive", lib.File)
  		}
  
  		off += l
  
  		pname = fmt.Sprintf("%s(%s)", lib.File, arhdr.name)
  		l = atolwhex(arhdr.size)
  		ldobj(ctxt, f, lib, l, pname, lib.File, ArchiveObj)
  	}
  
  out:
  	f.Close()
  }
  
  type Hostobj struct {
  	ld     func(*Link, *bio.Reader, string, int64, string)
  	pkg    string
  	pn     string
  	file   string
  	off    int64
  	length int64
  }
  
  var hostobj []Hostobj
  
  // These packages can use internal linking mode.
  // Others trigger external mode.
  var internalpkg = []string{
  	"crypto/x509",
  	"net",
  	"os/user",
  	"runtime/cgo",
  	"runtime/race",
  	"runtime/msan",
  }
  
  func ldhostobj(ld func(*Link, *bio.Reader, string, int64, string), f *bio.Reader, pkg string, length int64, pn string, file string) *Hostobj {
  	isinternal := false
  	for i := 0; i < len(internalpkg); i++ {
  		if pkg == internalpkg[i] {
  			isinternal = true
  			break
  		}
  	}
  
  	// DragonFly declares errno with __thread, which results in a symbol
  	// type of R_386_TLS_GD or R_X86_64_TLSGD. The Go linker does not
  	// currently know how to handle TLS relocations, hence we have to
  	// force external linking for any libraries that link in code that
  	// uses errno. This can be removed if the Go linker ever supports
  	// these relocation types.
  	if Headtype == objabi.Hdragonfly {
  		if pkg == "net" || pkg == "os/user" {
  			isinternal = false
  		}
  	}
  
  	if !isinternal {
  		externalobj = true
  	}
  
  	hostobj = append(hostobj, Hostobj{})
  	h := &hostobj[len(hostobj)-1]
  	h.ld = ld
  	h.pkg = pkg
  	h.pn = pn
  	h.file = file
  	h.off = f.Offset()
  	h.length = length
  	return h
  }
  
  func hostobjs(ctxt *Link) {
  	var h *Hostobj
  
  	for i := 0; i < len(hostobj); i++ {
  		h = &hostobj[i]
  		f, err := bio.Open(h.file)
  		if err != nil {
  			Exitf("cannot reopen %s: %v", h.pn, err)
  		}
  
  		f.Seek(h.off, 0)
  		h.ld(ctxt, f, h.pkg, h.length, h.pn)
  		f.Close()
  	}
  }
  
  // provided by lib9
  
  func rmtemp() {
  	os.RemoveAll(*flagTmpdir)
  }
  
  func hostlinksetup() {
  	if Linkmode != LinkExternal {
  		return
  	}
  
  	// For external link, record that we need to tell the external linker -s,
  	// and turn off -s internally: the external linker needs the symbol
  	// information for its final link.
  	debug_s = *FlagS
  	*FlagS = false
  
  	// create temporary directory and arrange cleanup
  	if *flagTmpdir == "" {
  		dir, err := ioutil.TempDir("", "go-link-")
  		if err != nil {
  			log.Fatal(err)
  		}
  		*flagTmpdir = dir
  		AtExit(rmtemp)
  	}
  
  	// change our output to temporary object file
  	coutbuf.f.Close()
  	mayberemoveoutfile()
  
  	p := filepath.Join(*flagTmpdir, "go.o")
  	var err error
  	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
  	if err != nil {
  		Exitf("cannot create %s: %v", p, err)
  	}
  
  	coutbuf.w = bufio.NewWriter(f)
  	coutbuf.f = f
  }
  
  // hostobjCopy creates a copy of the object files in hostobj in a
  // temporary directory.
  func hostobjCopy() (paths []string) {
  	var wg sync.WaitGroup
  	sema := make(chan struct{}, runtime.NumCPU()) // limit open file descriptors
  	for i, h := range hostobj {
  		h := h
  		dst := filepath.Join(*flagTmpdir, fmt.Sprintf("%06d.o", i))
  		paths = append(paths, dst)
  
  		wg.Add(1)
  		go func() {
  			sema <- struct{}{}
  			defer func() {
  				<-sema
  				wg.Done()
  			}()
  			f, err := os.Open(h.file)
  			if err != nil {
  				Exitf("cannot reopen %s: %v", h.pn, err)
  			}
  			if _, err := f.Seek(h.off, 0); err != nil {
  				Exitf("cannot seek %s: %v", h.pn, err)
  			}
  
  			w, err := os.Create(dst)
  			if err != nil {
  				Exitf("cannot create %s: %v", dst, err)
  			}
  			if _, err := io.CopyN(w, f, h.length); err != nil {
  				Exitf("cannot write %s: %v", dst, err)
  			}
  			if err := w.Close(); err != nil {
  				Exitf("cannot close %s: %v", dst, err)
  			}
  		}()
  	}
  	wg.Wait()
  	return paths
  }
  
  // writeGDBLinkerScript creates gcc linker script file in temp
  // directory. writeGDBLinkerScript returns created file path.
  // The script is used to work around gcc bug
  // (see https://golang.org/issue/20183 for details).
  func writeGDBLinkerScript() string {
  	name := "fix_debug_gdb_scripts.ld"
  	path := filepath.Join(*flagTmpdir, name)
  	src := `SECTIONS
  {
    .debug_gdb_scripts BLOCK(__section_alignment__) (NOLOAD) :
    {
      *(.debug_gdb_scripts)
    }
  }
  INSERT AFTER .debug_types;
  `
  	err := ioutil.WriteFile(path, []byte(src), 0666)
  	if err != nil {
  		Errorf(nil, "WriteFile %s failed: %v", name, err)
  	}
  	return path
  }
  
  // archive builds a .a archive from the hostobj object files.
  func (ctxt *Link) archive() {
  	if Buildmode != BuildmodeCArchive {
  		return
  	}
  
  	if *flagExtar == "" {
  		*flagExtar = "ar"
  	}
  
  	mayberemoveoutfile()
  
