# Packed Data¶ ↑

## Quick Reference¶ ↑

These tables summarize the directives for packing and unpacking.

### For Integers¶ ↑

    Directive     | Meaning
    --------------|---------------------------------------------------------------
    C             | 8-bit unsigned (unsigned char)
    S             | 16-bit unsigned, native endian (uint16_t)
    L             | 32-bit unsigned, native endian (uint32_t)
    Q             | 64-bit unsigned, native endian (uint64_t)
    J             | pointer width unsigned, native endian (uintptr_t)

    c             | 8-bit signed (signed char)
    s             | 16-bit signed, native endian (int16_t)
    l             | 32-bit signed, native endian (int32_t)
    q             | 64-bit signed, native endian (int64_t)
    j             | pointer width signed, native endian (intptr_t)

    S_ S!         | unsigned short, native endian
    I I_ I!       | unsigned int, native endian
    L_ L!         | unsigned long, native endian
    Q_ Q!         | unsigned long long, native endian
                  |   (raises ArgumentError if the platform has no long long type)
    J!            | uintptr_t, native endian (same with J)

    s_ s!         | signed short, native endian
    i i_ i!       | signed int, native endian
    l_ l!         | signed long, native endian
    q_ q!         | signed long long, native endian
                  |   (raises ArgumentError if the platform has no long long type)
    j!            | intptr_t, native endian (same with j)

    S> s> S!> s!> | each the same as the directive without >, but big endian
    L> l> L!> l!> |   S> is the same as n
    I!> i!>       |   L> is the same as N
    Q> q> Q!> q!> |
    J> j> J!> j!> |

    S< s< S!< s!< | each the same as the directive without <, but little endian
    L< l< L!< l!< |   S< is the same as v
    I!< i!<       |   L< is the same as V
    Q< q< Q!< q!< |
    J< j< J!< j!< |

    n             | 16-bit unsigned, network (big-endian) byte order
    N             | 32-bit unsigned, network (big-endian) byte order
    v             | 16-bit unsigned, VAX (little-endian) byte order
    V             | 32-bit unsigned, VAX (little-endian) byte order

    U             | UTF-8 character
    w             | BER-compressed integer

### For Floats¶ ↑

    Directive | Meaning
    ----------|--------------------------------------------------
    D d       | double-precision, native format
    F f       | single-precision, native format
    E         | double-precision, little-endian byte order
    e         | single-precision, little-endian byte order
    G         | double-precision, network (big-endian) byte order
    g         | single-precision, network (big-endian) byte order

### For Strings¶ ↑

    Directive | Meaning
    ----------|-----------------------------------------------------------------
    A         | arbitrary binary string (remove trailing nulls and ASCII spaces)
    a         | arbitrary binary string
    Z         | null-terminated string
    B         | bit string (MSB first)
    b         | bit string (LSB first)
    H         | hex string (high nibble first)
    h         | hex string (low nibble first)
    u         | UU-encoded string
    M         | quoted-printable, MIME encoding (see RFC2045)
    m         | base64 encoded string (RFC 2045) (default)
              |   (base64 encoded string (RFC 4648) if followed by 0)
    P         | pointer to a structure (fixed-length string)
    p         | pointer to a null-terminated string

### Additional Directives for Packing¶ ↑

    Directive | Meaning
    ----------|----------------------------------------------------------------
    @         | moves to absolute position
    X         | back up a byte
    x         | null byte

### Additional Directives for Unpacking¶ ↑

    Directive | Meaning
    ----------|----------------------------------------------------------------
    @         | skip to the offset given by the length argument
    X         | skip backward one byte
    x         | skip forward one byte

## Packing and Unpacking¶ ↑

Certain Ruby core methods deal with packing and unpacking data:

- Method `Array#pack`: Formats each element in array `self` into a binary string; returns that string.

- Method `String#unpack`: Extracts data from string `self`, forming objects that become the elements of a new array; returns that array.

- Method `String#unpack1`: Does the same, but unpacks and returns only the first extracted object.

Each of these methods accepts a string `template`, consisting of zero or more directive characters, each followed by zero or more modifier characters.

