= 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}[rdoc-ref:packed_data.rdoc@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 enocding}[https://en.wikipedia.org/wiki/X.690#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}[https://www.ietf.org/rfc/rfc2045.txt]):
["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}[https://www.ietf.org/rfc/rfc4648.txt]):
[""].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]