  	// Force the buffer to flush here so that external
  	// tools will see a complete file.
  	Cflush()
  	if err := coutbuf.f.Close(); err != nil {
  		Exitf("close: %v", err)
  	}
  	coutbuf.f = nil
  
  	argv := []string{*flagExtar, "-q", "-c", "-s", *flagOutfile}
  	argv = append(argv, filepath.Join(*flagTmpdir, "go.o"))
  	argv = append(argv, hostobjCopy()...)
  
  	if ctxt.Debugvlog != 0 {
  		ctxt.Logf("archive: %s\n", strings.Join(argv, " "))
  	}
  
  	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
  		Exitf("running %s failed: %v\n%s", argv[0], err, out)
  	}
  }
  
  func (l *Link) hostlink() {
  	if Linkmode != LinkExternal || nerrors > 0 {
  		return
  	}
  	if Buildmode == BuildmodeCArchive {
  		return
  	}
  
  	if *flagExtld == "" {
  		*flagExtld = "gcc"
  	}
  
  	var argv []string
  	argv = append(argv, *flagExtld)
  	argv = append(argv, hostlinkArchArgs()...)
  
  	if !*FlagS && !debug_s {
  		argv = append(argv, "-gdwarf-2")
  	} else if Headtype == objabi.Hdarwin {
  		// Recent versions of macOS print
  		//	ld: warning: option -s is obsolete and being ignored
  		// so do not pass any arguments.
  	} else {
  		argv = append(argv, "-s")
  	}
  
  	switch Headtype {
  	case objabi.Hdarwin:
  		argv = append(argv, "-Wl,-headerpad,1144")
  		if l.DynlinkingGo() {
  			argv = append(argv, "-Wl,-flat_namespace")
  		} else if !SysArch.InFamily(sys.ARM64) {
  			argv = append(argv, "-Wl,-no_pie")
  		}
  	case objabi.Hopenbsd:
  		argv = append(argv, "-Wl,-nopie")
  	case objabi.Hwindows:
  		if windowsgui {
  			argv = append(argv, "-mwindows")
  		} else {
  			argv = append(argv, "-mconsole")
  		}
  	}
  
  	switch Buildmode {
  	case BuildmodeExe:
  		if Headtype == objabi.Hdarwin {
  			argv = append(argv, "-Wl,-pagezero_size,4000000")
  		}
  	case BuildmodePIE:
  		if UseRelro() {
  			argv = append(argv, "-Wl,-z,relro")
  		}
  		argv = append(argv, "-pie")
  	case BuildmodeCShared:
  		if Headtype == objabi.Hdarwin {
  			argv = append(argv, "-dynamiclib")
  			if SysArch.Family != sys.AMD64 {
  				argv = append(argv, "-Wl,-read_only_relocs,suppress")
  			}
  		} else {
  			// ELF.
  			argv = append(argv, "-Wl,-Bsymbolic")
  			if UseRelro() {
  				argv = append(argv, "-Wl,-z,relro")
  			}
  			// Pass -z nodelete to mark the shared library as
  			// non-closeable: a dlclose will do nothing.
  			argv = append(argv, "-shared", "-Wl,-z,nodelete")
  		}
  	case BuildmodeShared:
  		if UseRelro() {
  			argv = append(argv, "-Wl,-z,relro")
  		}
  		argv = append(argv, "-shared")
  	case BuildmodePlugin:
  		if Headtype == objabi.Hdarwin {
  			argv = append(argv, "-dynamiclib")
  		} else {
  			if UseRelro() {
  				argv = append(argv, "-Wl,-z,relro")
  			}
  			argv = append(argv, "-shared")
  		}
  	}
  
  	if Iself && l.DynlinkingGo() {
  		// We force all symbol resolution to be done at program startup
  		// because lazy PLT resolution can use large amounts of stack at
  		// times we cannot allow it to do so.
  		argv = append(argv, "-Wl,-znow")
  
  		// Do not let the host linker generate COPY relocations. These
  		// can move symbols out of sections that rely on stable offsets
  		// from the beginning of the section (like STYPE).
  		argv = append(argv, "-Wl,-znocopyreloc")
  
  		if SysArch.InFamily(sys.ARM, sys.ARM64) {
  			// On ARM, the GNU linker will generate COPY relocations
  			// even with -znocopyreloc set.
  			// https://sourceware.org/bugzilla/show_bug.cgi?id=19962
  			//
  			// On ARM64, the GNU linker will fail instead of
  			// generating COPY relocations.
  			//
  			// In both cases, switch to gold.
  			argv = append(argv, "-fuse-ld=gold")
  
  			// If gold is not installed, gcc will silently switch
  			// back to ld.bfd. So we parse the version information
  			// and provide a useful error if gold is missing.
  			cmd := exec.Command(*flagExtld, "-fuse-ld=gold", "-Wl,--version")
  			if out, err := cmd.CombinedOutput(); err == nil {
  				if !bytes.Contains(out, []byte("GNU gold")) {
  					log.Fatalf("ARM external linker must be gold (issue #15696), but is not: %s", out)
  				}
  			}
  		}
  	}
  
  	if Iself && len(buildinfo) > 0 {
  		argv = append(argv, fmt.Sprintf("-Wl,--build-id=0x%x", buildinfo))
  	}
  
  	// On Windows, given -o foo, GCC will append ".exe" to produce
  	// "foo.exe".  We have decided that we want to honor the -o
  	// option. To make this work, we append a '.' so that GCC
  	// will decide that the file already has an extension. We
  	// only want to do this when producing a Windows output file
  	// on a Windows host.
  	outopt := *flagOutfile
  	if objabi.GOOS == "windows" && runtime.GOOS == "windows" && filepath.Ext(outopt) == "" {
  		outopt += "."
  	}
  	argv = append(argv, "-o")
  	argv = append(argv, outopt)
  
  	if rpath.val != "" {
  		argv = append(argv, fmt.Sprintf("-Wl,-rpath,%s", rpath.val))
  	}
  