Examples (directive `'C'` specifies ‘unsigned character’):

    [65].pack('C')      # => "A"  # One element, one directive.
    [65, 66].pack('CC') # => "AB" # Two elements, two directives.
    [65, 66].pack('C')  # => "A"  # Extra element is ignored.
    [65].pack('')       # => ""   # No directives.
    [65].pack('CC')               # Extra directive raises ArgumentError.

    'A'.unpack('C')   # => [65]      # One character, one directive.
    'AB'.unpack('CC') # => [65, 66]  # Two characters, two directives.
    'AB'.unpack('C')  # => [65]      # Extra character is ignored.
    'A'.unpack('CC')  # => [65, nil] # Extra directive generates nil.
    'AB'.unpack('')   # => []        # No directives.

The string `template` may contain any mixture of valid directives (directive `'c'` specifies ‘signed character’):

    [65, -1].pack('cC')  # => "A\xFF"
    "A\xFF".unpack('cC') # => [65, 255]

The string `template` may contain whitespace (which is ignored) and comments, each of which begins with character `'#'` and continues up to and including the next following newline:

    [0,1].pack("  C  #foo \n  C  ")    # => "\x00\x01"
    "\0\1".unpack("  C  #foo \n  C  ") # => [0, 1]

Any directive may be followed by either of these modifiers:

- `'*'` - The directive is to be applied as many times as needed:

      [65, 66].pack('C*') # => "AB"
      'AB'.unpack('C*')   # => [65, 66]

- `Integer` `count` - The directive is to be applied `count` times:

      [65, 66].pack('C2') # => "AB"
      [65, 66].pack('C3') # Raises ArgumentError.
      'AB'.unpack('C2')   # => [65, 66]
      'AB'.unpack('C3')   # => [65, 66, nil]

  Note: Directives in `%w[A a Z m]` use `count` differently; see String Directives.

If elements don’t fit the provided directive, only least significant bits are encoded:

    [257].pack("C").unpack("C") # => [1]

## Packing Method¶ ↑

Method `Array#pack` accepts optional keyword argument `buffer` that specifies the target string (instead of a new string):

    [65, 66].pack('C*', buffer: 'foo') # => "fooAB"

The method can accept a block:

    # Packed string is passed to the block.
    [65, 66].pack('C*') {|s| p s }    # => "AB"

## Unpacking Methods¶ ↑

Methods `String#unpack` and `String#unpack1` each accept an optional keyword argument `offset` that specifies an offset into the string:

    'ABC'.unpack('C*', offset: 1)  # => [66, 67]
    'ABC'.unpack1('C*', offset: 1) # => 66

Both methods can accept a block:

    # Each unpacked object is passed to the block.
    ret = []
    "ABCD".unpack("C*") {|c| ret << c }
    ret # => [65, 66, 67, 68]

    # The single unpacked object is passed to the block.
    'AB'.unpack1('C*') {|ele| p ele } # => 65

## Integer Directives¶ ↑

Each integer directive specifies the packing or unpacking for one element in the input or output array.

### 8-Bit Integer Directives¶ ↑

- `'c'` - 8-bit signed integer (like C `signed char`):

      [0, 1, 255].pack('c*')  # => "\x00\x01\xFF"
      s = [0, 1, -1].pack('c*') # => "\x00\x01\xFF"
      s.unpack('c*') # => [0, 1, -1]

- `'C'` - 8-bit unsigned integer (like C `unsigned char`):

      [0, 1, 255].pack('C*')    # => "\x00\x01\xFF"
      s = [0, 1, -1].pack('C*') # => "\x00\x01\xFF"
      s.unpack('C*')            # => [0, 1, 255]

### 16-Bit Integer Directives¶ ↑

- `'s'` - 16-bit signed integer, native-endian (like C `int16_t`):

      [513, -514].pack('s*')      # => "\x01\x02\xFE\xFD"
      s = [513, 65022].pack('s*') # => "\x01\x02\xFE\xFD"
      s.unpack('s*')              # => [513, -514]

