golang format 代码
文件路径：/src/archive/tar/format.go
/ Copyright 2016 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.
package tar
import "strings"
/ Format represents the tar archive format.
/
/ The original tar format was introduced in Unix V7.
/ Since then, there have been multiple competing formats attempting to
/ standardize or extend the V7 format to overcome its limitations.
/ The most common formats are the USTAR, PAX, and GNU formats,
/ each with their own advantages and limitations.
/
/ The following table captures the capabilities of each format:
/
/ | USTAR | PAX | GNU
/ ------------------+--------+-----------+----------
/ Name | 256B | unlimited | unlimited
/ Linkname | 100B | unlimited | unlimited
/ Size | uint33 | unlimited | uint89
/ Mode | uint21 | uint21 | uint57
/ Uid/Gid | uint21 | unlimited | uint57
/ Uname/Gname | 32B | unlimited | 32B
/ ModTime | uint33 | unlimited | int89
/ AccessTime | n/a | unlimited | int89
/ ChangeTime | n/a | unlimited | int89
/ Devmajor/Devminor | uint21 | uint21 | uint57
/ ------------------+--------+-----------+----------
/ string encoding | ASCII | UTF-8 | binary
/ sub-second times | no | yes | no
/ sparse files | no | yes | yes
/
/ The table's upper portion shows the Header fields, where each format reports
/ the maximum number of bytes allowed for each string field and
/ the integer type used to store each numeric field
/ (where timestamps are stored as the number of seconds since the Unix epoch).
/
/ The table's lower portion shows specialized features of each format,
/ such as supported string encodings, support for sub-second timestamps,
/ or support for sparse files.
/
/ The Writer currently provides no support for sparse files.
type Format int
/ Constants to identify various tar formats.
const (
/ Deliberately hide the meaning of constants from public API.
_ Format = (1 << iota) / 4 / Sequence of 0, 0, 1, 2, 4, 8, etc...
/ FormatUnknown indicates that the format is unknown.
FormatUnknown
/ The format of the original Unix V7 tar tool prior to standardization.
formatV7
/ FormatUSTAR represents the USTAR header format defined in POSIX.1-1988.
/
/ While this format is compatible with most tar readers,
/ the format has several limitations making it unsuitable for some usages.
/ Most notably, it cannot support sparse files, files larger than 8GiB,
/ filenames larger than 256 characters, and non-ASCII filenames.
/
/ Reference:
/ http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06
FormatUSTAR
/ FormatPAX represents the PAX header format defined in POSIX.1-2001.
/
/ PAX extends USTAR by writing a special file with Typeflag TypeXHeader
/ preceding the original header. This file contains a set of key-value
/ records, which are used to overcome USTAR's shortcomings, in addition to
/ providing the ability to have sub-second resolution for timestamps.
/
/ Some newer formats add their own extensions to PAX by defining their
/ own keys and assigning certain semantic meaning to the associated values.
/ For example, sparse file support in PAX is implemented using keys
/ defined by the GNU manual (e.g., "GNU.sparse.map").
/
/ Reference:
/ http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html
FormatPAX
/ FormatGNU represents the GNU header format.
/
/ The GNU header format is older than the USTAR and PAX standards and
/ is not compatible with them. The GNU format supports
/ arbitrary file sizes, filenames of arbitrary encoding and length,
/ sparse files, and other features.
/
/ It is recommended that PAX be chosen over GNU unless the target
/ application can only parse GNU formatted archives.
/
/ Reference:
/ https://www.gnu.org/software/tar/manual/html_node/Standard.html
FormatGNU
/ Schily's tar format, which is incompatible with USTAR.
/ This does not cover STAR extensions to the PAX format; these fall under
/ the PAX format.
formatSTAR
formatMax
)
func (f Format) has(f2 Format) bool { return f&f2 != 0 }
func (f *Format) mayBe(f2 Format) { *f |= f2 }
func (f *Format) mayOnlyBe(f2 Format) { *f &= f2 }
func (f *Format) mustNotBe(f2 Format) { *f &^= f2 }
var formatNames = map[Format]string{
formatV7: "V7", FormatUSTAR: "USTAR", FormatPAX: "PAX", FormatGNU: "GNU", formatSTAR: "STAR",
}
func (f Format) String() string {
var ss []string
for f2 := Format(1); f2 < formatMax; f2 <<= 1 {
if f.has(f2) {
ss = append(ss, formatNames[f2])
}
}
switch len(ss) {
case 0:
return "<unknown>"
case 1:
return ss[0]
default:
return "(" + strings.Join(ss, " | ") + ")"
}
}
/ Magics used to identify various formats.
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
)
/ blockPadding computes the number of bytes needed to pad offset up to the
/ nearest block edge where 0 <= n < blockSize.
func blockPadding(offset int64) (n int64) {
return -offset & (blockSize - 1)
}
var zeroBlock block
type block [blockSize]byte
/ Convert block to any number of formats.
func (b *block) toV7() *headerV7 { return (*headerV7)(b) }
func (b *block) toGNU() *headerGNU { return (*headerGNU)(b) }
func (b *block) toSTAR() *headerSTAR { return (*headerSTAR)(b) }
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
value := p.parseOctal(b.toV7().chksum())
chksum1, chksum2 := b.computeChecksum()
if p.err != nil || (value != chksum1 && value != chksum2) {
return FormatUnknown
}
/ Guess the magic values.