  	// Force global symbols to be exported for dlopen, etc.
  	if Iself {
  		argv = append(argv, "-rdynamic")
  	}
  
  	if strings.Contains(argv[0], "clang") {
  		argv = append(argv, "-Qunused-arguments")
  	}
  
  	argv = append(argv, filepath.Join(*flagTmpdir, "go.o"))
  	argv = append(argv, hostobjCopy()...)
  
  	if *FlagLinkshared {
  		seenDirs := make(map[string]bool)
  		seenLibs := make(map[string]bool)
  		addshlib := func(path string) {
  			dir, base := filepath.Split(path)
  			if !seenDirs[dir] {
  				argv = append(argv, "-L"+dir)
  				if !rpath.set {
  					argv = append(argv, "-Wl,-rpath="+dir)
  				}
  				seenDirs[dir] = true
  			}
  			base = strings.TrimSuffix(base, ".so")
  			base = strings.TrimPrefix(base, "lib")
  			if !seenLibs[base] {
  				argv = append(argv, "-l"+base)
  				seenLibs[base] = true
  			}
  		}
  		for _, shlib := range l.Shlibs {
  			addshlib(shlib.Path)
  			for _, dep := range shlib.Deps {
  				if dep == "" {
  					continue
  				}
  				libpath := findshlib(l, dep)
  				if libpath != "" {
  					addshlib(libpath)
  				}
  			}
  		}
  	}
  
  	argv = append(argv, ldflag...)
  
  	// When building a program with the default -buildmode=exe the
  	// gc compiler generates code requires DT_TEXTREL in a
  	// position independent executable (PIE). On systems where the
  	// toolchain creates PIEs by default, and where DT_TEXTREL
  	// does not work, the resulting programs will not run. See
  	// issue #17847. To avoid this problem pass -no-pie to the
  	// toolchain if it is supported.
  	if Buildmode == BuildmodeExe {
  		src := filepath.Join(*flagTmpdir, "trivial.c")
  		if err := ioutil.WriteFile(src, []byte("int main() { return 0; }"), 0666); err != nil {
  			Errorf(nil, "WriteFile trivial.c failed: %v", err)
  		}
  
  		// GCC uses -no-pie, clang uses -nopie.
  		for _, nopie := range []string{"-no-pie", "-nopie"} {
  			cmd := exec.Command(argv[0], nopie, "trivial.c")
  			cmd.Dir = *flagTmpdir
  			cmd.Env = append([]string{"LC_ALL=C"}, os.Environ()...)
  			out, err := cmd.CombinedOutput()
  			// GCC says "unrecognized command line option ‘-no-pie’"
  			// clang says "unknown argument: '-no-pie'"
  			supported := err == nil && !bytes.Contains(out, []byte("unrecognized")) && !bytes.Contains(out, []byte("unknown"))
  			if supported {
  				argv = append(argv, nopie)
  				break
  			}
  		}
  	}
  
  	for _, p := range strings.Fields(*flagExtldflags) {
  		argv = append(argv, p)
  
  		// clang, unlike GCC, passes -rdynamic to the linker
  		// even when linking with -static, causing a linker
  		// error when using GNU ld. So take out -rdynamic if
  		// we added it. We do it in this order, rather than
  		// only adding -rdynamic later, so that -*extldflags
  		// can override -rdynamic without using -static.
  		if Iself && p == "-static" {
  			for i := range argv {
  				if argv[i] == "-rdynamic" {
  					argv[i] = "-static"
  				}
  			}
  		}
  	}
  	if Headtype == objabi.Hwindows {
  		// use gcc linker script to work around gcc bug
  		// (see https://golang.org/issue/20183 for details).
  		p := writeGDBLinkerScript()
  		argv = append(argv, "-Wl,-T,"+p)
  		// libmingw32 and libmingwex have some inter-dependencies,
  		// so must use linker groups.
  		argv = append(argv, "-Wl,--start-group", "-lmingwex", "-lmingw32", "-Wl,--end-group")
  		argv = append(argv, peimporteddlls()...)
  	}
  
  	if l.Debugvlog != 0 {
  		l.Logf("%5.2f host link:", Cputime())
  		for _, v := range argv {
  			l.Logf(" %q", v)
  		}
  		l.Logf("\n")
  	}
  
  	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
  		Exitf("running %s failed: %v\n%s", argv[0], err, out)
  	} else if len(out) > 0 {
  		// always print external output even if the command is successful, so that we don't
  		// swallow linker warnings (see https://golang.org/issue/17935).
  		l.Logf("%s", out)
  	}
  
  	if !*FlagS && !*FlagW && !debug_s && Headtype == objabi.Hdarwin {
  		// Skip combining dwarf on arm.
  		if !SysArch.InFamily(sys.ARM, sys.ARM64) {
  			dsym := filepath.Join(*flagTmpdir, "go.dwarf")
  			if out, err := exec.Command("dsymutil", "-f", *flagOutfile, "-o", dsym).CombinedOutput(); err != nil {
  				Exitf("%s: running dsymutil failed: %v\n%s", os.Args[0], err, out)
  			}
  			// Skip combining if `dsymutil` didn't generate a file. See #11994.
  			if _, err := os.Stat(dsym); os.IsNotExist(err) {
  				return
  			}
  			// For os.Rename to work reliably, must be in same directory as outfile.
  			combinedOutput := *flagOutfile + "~"
  			if err := machoCombineDwarf(*flagOutfile, dsym, combinedOutput); err != nil {
  				Exitf("%s: combining dwarf failed: %v", os.Args[0], err)
  			}
  			os.Remove(*flagOutfile)
  			if err := os.Rename(combinedOutput, *flagOutfile); err != nil {
  				Exitf("%s: %v", os.Args[0], err)
  			}
  		}
  	}
  }
  