- `'S'` - 16-bit unsigned integer, native-endian (like C `uint16_t`):

      [513, -514].pack('S*')      # => "\x01\x02\xFE\xFD"
      s = [513, 65022].pack('S*') # => "\x01\x02\xFE\xFD"
      s.unpack('S*')              # => [513, 65022]

- `'n'` - 16-bit network integer, big-endian:

      s = [0, 1, -1, 32767, -32768, 65535].pack('n*')
      # => "\x00\x00\x00\x01\xFF\xFF\x7F\xFF\x80\x00\xFF\xFF"
      s.unpack('n*')
      # => [0, 1, 65535, 32767, 32768, 65535]

- `'v'` - 16-bit VAX integer, little-endian:

      s = [0, 1, -1, 32767, -32768, 65535].pack('v*')
      # => "\x00\x00\x01\x00\xFF\xFF\xFF\x7F\x00\x80\xFF\xFF"
      s.unpack('v*')
      # => [0, 1, 65535, 32767, 32768, 65535]

### 32-Bit Integer Directives¶ ↑

- `'l'` - 32-bit signed integer, native-endian (like C `int32_t`):

      s = [67305985, -50462977].pack('l*')
      # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
      s.unpack('l*')
      # => [67305985, -50462977]

- `'L'` - 32-bit unsigned integer, native-endian (like C `uint32_t`):

      s = [67305985, 4244504319].pack('L*')
      # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
      s.unpack('L*')
      # => [67305985, 4244504319]

- `'N'` - 32-bit network integer, big-endian:

      s = [0,1,-1].pack('N*')
      # => "\x00\x00\x00\x00\x00\x00\x00\x01\xFF\xFF\xFF\xFF"
      s.unpack('N*')
      # => [0, 1, 4294967295]

- `'V'` - 32-bit VAX integer, little-endian:

      s = [0,1,-1].pack('V*')
      # => "\x00\x00\x00\x00\x01\x00\x00\x00\xFF\xFF\xFF\xFF"
      s.unpack('v*')
      # => [0, 0, 1, 0, 65535, 65535]

### 64-Bit Integer Directives¶ ↑

- `'q'` - 64-bit signed integer, native-endian (like C `int64_t`):

      s = [578437695752307201, -506097522914230529].pack('q*')
      # => "\x01\x02\x03\x04\x05\x06\a\b\xFF\xFE\xFD\xFC\xFB\xFA\xF9\xF8"
      s.unpack('q*')
      # => [578437695752307201, -506097522914230529]

- `'Q'` - 64-bit unsigned integer, native-endian (like C `uint64_t`):

      s = [578437695752307201, 17940646550795321087].pack('Q*')
      # => "\x01\x02\x03\x04\x05\x06\a\b\xFF\xFE\xFD\xFC\xFB\xFA\xF9\xF8"
      s.unpack('Q*')
      # => [578437695752307201, 17940646550795321087]

### Platform-Dependent Integer Directives¶ ↑

- `'i'` - Platform-dependent width signed integer, native-endian (like C `int`):

      s = [67305985, -50462977].pack('i*')
      # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
      s.unpack('i*')
      # => [67305985, -50462977]

- `'I'` - Platform-dependent width unsigned integer, native-endian (like C `unsigned int`):

      s = [67305985, -50462977].pack('I*')
      # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
      s.unpack('I*')
      # => [67305985, 4244504319]

- `'j'` - Pointer-width signed integer, native-endian (like C `intptr_t`):

      s = [67305985, -50462977].pack('j*')
      # => "\x01\x02\x03\x04\x00\x00\x00\x00\xFF\xFE\xFD\xFC\xFF\xFF\xFF\xFF"
      s.unpack('j*')
      # => [67305985, -50462977]

- `'J'` - Pointer-width unsigned integer, native-endian (like C `uintptr_t`):

      s = [67305985, 4244504319].pack('J*')
      # => "\x01\x02\x03\x04\x00\x00\x00\x00\xFF\xFE\xFD\xFC\x00\x00\x00\x00"
      s.unpack('J*')
      # => [67305985, 4244504319]

### Other Integer Directives¶ ↑

- `'U'` - UTF-8 character:

      s = [4194304].pack('U*')
      # => "\xF8\x90\x80\x80\x80"
      s.unpack('U*')
      # => [4194304]

- `'w'` - BER-encoded integer (see BER encoding):

      s = [1073741823].pack('w*')
      # => "\x83\xFF\xFF\xFF\x7F"
      s.unpack('w*')
      # => [1073741823]

### Modifiers for Integer Directives¶ ↑

For the following directives, `'!'` or `'_'` modifiers may be suffixed as underlying platform’s native size.

