linker in external linking mode.

By default, cmd/link will decide the linking mode as follows: if the only
packages using cgo are those on a list of known standard library
packages (net, os/user, runtime/cgo), cmd/link will use internal linking
mode. Otherwise, there are non-standard cgo packages involved, and cmd/link
will use external linking mode. The first rule means that a build of

		}
	}

	// Check basic fields.
	var blk block
	v7 := blk.toV7()
	ustar := blk.toUSTAR()
	gnu := blk.toGNU()
	verifyString(h.Name, len(v7.name()), "Name", paxPath)
	verifyString(h.Linkname, len(v7.linkName()), "Linkname", paxLinkpath)
	verifyString(h.Uname, len(ustar.userName()), "Uname", paxUname)
	verifyString(h.Gname, len(ustar.groupName()), "Gname", paxGname)
	verifyNumeric(h.Mode, len(v7.mode()), "Mode", paxNone)

			Typeflag:   '0',
			Uname:      "alex",
			Gname:      "wheel",
			AccessTime: time.Unix(1389782991, 419875220),
			ChangeTime: time.Unix(1389782956, 794414986),
			Xattrs: map[string]string{
				"user.key":  "value",
				"user.key2": "value2",
				// Interestingly, selinux encodes the terminating null inside the xattr
				"security.selinux": "unconfined_u:object_r:default_t:s0\x00",
			},
			PAXRecords: map[string]string{

const (
	magicGNU, versionGNU     = "ustar ", " \x00"
	magicUSTAR, versionUSTAR = "ustar\x00", "00"
	trailerSTAR              = "tar\x00"
)

// Size constants from various tar specifications.
const (
	blockSize  = 512 // Size of each block in a tar stream
	nameSize   = 100 // Max length of the name field in USTAR format
	prefixSize = 155 // Max length of the prefix field in USTAR format

// data and a flag, atEOF, that reports whether the [Reader] has no more data
// to give. The return values are the number of bytes to advance the input
// and the next token to return to the user, if any, plus an error, if any.
//
// Scanning stops if the function returns an error, in which case some of
// the input may be discarded. If that error is [ErrFinalToken], scanning

	// and must use forward slashes instead of back slashes. A trailing slash
	// indicates that this file is a directory and should have no data.
	Name string

	// Comment is any arbitrary user-defined string shorter than 64KiB.
	Comment string

	// NonUTF8 indicates that Name and Comment are not encoded in UTF-8.
	//
	// By specification, the only other encoding permitted should be CP-437,

</li>

<li>
<code>SB</code>: Static base pointer: global symbols.
</li>

<li>
<code>SP</code>: Stack pointer: the highest address within the local stack frame.
</li>

</ul>

<p>
All user-defined symbols are written as offsets to the pseudo-registers
<code>FP</code> (arguments and locals) and <code>SB</code> (globals).
</p>

<p>

	fmtNum(v7.uid(), int64(hdr.Uid))
	fmtNum(v7.gid(), int64(hdr.Gid))
	fmtNum(v7.size(), hdr.Size)
	fmtNum(v7.modTime(), modTime.Unix())

	ustar := tw.blk.toUSTAR()
	fmtStr(ustar.userName(), hdr.Uname)
	fmtStr(ustar.groupName(), hdr.Gname)
	fmtNum(ustar.devMajor(), hdr.Devmajor)
	fmtNum(ustar.devMinor(), hdr.Devminor)

	return &tw.blk
}

// writeRawFile writes a minimal file with the given name and flag type.

				Gname:    "guillaume",
				ModTime:  time.Unix(1399583047, 0),
			}, nil},
		},
	}, {
		// This file was produced using GNU tar v1.17.
		//	gnutar -b 4 --format=ustar (longname/)*15 + file.txt
		file: "testdata/ustar.tar",
		tests: []testFnc{
			testHeader{Header{
				Typeflag: TypeReg,
				Name:     strings.Repeat("longname/", 15) + "file.txt",
				Size:     6,
				Mode:     0644,

	if format > formatV7 {
		ustar := tr.blk.toUSTAR()
		hdr.Uname = p.parseString(ustar.userName())
		hdr.Gname = p.parseString(ustar.groupName())
		hdr.Devmajor = p.parseNumeric(ustar.devMajor())
		hdr.Devminor = p.parseNumeric(ustar.devMinor())

		var prefix string
		switch {
		case format.has(FormatUSTAR | FormatPAX):
			hdr.Format = format
			ustar := tr.blk.toUSTAR()