  // hostlinkArchArgs returns arguments to pass to the external linker
  // based on the architecture.
  func hostlinkArchArgs() []string {
  	switch SysArch.Family {
  	case sys.I386:
  		return []string{"-m32"}
  	case sys.AMD64, sys.PPC64, sys.S390X:
  		return []string{"-m64"}
  	case sys.ARM:
  		return []string{"-marm"}
  	case sys.ARM64:
  		// nothing needed
  	case sys.MIPS64:
  		return []string{"-mabi=64"}
  	case sys.MIPS:
  		return []string{"-mabi=32"}
  	}
  	return nil
  }
  
  // ldobj loads an input object. If it is a host object (an object
  // compiled by a non-Go compiler) it returns the Hostobj pointer. If
  // it is a Go object, it returns nil.
  func ldobj(ctxt *Link, f *bio.Reader, lib *Library, length int64, pn string, file string, whence int) *Hostobj {
  	pkg := objabi.PathToPrefix(lib.Pkg)
  
  	eof := f.Offset() + length
  	start := f.Offset()
  	c1 := bgetc(f)
  	c2 := bgetc(f)
  	c3 := bgetc(f)
  	c4 := bgetc(f)
  	f.Seek(start, 0)
  
  	magic := uint32(c1)<<24 | uint32(c2)<<16 | uint32(c3)<<8 | uint32(c4)
  	if magic == 0x7f454c46 { // \x7F E L F
  		return ldhostobj(ldelf, f, pkg, length, pn, file)
  	}
  
  	if magic&^1 == 0xfeedface || magic&^0x01000000 == 0xcefaedfe {
  		return ldhostobj(ldmacho, f, pkg, length, pn, file)
  	}
  
  	if c1 == 0x4c && c2 == 0x01 || c1 == 0x64 && c2 == 0x86 {
  		return ldhostobj(ldpe, f, pkg, length, pn, file)
  	}
  
  	/* check the header */
  	line, err := f.ReadString('\n')
  	if err != nil {
  		Errorf(nil, "truncated object file: %s: %v", pn, err)
  		return nil
  	}
  
  	if !strings.HasPrefix(line, "go object ") {
  		if strings.HasSuffix(pn, ".go") {
  			Exitf("%s: uncompiled .go source file", pn)
  			return nil
  		}
  
  		if line == SysArch.Name {
  			// old header format: just $GOOS
  			Errorf(nil, "%s: stale object file", pn)
  			return nil
  		}
  
  		Errorf(nil, "%s: not an object file", pn)
  		return nil
  	}
  
  	// First, check that the basic GOOS, GOARCH, and Version match.
  	t := fmt.Sprintf("%s %s %s ", objabi.GOOS, objabi.GOARCH, objabi.Version)
  
  	line = strings.TrimRight(line, "\n")
  	if !strings.HasPrefix(line[10:]+" ", t) && !*flagF {
  		Errorf(nil, "%s: object is [%s] expected [%s]", pn, line[10:], t)
  		return nil
  	}
  
  	// Second, check that longer lines match each other exactly,
  	// so that the Go compiler and write additional information
  	// that must be the same from run to run.
  	if len(line) >= len(t)+10 {
  		if theline == "" {
  			theline = line[10:]
  		} else if theline != line[10:] {
  			Errorf(nil, "%s: object is [%s] expected [%s]", pn, line[10:], theline)
  			return nil
  		}
  	}
  
  	/* skip over exports and other info -- ends with \n!\n */
  	import0 := f.Offset()
  
  	c1 = '\n' // the last line ended in \n
  	c2 = bgetc(f)
  	c3 = bgetc(f)
  	for c1 != '\n' || c2 != '!' || c3 != '\n' {
  		c1 = c2
  		c2 = c3
  		c3 = bgetc(f)
  		if c3 == -1 {
  			Errorf(nil, "truncated object file: %s", pn)
  			return nil
  		}
  	}
  
  	import1 := f.Offset()
  
  	f.Seek(import0, 0)
  	ldpkg(ctxt, f, pkg, import1-import0-2, pn, whence) // -2 for !\n
  	f.Seek(import1, 0)
  
  	LoadObjFile(ctxt, f, lib, eof-f.Offset(), pn)
  	return nil
  }
  
  func readelfsymboldata(ctxt *Link, f *elf.File, sym *elf.Symbol) []byte {
  	data := make([]byte, sym.Size)
  	sect := f.Sections[sym.Section]
  	if sect.Type != elf.SHT_PROGBITS && sect.Type != elf.SHT_NOTE {
  		Errorf(nil, "reading %s from non-data section", sym.Name)
  	}
  	n, err := sect.ReadAt(data, int64(sym.Value-sect.Addr))
  	if uint64(n) != sym.Size {
  		Errorf(nil, "reading contents of %s: %v", sym.Name, err)
  	}
  	return data
  }
  
  func readwithpad(r io.Reader, sz int32) ([]byte, error) {
  	data := make([]byte, Rnd(int64(sz), 4))
  	_, err := io.ReadFull(r, data)
  	if err != nil {
  		return nil, err
  	}
  	data = data[:sz]
  	return data, nil
  }
  