- `'i'`, `'I'` - C `int`, always native size.

- `'s'`, `'S'` - C `short`.

- `'l'`, `'L'` - C `long`.

- `'q'`, `'Q'` - C `long long`, if available.

- `'j'`, `'J'` - C `intptr_t`, always native size.

Native size modifiers are silently ignored for always native size directives.

The endian modifiers also may be suffixed in the directives above:

- `'>'` - Big-endian.

- `'<'` - Little-endian.

## Float Directives¶ ↑

Each float directive specifies the packing or unpacking for one element in the input or output array.

### Single-Precision Float Directives¶ ↑

- `'F'` or `'f'` - Native format:

      s = [3.0].pack('F') # => "\x00\x00@@"
      s.unpack('F')       # => [3.0]

- `'e'` - Little-endian:

      s = [3.0].pack('e') # => "\x00\x00@@"
      s.unpack('e')       # => [3.0]

- `'g'` - Big-endian:

      s = [3.0].pack('g') # => "@@\x00\x00"
      s.unpack('g')       # => [3.0]

### Double-Precision Float Directives¶ ↑

- `'D'` or `'d'` - Native format:

      s = [3.0].pack('D') # => "\x00\x00\x00\x00\x00\x00\b@"
      s.unpack('D')       # => [3.0]

- `'E'` - Little-endian:

      s = [3.0].pack('E') # => "\x00\x00\x00\x00\x00\x00\b@"
      s.unpack('E')       # => [3.0]

- `'G'` - Big-endian:

      s = [3.0].pack('G') # => "@\b\x00\x00\x00\x00\x00\x00"
      s.unpack('G')       # => [3.0]

A float directive may be infinity or not-a-number:

    inf = 1.0/0.0                  # => Infinity
    [inf].pack('f')                # => "\x00\x00\x80\x7F"
    "\x00\x00\x80\x7F".unpack('f') # => [Infinity]

    nan = inf/inf                  # => NaN
    [nan].pack('f')                # => "\x00\x00\xC0\x7F"
    "\x00\x00\xC0\x7F".unpack('f') # => [NaN]

## String Directives¶ ↑

Each string directive specifies the packing or unpacking for one byte in the input or output string.

### Binary String Directives¶ ↑

- `'A'` - Arbitrary binary string (space padded; count is width); `nil` is treated as the empty string:

      ['foo'].pack('A')    # => "f"
      ['foo'].pack('A*')   # => "foo"
      ['foo'].pack('A2')   # => "fo"
      ['foo'].pack('A4')   # => "foo "
      [nil].pack('A')      # => " "
      [nil].pack('A*')     # => ""
      [nil].pack('A2')     # => "  "
      [nil].pack('A4')     # => "    "

      "foo\0".unpack('A')      # => ["f"]
      "foo\0".unpack('A4')     # => ["foo"]
      "foo\0bar".unpack('A10') # => ["foo\x00bar"] # Reads past "\0".
      "foo ".unpack('A')       # => ["f"]
      "foo ".unpack('A4')      # => ["foo"]
      "foo".unpack('A4')       # => ["foo"]

      russian = "\u{442 435 441 442}" # => "тест"
      russian.size         # => 4
      russian.bytesize     # => 8
      [russian].pack('A')  # => "\xD1"
      [russian].pack('A*') # => "\xD1\x82\xD0\xB5\xD1\x81\xD1\x82"
      russian.unpack('A')  # => ["\xD1"]
      russian.unpack('A2') # => ["\xD1\x82"]
      russian.unpack('A4') # => ["\xD1\x82\xD0\xB5"]
      russian.unpack('A*') # => ["\xD1\x82\xD0\xB5\xD1\x81\xD1\x82"]

