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binary(n)		     Tcl Built-In Commands		     binary(n)

______________________________________________________________________________

NAME
       binary - Insert and extract fields from binary strings

SYNOPSIS
       binary format formatString ?arg arg ...?
       binary scan string formatString ?varName varName ...?
_________________________________________________________________

DESCRIPTION
       This  command  provides	facilities  for manipulating binary data.  The
       first form, binary format, creates a binary string from normal Tcl val‐
       ues.   For  example,  given the values 16 and 22, on a 32-bit architec‐
       ture, it might produce an 8-byte binary string consisting of two 4-byte
       integers, one for each of the numbers.  The second form of the command,
       binary scan, does the opposite: it extracts data from a	binary	string
       and returns it as ordinary Tcl string values.

BINARY FORMAT
       The  binary  format  command  generates a binary string whose layout is
       specified by the formatString and whose contents come  from  the	 addi‐
       tional arguments.  The resulting binary value is returned.

       The  formatString  consists  of a sequence of zero or more field speci‐
       fiers separated by zero or more spaces.	Each field specifier is a sin‐
       gle  type  character followed by an optional flag character followed by
       an optional numeric count.  Most field specifiers consume one  argument
       to  obtain the value to be formatted.  The type character specifies how
       the value is to be formatted.  The count typically indicates  how  many
       items  of the specified type are taken from the value.  If present, the
       count is a non-negative decimal integer or *, which normally  indicates
       that  all  of  the items in the value are to be used.  If the number of
       arguments does not match the number of fields in the format string that
       consume	arguments,  then  an error is generated. The flag character is
       ignored for for binary format.

       Here is a small example to clarify the relation between the field spec‐
       ifiers and the arguments:
	      binary format d3d {1.0 2.0 3.0 4.0} 0.1

       The  first argument is a list of four numbers, but because of the count
       of 3 for the associated field specifier, only the first three  will  be
       used.  The  second  argument is associated with the second field speci‐
       fier. The resulting binary string contains the four numbers  1.0,  2.0,
       3.0 and 0.1.

       Each type-count pair moves an imaginary cursor through the binary data,
       storing bytes at the current position and advancing the cursor to  just
       after  the  last byte stored.  The cursor is initially at position 0 at
       the beginning of the data.  The type may be any one  of	the  following
       characters:

       a    Stores  a byte string of length count in the output string.	 Every
	    character is taken as modulo 256 (i.e. the low byte of every char‐
	    acter  is used, and the high byte discarded) so when storing char‐
	    acter  strings  not	 wholly	 expressible  using   the   characters
	    \u0000-\u00ff, the encoding convertto command should be used first
	    to change the string into an external representation if this trun‐
	    cation  is not desired (i.e. if the characters are not part of the
	    ISO 8859-1 character set.)	If arg has  fewer  than	 count	bytes,
	    then  additional zero bytes are used to pad out the field.	If arg
	    is longer than the specified length, the extra characters will  be
	    ignored.  If count is *, then all of the bytes in arg will be for‐
	    matted.  If count is omitted, then one character will  be  format‐
	    ted.  For example,
		   binary format a7a*a alpha bravo charlie
	    will return a string equivalent to alpha\000\000bravoc,
		   binary format a* [encoding convertto utf-8 \u20ac]
	    will  return  a  string  equivalent	 to \342\202\254 (which is the
	    UTF-8 byte sequence for a Euro-currency character) and
		   binary format a* [encoding convertto iso8859-15 \u20ac]
	    will return a string equivalent to \244 (which is the ISO  8859-15
	    byte  sequence for a Euro-currency character). Contrast these last
	    two with:
		   binary format a* \u20ac
	    which returns a string equivalent to \254 (i.e. \xac) by  truncat‐
	    ing the high-bits of the character, and which is probably not what
	    is desired.

       A    This form is the same as a except that spaces are used for padding
	    instead of nulls.  For example,
		   binary format A6A*A alpha bravo charlie
	    will return alpha bravoc.

       b    Stores a string of count binary digits in low-to-high order within
	    each byte in the output string.  Arg must contain a sequence of  1
	    and	 0  characters.	  The  resulting bytes are emitted in first to
	    last order with the bits  being  formatted	in  low-to-high	 order
	    within  each byte.	If arg has fewer than count digits, then zeros
	    will be used for the remaining bits.  If arg  has  more  than  the
	    specified  number of digits, the extra digits will be ignored.  If
	    count is *, then all of the digits in arg will be  formatted.   If
	    count is omitted, then one digit will be formatted.	 If the number
	    of bits formatted does not end at a byte boundary,	the  remaining
	    bits of the last byte will be zeros.  For example,
		   binary format b5b* 11100 111000011010
	    will return a string equivalent to \x07\x87\x05.

       B    This  form	is  the	 same  as b except that the bits are stored in
	    high-to-low order within each byte.	 For example,
		   binary format B5B* 11100 111000011010
	    will return a string equivalent to \xe0\xe1\xa0.

