Packed Data

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 signed 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]

Pointer Directives

• 'j' - 64-bit 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' - 64-bit 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 enocding):

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

Modifiers for Integer Directives

For directives in 'i', 'I', 's', 'S', 'l', 'L', 'q', 'Q', 'j', and 'J', these modifiers may be suffixed:

• '!' or '_' - Underlying platform’s native size.

• '>' - 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:

  [0].pack("U")          # => "\u0000"
  [0x3fffffff].pack("U") # => "\xFC\xBF\xBF\xBF\xBF\xBF"
  [0x40000000].pack("U") # => "\xFD\x80\x80\x80\x80\x80"
  [0x7fffffff].pack("U") # => "\xFD\xBF\xBF\xBF\xBF\xBF"

Offset Directives

• '@' - Begin packing at the given byte offset; for packing, null fill 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"
  
  "\x01\x00\x00\x02".unpack("C@3C") # => [1, 2]
  "\x00".unpack("@1C")              # => [nil]

• 'X' - Back up a byte:

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

Null Byte Direcive

• 'x' - Null byte:

  [].pack("x0")                # => ""
  [].pack("x")                 # => "\x00"
  [].pack("x8")                # => "\x00\x00\x00\x00\x00\x00\x00\x00"
  "\x00\x00\x02".unpack("CxC") # => [0, 2]