- `'a'` - Arbitrary binary string (null padded; count is width):

      ["foo"].pack('a')    # => "f"
      ["foo"].pack('a*')   # => "foo"
      ["foo"].pack('a2')   # => "fo"
      ["foo\0"].pack('a4') # => "foo\x00"
      [nil].pack('a')      # => "\x00"
      [nil].pack('a*')     # => ""
      [nil].pack('a2')     # => "\x00\x00"
      [nil].pack('a4')     # => "\x00\x00\x00\x00"

      "foo\0".unpack('a')     # => ["f"]
      "foo\0".unpack('a4')    # => ["foo\x00"]
      "foo ".unpack('a4')     # => ["foo "]
      "foo".unpack('a4')      # => ["foo"]
      "foo\0bar".unpack('a4') # => ["foo\x00"] # Reads past "\0".

- `'Z'` - Same as `'a'`, except that null is added or ignored with `'*'`:

      ["foo"].pack('Z*')   # => "foo\x00"
      [nil].pack('Z*')     # => "\x00"

      "foo\0".unpack('Z*')    # => ["foo"]
      "foo".unpack('Z*')      # => ["foo"]
      "foo\0bar".unpack('Z*') # => ["foo"] # Does not read past "\0".

### Bit String Directives¶ ↑

- `'B'` - Bit string (high byte first):

      ['11111111' + '00000000'].pack('B*') # => "\xFF\x00"
      ['10000000' + '01000000'].pack('B*') # => "\x80@"

      ['1'].pack('B0') # => ""
      ['1'].pack('B1') # => "\x80"
      ['1'].pack('B2') # => "\x80\x00"
      ['1'].pack('B3') # => "\x80\x00"
      ['1'].pack('B4') # => "\x80\x00\x00"
      ['1'].pack('B5') # => "\x80\x00\x00"
      ['1'].pack('B6') # => "\x80\x00\x00\x00"

      "\xff\x00".unpack("B*") # => ["1111111100000000"]
      "\x01\x02".unpack("B*") # => ["0000000100000010"]

      "".unpack("B0")     # => [""]
      "\x80".unpack("B1") # => ["1"]
      "\x80".unpack("B2") # => ["10"]
      "\x80".unpack("B3") # => ["100"]

- `'b'` - Bit string (low byte first):

      ['11111111' + '00000000'].pack('b*') # => "\xFF\x00"
      ['10000000' + '01000000'].pack('b*') # => "\x01\x02"

      ['1'].pack('b0') # => ""
      ['1'].pack('b1') # => "\x01"
      ['1'].pack('b2') # => "\x01\x00"
      ['1'].pack('b3') # => "\x01\x00"
      ['1'].pack('b4') # => "\x01\x00\x00"
      ['1'].pack('b5') # => "\x01\x00\x00"
      ['1'].pack('b6') # => "\x01\x00\x00\x00"

      "\xff\x00".unpack("b*") # => ["1111111100000000"]
      "\x01\x02".unpack("b*") # => ["1000000001000000"]

      "".unpack("b0")     # => [""]
      "\x01".unpack("b1") # => ["1"]
      "\x01".unpack("b2") # => ["10"]
      "\x01".unpack("b3") # => ["100"]

### Hex String Directives¶ ↑

- `'H'` - Hex string (high nibble first):

      ['10ef'].pack('H*')    # => "\x10\xEF"
      ['10ef'].pack('H0')    # => ""
      ['10ef'].pack('H3')    # => "\x10\xE0"
      ['10ef'].pack('H5')    # => "\x10\xEF\x00"

      ['fff'].pack('H3')    # => "\xFF\xF0"
      ['fff'].pack('H4')    # => "\xFF\xF0"
      ['fff'].pack('H5')    # => "\xFF\xF0\x00"
      ['fff'].pack('H6')    # => "\xFF\xF0\x00"
      ['fff'].pack('H7')    # => "\xFF\xF0\x00\x00"
      ['fff'].pack('H8')    # => "\xFF\xF0\x00\x00"