  func readnote(f *elf.File, name []byte, typ int32) ([]byte, error) {
  	for _, sect := range f.Sections {
  		if sect.Type != elf.SHT_NOTE {
  			continue
  		}
  		r := sect.Open()
  		for {
  			var namesize, descsize, noteType int32
  			err := binary.Read(r, f.ByteOrder, &namesize)
  			if err != nil {
  				if err == io.EOF {
  					break
  				}
  				return nil, fmt.Errorf("read namesize failed: %v", err)
  			}
  			err = binary.Read(r, f.ByteOrder, &descsize)
  			if err != nil {
  				return nil, fmt.Errorf("read descsize failed: %v", err)
  			}
  			err = binary.Read(r, f.ByteOrder, &noteType)
  			if err != nil {
  				return nil, fmt.Errorf("read type failed: %v", err)
  			}
  			noteName, err := readwithpad(r, namesize)
  			if err != nil {
  				return nil, fmt.Errorf("read name failed: %v", err)
  			}
  			desc, err := readwithpad(r, descsize)
  			if err != nil {
  				return nil, fmt.Errorf("read desc failed: %v", err)
  			}
  			if string(name) == string(noteName) && typ == noteType {
  				return desc, nil
  			}
  		}
  	}
  	return nil, nil
  }
  
  func findshlib(ctxt *Link, shlib string) string {
  	for _, libdir := range ctxt.Libdir {
  		libpath := filepath.Join(libdir, shlib)
  		if _, err := os.Stat(libpath); err == nil {
  			return libpath
  		}
  	}
  	Errorf(nil, "cannot find shared library: %s", shlib)
  	return ""
  }
  
  func ldshlibsyms(ctxt *Link, shlib string) {
  	libpath := findshlib(ctxt, shlib)
  	if libpath == "" {
  		return
  	}
  	for _, processedlib := range ctxt.Shlibs {
  		if processedlib.Path == libpath {
  			return
  		}
  	}
  	if ctxt.Debugvlog > 1 {
  		ctxt.Logf("%5.2f ldshlibsyms: found library with name %s at %s\n", Cputime(), shlib, libpath)
  	}
  
  	f, err := elf.Open(libpath)
  	if err != nil {
  		Errorf(nil, "cannot open shared library: %s", libpath)
  		return
  	}
  
  	hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
  	if err != nil {
  		Errorf(nil, "cannot read ABI hash from shared library %s: %v", libpath, err)
  		return
  	}
  
  	depsbytes, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GODEPS_TAG)
  	if err != nil {
  		Errorf(nil, "cannot read dep list from shared library %s: %v", libpath, err)
  		return
  	}
  	deps := strings.Split(string(depsbytes), "\n")
  
  	syms, err := f.DynamicSymbols()
  	if err != nil {
  		Errorf(nil, "cannot read symbols from shared library: %s", libpath)
  		return
  	}
  	gcdataLocations := make(map[uint64]*Symbol)
  	for _, elfsym := range syms {
  		if elf.ST_TYPE(elfsym.Info) == elf.STT_NOTYPE || elf.ST_TYPE(elfsym.Info) == elf.STT_SECTION {
  			continue
  		}
  		lsym := ctxt.Syms.Lookup(elfsym.Name, 0)
  		// Because loadlib above loads all .a files before loading any shared
  		// libraries, any non-dynimport symbols we find that duplicate symbols
  		// already loaded should be ignored (the symbols from the .a files
  		// "win").
  		if lsym.Type != 0 && lsym.Type != SDYNIMPORT {
  			continue
  		}
  		lsym.Type = SDYNIMPORT
  		lsym.ElfType = elf.ST_TYPE(elfsym.Info)
  		lsym.Size = int64(elfsym.Size)
  		if elfsym.Section != elf.SHN_UNDEF {
  			// Set .File for the library that actually defines the symbol.
  			lsym.File = libpath
  			// The decodetype_* functions in decodetype.go need access to
  			// the type data.
  			if strings.HasPrefix(lsym.Name, "type.") && !strings.HasPrefix(lsym.Name, "type..") {
  				lsym.P = readelfsymboldata(ctxt, f, &elfsym)
  				gcdataLocations[elfsym.Value+2*uint64(SysArch.PtrSize)+8+1*uint64(SysArch.PtrSize)] = lsym
  			}
  		}
  	}
  	gcdataAddresses := make(map[*Symbol]uint64)
  	if SysArch.Family == sys.ARM64 {
  		for _, sect := range f.Sections {
  			if sect.Type == elf.SHT_RELA {
  				var rela elf.Rela64
  				rdr := sect.Open()
  				for {
  					err := binary.Read(rdr, f.ByteOrder, &rela)
  					if err == io.EOF {
  						break
  					} else if err != nil {
  						Errorf(nil, "reading relocation failed %v", err)
  						return
  					}
  					t := elf.R_AARCH64(rela.Info & 0xffff)
  					if t != elf.R_AARCH64_RELATIVE {
  						continue
  					}
  					if lsym, ok := gcdataLocations[rela.Off]; ok {
  						gcdataAddresses[lsym] = uint64(rela.Addend)
  					}
  				}
  			}
  		}
  	}
  
  	ctxt.Shlibs = append(ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps, File: f, gcdataAddresses: gcdataAddresses})
  }
  
  func addsection(seg *Segment, name string, rwx int) *Section {
  	sect := new(Section)
  	sect.Rwx = uint8(rwx)
  	sect.Name = name
  	sect.Seg = seg
  	sect.Align = int32(SysArch.PtrSize) // everything is at least pointer-aligned
  	seg.Sections = append(seg.Sections, sect)
  	return sect
  }
  
  func Le16(b []byte) uint16 {
  	return uint16(b[0]) | uint16(b[1])<<8
  }
  
  func Le32(b []byte) uint32 {
  	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
  }
  
  func Le64(b []byte) uint64 {
  	return uint64(Le32(b)) | uint64(Le32(b[4:]))<<32
  }
  
  func Be16(b []byte) uint16 {
  	return uint16(b[0])<<8 | uint16(b[1])
  }
  
  func Be32(b []byte) uint32 {
  	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])
  }
  
  type chain struct {
  	sym   *Symbol
  	up    *chain
  	limit int // limit on entry to sym
  }
  
  var morestack *Symbol
  
  // TODO: Record enough information in new object files to
  // allow stack checks here.
  
