func (b *block) toUSTAR() *headerUSTAR { return (*headerUSTAR)(b) }
func (b *block) toSparse() sparseArray { return sparseArray(b[:]) }

// getFormat checks that the block is a valid tar header based on the checksum.
// It then attempts to guess the specific format based on magic values.
// If the checksum fails, then FormatUnknown is returned.
func (b *block) getFormat() Format {
	// Verify checksum.
	var p parser

	}
	b = b[22:] // skip over most of the header
	filenameLen := int(b.uint16())
	extraLen := int(b.uint16())
	return int64(fileHeaderLen + filenameLen + extraLen), nil
}

// readDirectoryHeader attempts to read a directory header from r.
// It returns io.ErrUnexpectedEOF if it cannot read a complete header,
// and ErrFormat if it doesn't find a valid header signature.
func readDirectoryHeader(f *File, r io.Reader) error {

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build ignore

// detect attempts to autodetect the correct
// values of the environment variables
// used by go_ios_exec.
// detect shells out to ideviceinfo, a third party program that can
// be obtained by following the instructions at
// https://github.com/libimobiledevice/libimobiledevice.

		nproc = 7
	}
	if setNproc {
		var rlim syscall.Rlimit
		if syscall.Getrlimit(nproc, &rlim) == nil {
			max := int(rlim.Cur) / (threads + 5)
			if attempts > max {
				t.Logf("lowering attempts from %d to %d for RLIMIT_NPROC", attempts, max)
				attempts = max
			}
		}
	}

	if os.Getenv("test18146") == "exec" {
		runtime.GOMAXPROCS(1)
		for n := threads; n > 0; n-- {
			go func() {
				for {

					f.Preamble += commentText(cg) + "\n"
					f.Preamble += "#line 1 \"cgo-generated-wrapper\"\n"
				}
			}

		case *ast.FuncDecl:
			// Also, reject attempts to declare methods on C.T or *C.T.
			// (The generated code would otherwise accept this
			// invalid input; see issue #57926.)
			if decl.Recv != nil && len(decl.Recv.List) > 0 {
				recvType := decl.Recv.List[0].Type

Also for security reasons, only a limited set of characters are
permitted, notably alphanumeric characters and a few symbols, such as
'.', that will not be interpreted in unexpected ways. Attempts to use
forbidden characters will get a "malformed #cgo argument" error.

When building, the CGO_CFLAGS, CGO_CPPFLAGS, CGO_CXXFLAGS, CGO_FFLAGS and
CGO_LDFLAGS environment variables are added to the flags derived from

		0xff, 0xff, 0xff, 0x20, 0x00,
	}
	defer func() {
		if r := recover(); r != nil {
			t.Fatalf("NewReader panicked: %s", r)
		}
	}()
	// Previously, this would trigger a panic as we attempt to read from
	// an io.SectionReader which would access a slice at a negative offset
	// as the section reader offset & size were < 0.
	NewReader(bytes.NewReader(data), int64(len(data))+1875)

		}
		return int64(x)
	}

	// Normal case is base-8 (octal) format.
	return p.parseOctal(b)
}

// formatNumeric encodes x into b using base-8 (octal) encoding if possible.
// Otherwise it will attempt to use base-256 (binary) encoding.
func (f *formatter) formatNumeric(b []byte, x int64) {
	if fitsInOctal(len(b), x) {
		f.formatOctal(b, x)
		return
	}

	if fitsInBase256(len(b), x) {

			// type and source locations of constant strings)
			// like the second arg in the call below:
			//
			//     myfunction(42, "foo")
			//
			// If a var has no name we won't see attempts to
			// refer to it via "C.<name>", so skip these vars
			//
			// See issue 53000 for more context.
			if name == "" {
				break
			}
			typOff, _ := e.Val(dwarf.AttrType).(dwarf.Offset)

			if cnt != v.cnt {
				t.Errorf("test %d, NewReader(%s) with %s discard: got %d headers, want %d headers",
					i, s1, s2, cnt, v.cnt)
			}
		}
	}
}

// TestReadHeaderOnly tests that Reader does not attempt to read special
// header-only files.
func TestReadHeaderOnly(t *testing.T) {
	f, err := os.Open("testdata/hdr-only.tar")
	if err != nil {
		t.Fatalf("unexpected error: %v", err)
	}
	defer f.Close()