      "\x10\xef".unpack('H*')    # => ["10ef"]
      "\x10\xef".unpack('H0')    # => [""]
      "\x10\xef".unpack('H1')    # => ["1"]
      "\x10\xef".unpack('H2')    # => ["10"]
      "\x10\xef".unpack('H3')    # => ["10e"]
      "\x10\xef".unpack('H4')    # => ["10ef"]
      "\x10\xef".unpack('H5')    # => ["10ef"]

- `'h'` - Hex string (low nibble first):

      ['10ef'].pack('h*') # => "\x01\xFE"
      ['10ef'].pack('h0') # => ""
      ['10ef'].pack('h3') # => "\x01\x0E"
      ['10ef'].pack('h5') # => "\x01\xFE\x00"

      ['fff'].pack('h3') # => "\xFF\x0F"
      ['fff'].pack('h4') # => "\xFF\x0F"
      ['fff'].pack('h5') # => "\xFF\x0F\x00"
      ['fff'].pack('h6') # => "\xFF\x0F\x00"
      ['fff'].pack('h7') # => "\xFF\x0F\x00\x00"
      ['fff'].pack('h8') # => "\xFF\x0F\x00\x00"

      "\x01\xfe".unpack('h*') # => ["10ef"]
      "\x01\xfe".unpack('h0') # => [""]
      "\x01\xfe".unpack('h1') # => ["1"]
      "\x01\xfe".unpack('h2') # => ["10"]
      "\x01\xfe".unpack('h3') # => ["10e"]
      "\x01\xfe".unpack('h4') # => ["10ef"]
      "\x01\xfe".unpack('h5') # => ["10ef"]

### Pointer String Directives¶ ↑

- `'P'` - Pointer to a structure (fixed-length string):

      s = ['abc'].pack('P')  # => "\xE0O\x7F\xE5\xA1\x01\x00\x00"
      s.unpack('P*')         # => ["abc"]
      ".".unpack("P")        # => []
      ("\0" * 8).unpack("P") # => [nil]
      [nil].pack("P")        # => "\x00\x00\x00\x00\x00\x00\x00\x00"

- `'p'` - Pointer to a null-terminated string:

      s = ['abc'].pack('p')  # => "(\xE4u\xE5\xA1\x01\x00\x00"
      s.unpack('p*')         # => ["abc"]
      ".".unpack("p")        # => []
      ("\0" * 8).unpack("p") # => [nil]
      [nil].pack("p")        # => "\x00\x00\x00\x00\x00\x00\x00\x00"

### Other String Directives¶ ↑

- `'M'` - Quoted printable, MIME encoding; text mode, but input must use LF and output LF; (see RFC 2045):

      ["a b c\td \ne"].pack('M') # => "a b c\td =\n\ne=\n"
      ["\0"].pack('M')           # => "=00=\n"

      ["a"*1023].pack('M') == ("a"*73+"=\n")*14+"a=\n"     # => true
      ("a"*73+"=\na=\n").unpack('M') == ["a"*74]           # => true
      (("a"*73+"=\n")*14+"a=\n").unpack('M') == ["a"*1023] # => true

      "a b c\td =\n\ne=\n".unpack('M')    # => ["a b c\td \ne"]
      "=00=\n".unpack('M')    # => ["\x00"]

      "pre=31=32=33after".unpack('M') # => ["pre123after"]
      "pre=\nafter".unpack('M')       # => ["preafter"]
      "pre=\r\nafter".unpack('M')     # => ["preafter"]
      "pre=".unpack('M')              # => ["pre="]
      "pre=\r".unpack('M')            # => ["pre=\r"]
      "pre=hoge".unpack('M')          # => ["pre=hoge"]
      "pre==31after".unpack('M')      # => ["pre==31after"]
      "pre===31after".unpack('M')     # => ["pre===31after"]

- `'m'` - Base64 encoded string; count specifies input bytes between each newline, rounded down to nearest multiple of 3; if count is zero, no newlines are added; (see RFC 4648):