  func haslinkregister(ctxt *Link) bool {
  	return ctxt.FixedFrameSize() != 0
  }
  
  func callsize(ctxt *Link) int {
  	if haslinkregister(ctxt) {
  		return 0
  	}
  	return SysArch.RegSize
  }
  
  func (ctxt *Link) dostkcheck() {
  	var ch chain
  
  	morestack = ctxt.Syms.Lookup("runtime.morestack", 0)
  
  	// Every splitting function ensures that there are at least StackLimit
  	// bytes available below SP when the splitting prologue finishes.
  	// If the splitting function calls F, then F begins execution with
  	// at least StackLimit - callsize() bytes available.
  	// Check that every function behaves correctly with this amount
  	// of stack, following direct calls in order to piece together chains
  	// of non-splitting functions.
  	ch.up = nil
  
  	ch.limit = objabi.StackLimit - callsize(ctxt)
  
  	// Check every function, but do the nosplit functions in a first pass,
  	// to make the printed failure chains as short as possible.
  	for _, s := range ctxt.Textp {
  		// runtime.racesymbolizethunk is called from gcc-compiled C
  		// code running on the operating system thread stack.
  		// It uses more than the usual amount of stack but that's okay.
  		if s.Name == "runtime.racesymbolizethunk" {
  			continue
  		}
  
  		if s.Attr.NoSplit() {
  			ch.sym = s
  			stkcheck(ctxt, &ch, 0)
  		}
  	}
  
  	for _, s := range ctxt.Textp {
  		if !s.Attr.NoSplit() {
  			ch.sym = s
  			stkcheck(ctxt, &ch, 0)
  		}
  	}
  }
  
  func stkcheck(ctxt *Link, up *chain, depth int) int {
  	limit := up.limit
  	s := up.sym
  
  	// Don't duplicate work: only need to consider each
  	// function at top of safe zone once.
  	top := limit == objabi.StackLimit-callsize(ctxt)
  	if top {
  		if s.Attr.StackCheck() {
  			return 0
  		}
  		s.Attr |= AttrStackCheck
  	}
  
  	if depth > 100 {
  		Errorf(s, "nosplit stack check too deep")
  		stkbroke(ctxt, up, 0)
  		return -1
  	}
  
  	if s.Attr.External() || s.FuncInfo == nil {
  		// external function.
  		// should never be called directly.
  		// onlyctxt.Diagnose the direct caller.
  		// TODO(mwhudson): actually think about this.
  		if depth == 1 && s.Type != SXREF && !ctxt.DynlinkingGo() &&
  			Buildmode != BuildmodeCArchive && Buildmode != BuildmodePIE && Buildmode != BuildmodeCShared && Buildmode != BuildmodePlugin {
  
  			Errorf(s, "call to external function")
  		}
  		return -1
  	}
  
  	if limit < 0 {
  		stkbroke(ctxt, up, limit)
  		return -1
  	}
  
  	// morestack looks like it calls functions,
  	// but it switches the stack pointer first.
  	if s == morestack {
  		return 0
  	}
  
  	var ch chain
  	ch.up = up
  
  	if !s.Attr.NoSplit() {
  		// Ensure we have enough stack to call morestack.
  		ch.limit = limit - callsize(ctxt)
  		ch.sym = morestack
  		if stkcheck(ctxt, &ch, depth+1) < 0 {
  			return -1
  		}
  		if !top {
  			return 0
  		}
  		// Raise limit to allow frame.
  		locals := int32(0)
  		if s.FuncInfo != nil {
  			locals = s.FuncInfo.Locals
  		}
  		limit = int(objabi.StackLimit+locals) + int(ctxt.FixedFrameSize())
  	}
  
  	// Walk through sp adjustments in function, consuming relocs.
  	ri := 0
  
  	endr := len(s.R)
  	var ch1 chain
  	var pcsp Pciter
  	var r *Reloc
  	for pciterinit(ctxt, &pcsp, &s.FuncInfo.Pcsp); pcsp.done == 0; pciternext(&pcsp) {
  		// pcsp.value is in effect for [pcsp.pc, pcsp.nextpc).
  
  		// Check stack size in effect for this span.
  		if int32(limit)-pcsp.value < 0 {
  			stkbroke(ctxt, up, int(int32(limit)-pcsp.value))
  			return -1
  		}
  
  		// Process calls in this span.
  		for ; ri < endr && uint32(s.R[ri].Off) < pcsp.nextpc; ri++ {
  			r = &s.R[ri]
  			switch r.Type {
  			// Direct call.
  			case objabi.R_CALL, objabi.R_CALLARM, objabi.R_CALLARM64, objabi.R_CALLPOWER, objabi.R_CALLMIPS:
  				ch.limit = int(int32(limit) - pcsp.value - int32(callsize(ctxt)))
  				ch.sym = r.Sym
  				if stkcheck(ctxt, &ch, depth+1) < 0 {
  					return -1
  				}
  
  			// Indirect call. Assume it is a call to a splitting function,
  			// so we have to make sure it can call morestack.
  			// Arrange the data structures to report both calls, so that
  			// if there is an error, stkprint shows all the steps involved.
  			case objabi.R_CALLIND:
  				ch.limit = int(int32(limit) - pcsp.value - int32(callsize(ctxt)))
  
  				ch.sym = nil
  				ch1.limit = ch.limit - callsize(ctxt) // for morestack in called prologue
  				ch1.up = &ch
  				ch1.sym = morestack
  				if stkcheck(ctxt, &ch1, depth+2) < 0 {
  					return -1
  				}
  			}
  		}
  	}
  
  	return 0
  }
  
  func stkbroke(ctxt *Link, ch *chain, limit int) {
  	Errorf(ch.sym, "nosplit stack overflow")
  	stkprint(ctxt, ch, limit)
  }
  
  func stkprint(ctxt *Link, ch *chain, limit int) {
  	var name string
  
  	if ch.sym != nil {
  		name = ch.sym.Name
  		if ch.sym.Attr.NoSplit() {
  			name += " (nosplit)"
  		}
  	} else {
  		name = "function pointer"
  	}
  