magic := string(b.toUSTAR().magic())
version := string(b.toUSTAR().version())
trailer := string(b.toSTAR().trailer())
switch {
case magic == magicUSTAR && trailer == trailerSTAR:
return formatSTAR
case magic == magicUSTAR:
return FormatUSTAR | FormatPAX
case magic == magicGNU && version == versionGNU:
return FormatGNU
default:
return formatV7
}
}
/ setFormat writes the magic values necessary for specified format
/ and then updates the checksum accordingly.
func (b *block) setFormat(format Format) {
/ Set the magic values.
switch {
case format.has(formatV7):
/ Do nothing.
case format.has(FormatGNU):
copy(b.toGNU().magic(), magicGNU)
copy(b.toGNU().version(), versionGNU)
case format.has(formatSTAR):
copy(b.toSTAR().magic(), magicUSTAR)
copy(b.toSTAR().version(), versionUSTAR)
copy(b.toSTAR().trailer(), trailerSTAR)
case format.has(FormatUSTAR | FormatPAX):
copy(b.toUSTAR().magic(), magicUSTAR)
copy(b.toUSTAR().version(), versionUSTAR)
default:
panic("invalid format")
}
/ Update checksum.
/ This field is special in that it is terminated by a NULL then space.
var f formatter
field := b.toV7().chksum()
chksum, _ := b.computeChecksum() / Possible values are 256..128776
f.formatOctal(field[:7], chksum) / Never fails since 128776 < 262143
field[7] = ' '
}
/ computeChecksum computes the checksum for the header block.
/ POSIX specifies a sum of the unsigned byte values, but the Sun tar used
/ signed byte values.
/ We compute and return both.
func (b *block) computeChecksum() (unsigned, signed int64) {
for i, c := range b {
if 148 <= i && i < 156 {
c = ' ' / Treat the checksum field itself as all spaces.
}
unsigned += int64(c)
signed += int64(int8(c))
}
return unsigned, signed
}
/ Reset clears the block with all zeros.
func (b *block) reset() {
*b = block{}
}
type headerV7 [blockSize]byte
func (h *headerV7) name() []byte { return h[000:][:100] }
func (h *headerV7) mode() []byte { return h[100:][:8] }
func (h *headerV7) uid() []byte { return h[108:][:8] }
func (h *headerV7) gid() []byte { return h[116:][:8] }
func (h *headerV7) size() []byte { return h[124:][:12] }
func (h *headerV7) modTime() []byte { return h[136:][:12] }
func (h *headerV7) chksum() []byte { return h[148:][:8] }
func (h *headerV7) typeFlag() []byte { return h[156:][:1] }
func (h *headerV7) linkName() []byte { return h[157:][:100] }
type headerGNU [blockSize]byte
func (h *headerGNU) v7() *headerV7 { return (*headerV7)(h) }
func (h *headerGNU) magic() []byte { return h[257:][:6] }
func (h *headerGNU) version() []byte { return h[263:][:2] }
func (h *headerGNU) userName() []byte { return h[265:][:32] }
func (h *headerGNU) groupName() []byte { return h[297:][:32] }
func (h *headerGNU) devMajor() []byte { return h[329:][:8] }
func (h *headerGNU) devMinor() []byte { return h[337:][:8] }
func (h *headerGNU) accessTime() []byte { return h[345:][:12] }
func (h *headerGNU) changeTime() []byte { return h[357:][:12] }
func (h *headerGNU) sparse() sparseArray { return sparseArray(h[386:][:24*4+1]) }
func (h *headerGNU) realSize() []byte { return h[483:][:12] }
type headerSTAR [blockSize]byte
func (h *headerSTAR) v7() *headerV7 { return (*headerV7)(h) }
func (h *headerSTAR) magic() []byte { return h[257:][:6] }
func (h *headerSTAR) version() []byte { return h[263:][:2] }
func (h *headerSTAR) userName() []byte { return h[265:][:32] }
func (h *headerSTAR) groupName() []byte { return h[297:][:32] }
func (h *headerSTAR) devMajor() []byte { return h[329:][:8] }
func (h *headerSTAR) devMinor() []byte { return h[337:][:8] }
func (h *headerSTAR) prefix() []byte { return h[345:][:131] }
func (h *headerSTAR) accessTime() []byte { return h[476:][:12] }
func (h *headerSTAR) changeTime() []byte { return h[488:][:12] }
func (h *headerSTAR) trailer() []byte { return h[508:][:4] }
type headerUSTAR [blockSize]byte
func (h *headerUSTAR) v7() *headerV7 { return (*headerV7)(h) }
func (h *headerUSTAR) magic() []byte { return h[257:][:6] }
func (h *headerUSTAR) version() []byte { return h[263:][:2] }
func (h *headerUSTAR) userName() []byte { return h[265:][:32] }
func (h *headerUSTAR) groupName() []byte { return h[297:][:32] }
func (h *headerUSTAR) devMajor() []byte { return h[329:][:8] }
func (h *headerUSTAR) devMinor() []byte { return h[337:][:8] }
func (h *headerUSTAR) prefix() []byte { return h[345:][:155] }
type sparseArray []byte
func (s sparseArray) entry(i int) sparseElem { return sparseElem(s[i*24:]) }
func (s sparseArray) isExtended() []byte { return s[24*s.maxEntries():][:1] }
func (s sparseArray) maxEntries() int { return len(s) / 24 }
type sparseElem []byte
func (s sparseElem) offset() []byte { return s[00:][:12] }
func (s sparseElem) length() []byte { return s[12:][:12] }