      [""].pack('m')             # => ""
      ["\0"].pack('m')           # => "AA==\n"
      ["\0\0"].pack('m')         # => "AAA=\n"
      ["\0\0\0"].pack('m')       # => "AAAA\n"
      ["\377"].pack('m')         # => "/w==\n"
      ["\377\377"].pack('m')     # => "//8=\n"
      ["\377\377\377"].pack('m') # => "////\n"

      "".unpack('m')       # => [""]
      "AA==\n".unpack('m') # => ["\x00"]
      "AAA=\n".unpack('m') # => ["\x00\x00"]
      "AAAA\n".unpack('m') # => ["\x00\x00\x00"]
      "/w==\n".unpack('m') # => ["\xFF"]
      "//8=\n".unpack('m') # => ["\xFF\xFF"]
      "////\n".unpack('m') # => ["\xFF\xFF\xFF"]
      "A\n".unpack('m')    # => [""]
      "AA\n".unpack('m')   # => ["\x00"]
      "AA=\n".unpack('m')  # => ["\x00"]
      "AAA\n".unpack('m')  # => ["\x00\x00"]

      [""].pack('m0')             # => ""
      ["\0"].pack('m0')           # => "AA=="
      ["\0\0"].pack('m0')         # => "AAA="
      ["\0\0\0"].pack('m0')       # => "AAAA"
      ["\377"].pack('m0')         # => "/w=="
      ["\377\377"].pack('m0')     # => "//8="
      ["\377\377\377"].pack('m0') # => "////"

      "".unpack('m0')     # => [""]
      "AA==".unpack('m0') # => ["\x00"]
      "AAA=".unpack('m0') # => ["\x00\x00"]
      "AAAA".unpack('m0') # => ["\x00\x00\x00"]
      "/w==".unpack('m0') # => ["\xFF"]
      "//8=".unpack('m0') # => ["\xFF\xFF"]
      "////".unpack('m0') # => ["\xFF\xFF\xFF"]

- `'u'` - UU-encoded string:

      [""].pack("u")        # => ""
      ["a"].pack("u")       # => "!80``\n"
      ["aaa"].pack("u")     # => "#86%A\n"

      "".unpack("u")        # => [""]
      "#86)C\n".unpack("u") # => ["abc"]

## Offset Directives¶ ↑

- `'@'` - Begin packing at the given byte offset; for packing, null fill or shrink if necessary:

      [1, 2].pack("C@0C")     # => "\x02"
      [1, 2].pack("C@1C")     # => "\x01\x02"
      [1, 2].pack("C@5C")     # => "\x01\x00\x00\x00\x00\x02"
      [*1..5].pack("CCCC@2C") # => "\x01\x02\x05"

  For unpacking, cannot to move to outside the string:

      "\x01\x00\x00\x02".unpack("C@3C") # => [1, 2]
      "\x00".unpack("@1C")              # => [nil]
      "\x00".unpack("@2C")              # Raises ArgumentError.

- `'X'` - For packing, shrink for the given byte offset:

      [0, 1, 2].pack("CCXC")    # => "\x00\x02"
      [0, 1, 2].pack("CCX2C")   # => "\x02"

  For unpacking; rewind unpacking position for the given byte offset:

      "\x00\x02".unpack("CCXC") # => [0, 2, 2]

  Cannot to move to outside the string:

      [0, 1, 2].pack("CCX3C")   # Raises ArgumentError.
      "\x00\x02".unpack("CX3C") # Raises ArgumentError.

- `'x'` - Begin packing at after the given byte offset; for packing, null fill if necessary:

      [].pack("x0")                # => ""
      [].pack("x")                 # => "\x00"
      [].pack("x8")                # => "\x00\x00\x00\x00\x00\x00\x00\x00"

  For unpacking, cannot to move to outside the string:

      "\x00\x00\x02".unpack("CxC") # => [0, 2]
      "\x00\x00\x02".unpack("x3C") # => [nil]
      "\x00\x00\x02".unpack("x4C") # Raises ArgumentError