  	if ch.up == nil {
  		// top of chain.  ch->sym != nil.
  		if ch.sym.Attr.NoSplit() {
  			fmt.Printf("\t%d\tassumed on entry to %s\n", ch.limit, name)
  		} else {
  			fmt.Printf("\t%d\tguaranteed after split check in %s\n", ch.limit, name)
  		}
  	} else {
  		stkprint(ctxt, ch.up, ch.limit+callsize(ctxt))
  		if !haslinkregister(ctxt) {
  			fmt.Printf("\t%d\ton entry to %s\n", ch.limit, name)
  		}
  	}
  
  	if ch.limit != limit {
  		fmt.Printf("\t%d\tafter %s uses %d\n", limit, name, ch.limit-limit)
  	}
  }
  
  func Cflush() {
  	if err := coutbuf.w.Flush(); err != nil {
  		Exitf("flushing %s: %v", coutbuf.f.Name(), err)
  	}
  }
  
  func Cseek(p int64) {
  	if p == coutbuf.off {
  		return
  	}
  	Cflush()
  	if _, err := coutbuf.f.Seek(p, 0); err != nil {
  		Exitf("seeking in output [0, 1]: %v", err)
  	}
  	coutbuf.off = p
  }
  
  func Cwritestring(s string) {
  	coutbuf.WriteString(s)
  }
  
  func Cwrite(p []byte) {
  	coutbuf.Write(p)
  }
  
  func Cput(c uint8) {
  	coutbuf.w.WriteByte(c)
  	coutbuf.off++
  }
  
  func usage() {
  	fmt.Fprintf(os.Stderr, "usage: link [options] main.o\n")
  	objabi.Flagprint(2)
  	Exit(2)
  }
  
  func doversion() {
  	Exitf("version %s", objabi.Version)
  }
  
  type SymbolType int8
  
  const (
  	TextSym      SymbolType = 'T'
  	DataSym                 = 'D'
  	BSSSym                  = 'B'
  	UndefinedSym            = 'U'
  	TLSSym                  = 't'
  	FileSym                 = 'f'
  	FrameSym                = 'm'
  	ParamSym                = 'p'
  	AutoSym                 = 'a'
  )
  
  func genasmsym(ctxt *Link, put func(*Link, *Symbol, string, SymbolType, int64, *Symbol)) {
  	// These symbols won't show up in the first loop below because we
  	// skip STEXT symbols. Normal STEXT symbols are emitted by walking textp.
  	s := ctxt.Syms.Lookup("runtime.text", 0)
  	if s.Type == STEXT {
  		put(ctxt, s, s.Name, TextSym, s.Value, nil)
  	}
  
  	n := 0
  
  	// Generate base addresses for all text sections if there are multiple
  	for _, sect := range Segtext.Sections {
  		if n == 0 {
  			n++
  			continue
  		}
  		if sect.Name != ".text" {
  			break
  		}
  		s = ctxt.Syms.ROLookup(fmt.Sprintf("runtime.text.%d", n), 0)
  		if s == nil {
  			break
  		}
  		if s.Type == STEXT {
  			put(ctxt, s, s.Name, TextSym, s.Value, nil)
  		}
  		n++
  	}
  
  	s = ctxt.Syms.Lookup("runtime.etext", 0)
  	if s.Type == STEXT {
  		put(ctxt, s, s.Name, TextSym, s.Value, nil)
  	}
  
  	for _, s := range ctxt.Syms.Allsym {
  		if s.Attr.NotInSymbolTable() {
  			continue
  		}
  		if (s.Name == "" || s.Name[0] == '.') && s.Version == 0 && s.Name != ".rathole" && s.Name != ".TOC." {
  			continue
  		}
  		switch s.Type & SMASK {
  		case SCONST,
  			SRODATA,
  			SSYMTAB,
  			SPCLNTAB,
  			SINITARR,
  			SDATA,
  			SNOPTRDATA,
  			SELFROSECT,
  			SMACHOGOT,
  			STYPE,
  			SSTRING,
  			SGOSTRING,
  			SGOFUNC,
  			SGCBITS,
  			STYPERELRO,
  			SSTRINGRELRO,
  			SGOSTRINGRELRO,
  			SGOFUNCRELRO,
  			SGCBITSRELRO,
  			SRODATARELRO,
  			STYPELINK,
  			SITABLINK,
  			SWINDOWS:
  			if !s.Attr.Reachable() {
  				continue
  			}
  			put(ctxt, s, s.Name, DataSym, Symaddr(s), s.Gotype)
  
  		case SBSS, SNOPTRBSS:
  			if !s.Attr.Reachable() {
  				continue
  			}
  			if len(s.P) > 0 {
  				Errorf(s, "should not be bss (size=%d type=%v special=%v)", len(s.P), s.Type, s.Attr.Special())
  			}
  			put(ctxt, s, s.Name, BSSSym, Symaddr(s), s.Gotype)
  
  		case SFILE:
  			put(ctxt, nil, s.Name, FileSym, s.Value, nil)
  
  		case SHOSTOBJ:
  			if Headtype == objabi.Hwindows || Iself {
  				put(ctxt, s, s.Name, UndefinedSym, s.Value, nil)
  			}
  
  		case SDYNIMPORT:
  			if !s.Attr.Reachable() {
  				continue
  			}
  			put(ctxt, s, s.Extname, UndefinedSym, 0, nil)
  
  		case STLSBSS:
  			if Linkmode == LinkExternal {
  				put(ctxt, s, s.Name, TLSSym, Symaddr(s), s.Gotype)
  			}
  		}
  	}
  
  	var off int32
  	for _, s := range ctxt.Textp {
  		put(ctxt, s, s.Name, TextSym, s.Value, s.Gotype)
  
  		locals := int32(0)
  		if s.FuncInfo != nil {
  			locals = s.FuncInfo.Locals
  		}
  		// NOTE(ality): acid can't produce a stack trace without .frame symbols
  		put(ctxt, nil, ".frame", FrameSym, int64(locals)+int64(SysArch.PtrSize), nil)
  
  		if s.FuncInfo == nil {
  			continue
  		}
  		for _, a := range s.FuncInfo.Autom {
  			// Emit a or p according to actual offset, even if label is wrong.
  			// This avoids negative offsets, which cannot be encoded.
  			if a.Name != objabi.A_AUTO && a.Name != objabi.A_PARAM {
  				continue
  			}
  
  			// compute offset relative to FP
  			if a.Name == objabi.A_PARAM {
  				off = a.Aoffset
  			} else {
  				off = a.Aoffset - int32(SysArch.PtrSize)
  			}
  
  			// FP
  			if off >= 0 {
  				put(ctxt, nil, a.Asym.Name, ParamSym, int64(off), a.Gotype)
  				continue
  			}
  
  			// SP
  			if off <= int32(-SysArch.PtrSize) {
  				put(ctxt, nil, a.Asym.Name, AutoSym, -(int64(off) + int64(SysArch.PtrSize)), a.Gotype)
  				continue
  			}
  			// Otherwise, off is addressing the saved program counter.
  			// Something underhanded is going on. Say nothing.
  		}
  	}
  
  	if ctxt.Debugvlog != 0 || *flagN {
  		ctxt.Logf("%5.2f symsize = %d\n", Cputime(), uint32(Symsize))
  	}
  }
  
  func Symaddr(s *Symbol) int64 {
  	if !s.Attr.Reachable() {
  		Errorf(s, "unreachable symbol in symaddr")
  	}
  	return s.Value
  }
  
  func (ctxt *Link) xdefine(p string, t SymKind, v int64) {
  	s := ctxt.Syms.Lookup(p, 0)
  	s.Type = t
  	s.Value = v
  	s.Attr |= AttrReachable
  	s.Attr |= AttrSpecial
  	s.Attr |= AttrLocal
  }
  
  func datoff(s *Symbol, addr int64) int64 {
  	if uint64(addr) >= Segdata.Vaddr {
  		return int64(uint64(addr) - Segdata.Vaddr + Segdata.Fileoff)
  	}
  	if uint64(addr) >= Segtext.Vaddr {
  		return int64(uint64(addr) - Segtext.Vaddr + Segtext.Fileoff)
  	}
  	Errorf(s, "invalid datoff %#x", addr)
  	return 0
  }
  
  func Entryvalue(ctxt *Link) int64 {
  	a := *flagEntrySymbol
  	if a[0] >= '0' && a[0] <= '9' {
  		return atolwhex(a)
  	}
  	s := ctxt.Syms.Lookup(a, 0)
  	if s.Type == 0 {
  		return *FlagTextAddr
  	}
  	if s.Type != STEXT {
  		Errorf(s, "entry not text")
  	}
  	return s.Value
  }
  
  func undefsym(ctxt *Link, s *Symbol) {
  	var r *Reloc
  
  	for i := 0; i < len(s.R); i++ {
  		r = &s.R[i]
  		if r.Sym == nil { // happens for some external ARM relocs
  			continue
  		}
  		if r.Sym.Type == Sxxx || r.Sym.Type == SXREF {
  			Errorf(s, "undefined: %q", r.Sym.Name)
  		}
  		if !r.Sym.Attr.Reachable() && r.Type != objabi.R_WEAKADDROFF {
  			Errorf(s, "relocation target %q", r.Sym.Name)
  		}
  	}
  }
  
  func (ctxt *Link) undef() {
  	for _, s := range ctxt.Textp {
  		undefsym(ctxt, s)
  	}
  	for _, s := range datap {
  		undefsym(ctxt, s)
  	}
  	if nerrors > 0 {
  		errorexit()
  	}
  }
  
  func (ctxt *Link) callgraph() {
  	if !*FlagC {
  		return
  	}
  
  	var i int
  	var r *Reloc
  	for _, s := range ctxt.Textp {
  		for i = 0; i < len(s.R); i++ {
  			r = &s.R[i]
  			if r.Sym == nil {
  				continue
  			}
  			if (r.Type == objabi.R_CALL || r.Type == objabi.R_CALLARM || r.Type == objabi.R_CALLPOWER || r.Type == objabi.R_CALLMIPS) && r.Sym.Type == STEXT {
  				ctxt.Logf("%s calls %s\n", s.Name, r.Sym.Name)
  			}
  		}
  	}
  }
  
  func Rnd(v int64, r int64) int64 {
  	if r <= 0 {
  		return v
  	}
  	v += r - 1
  	c := v % r
  	if c < 0 {
  		c += r
  	}
  	v -= c
  	return v
  }
  
  func bgetc(r *bio.Reader) int {
  	c, err := r.ReadByte()
  	if err != nil {
  		if err != io.EOF {
  			log.Fatalf("reading input: %v", err)
  		}
  		return -1
  	}
  	return int(c)
  }
  
  type markKind uint8 // for postorder traversal
  const (
  	unvisited markKind = iota
  	visiting
  	visited
  )
  
  func postorder(libs []*Library) []*Library {
  	order := make([]*Library, 0, len(libs)) // hold the result
  	mark := make(map[*Library]markKind, len(libs))
  	for _, lib := range libs {
  		dfs(lib, mark, &order)
  	}
  	return order
  }
  
  func dfs(lib *Library, mark map[*Library]markKind, order *[]*Library) {
  	if mark[lib] == visited {
  		return
  	}
  	if mark[lib] == visiting {
  		panic("found import cycle while visiting " + lib.Pkg)
  	}
  	mark[lib] = visiting
  	for _, i := range lib.imports {
  		dfs(i, mark, order)
  	}
  	mark[lib] = visited
  	*order = append(*order, lib)
  }
  

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