       H    Stores a string of count hexadecimal digits in high-to-low	within
	    each  byte	in  the output string.	Arg must contain a sequence of
	    characters in the  set  “0123456789abcdefABCDEF”.	The  resulting
	    bytes are emitted in first to last order with the hex digits being
	    formatted in high-to-low order within each byte.  If arg has fewer
	    than  count digits, then zeros will be used for the remaining dig‐
	    its.  If arg has more than the specified  number  of  digits,  the
	    extra digits will be ignored.  If count is *, then all of the dig‐
	    its in arg will be formatted.  If count is omitted, then one digit
	    will be formatted.	If the number of digits formatted does not end
	    at a byte boundary, the remaining bits of the last	byte  will  be
	    zeros.  For example,
		   binary format H3H*H2 ab DEF 987
	    will return a string equivalent to \xab\x00\xde\xf0\x98.

       h    This  form	is  the same as H except that the digits are stored in
	    low-to-high order within each byte. This is seldom	required.  For
	    example,
		   binary format h3h*h2 AB def 987
	    will return a string equivalent to \xba\x00\xed\x0f\x89.

       c    Stores  one or more 8-bit integer values in the output string.  If
	    no count is specified, then arg must consist of an integer	value.
	    If	count  is  specified, arg must consist of a list containing at
	    least that many integers. The low-order 8 bits of each integer are
	    stored as a one-byte value at the cursor position.	If count is *,
	    then all of the integers in the list are formatted. If the	number
	    of elements in the list is greater than count, then the extra ele‐
	    ments are ignored.	For example,
		   binary format c3cc* {3 -3 128 1} 260 {2 5}
	    will  return  a  string  equivalent	 to  \x03\xfd\x80\x04\x02\x05,
	    whereas
		   binary format c {2 5}
	    will generate an error.

       s    This  form	is  the	 same  as  c except that it stores one or more
	    16-bit integers in little-endian byte order in the output  string.
	    The	 low-order  16-bits  of	 each integer are stored as a two-byte
	    value at the cursor	 position  with	 the  least  significant  byte
	    stored first.  For example,
		   binary format s3 {3 -3 258 1}
	    will return a string equivalent to \x03\x00\xfd\xff\x02\x01.

       S    This  form	is  the	 same  as  s except that it stores one or more
	    16-bit integers in big-endian byte order  in  the  output  string.
	    For example,
		   binary format S3 {3 -3 258 1}
	    will return a string equivalent to \x00\x03\xff\xfd\x01\x02.

       t    This  form	(mnemonically tiny) is the same as s and S except that │
	    it stores the 16-bit integers in the output string in  the	native │
	    byte  order	 of  the  machine where the Tcl script is running.  To │
	    determine what the native byte order of the machine is,  refer  to │
	    the byteOrder element of the tcl_platform array.

       i    This  form	is  the	 same  as  c except that it stores one or more
	    32-bit integers in little-endian byte order in the output  string.
	    The	 low-order  32-bits  of each integer are stored as a four-byte
	    value at the cursor	 position  with	 the  least  significant  byte
	    stored first.  For example,
		   binary format i3 {3 -3 65536 1}
	    will	return	      a	      string	   equivalent	    to
	    \x03\x00\x00\x00\xfd\xff\xff\xff\x00\x00\x01\x00

       I    This form is the same as i except that it stores one or  more  one
	    or	more  32-bit  integers	in big-endian byte order in the output
	    string.  For example,
		   binary format I3 {3 -3 65536 1}
	    will       return	    a	    string	  equivalent	    to
	    \x00\x00\x00\x03\xff\xff\xff\xfd\x00\x01\x00\x00

       n    This  form	(mnemonically number or normal) is the same as i and I │
	    except that it stores the 32-bit integers in the output string  in │
	    the	 native byte order of the machine where the Tcl script is run‐ │
	    ning.  To determine what the native byte order of the machine  is, │
	    refer to the byteOrder element of the tcl_platform array.

       w    This  form	is  the	 same  as  c except that it stores one or more
	    64-bit integers in little-endian byte order in the output  string.
	    The	 low-order 64-bits of each integer are stored as an eight-byte
	    value at the cursor	 position  with	 the  least  significant  byte
	    stored first.  For example,
		   binary format w 7810179016327718216
	    will return the string HelloTcl

       W    This  form	is the same as w except that it stores one or more one
	    or more 64-bit integers in big-endian byte	order  in  the	output
	    string.  For example,
		   binary format Wc 4785469626960341345 110
	    will return the string BigEndian

       m    This  form	(mnemonically  the mirror of w) is the same as w and W │
	    except that it stores the 64-bit integers in the output string  in │
	    the	 native byte order of the machine where the Tcl script is run‐ │
	    ning.  To determine what the native byte order of the machine  is, │
	    refer to the byteOrder element of the tcl_platform array.

       f    This  form	is the same as c except that it stores one or more one
	    or more single-precision floating point numbers in	the  machine's
	    native  representation  in the output string.  This representation
	    is not portable across architectures, so it should not be used  to
	    communicate	 floating  point numbers across the network.  The size
	    of a floating point number may vary across architectures,  so  the
	    number  of	bytes that are generated may vary.  If the value over‐
	    flows the machine's	 native	 representation,  then	the  value  of
	    FLT_MAX  as	 defined  by the system will be used instead.  Because
	    Tcl uses double-precision floating point numbers internally, there
	    may	 be  some loss of precision in the conversion to single-preci‐
	    sion.  For example, on a Windows system running on an  Intel  Pen‐
	    tium processor,
		   binary format f2 {1.6 3.4}
	    will	return	      a	      string	   equivalent	    to
	    \xcd\xcc\xcc\x3f\x9a\x99\x59\x40.

       r    This form (mnemonically real) is the same  as  f  except  that  it │
	    stores  the	 single-precision  floating  point  numbers in little- │
	    endian order.  This conversion  only  produces  meaningful	output │
	    when  used on machines which use the IEEE floating point represen‐ │
	    tation (very common, but not universal.)

       R    This form is the same as r except that it stores the single-preci‐ │
	    sion floating point numbers in big-endian order.

       d    This  form	is the same as f except that it stores one or more one
	    or more double-precision floating point numbers in	the  machine's
	    native  representation  in	the  output string.  For example, on a
	    Windows system running on an Intel Pentium processor,
		   binary format d1 {1.6}
	    will       return	    a	    string	  equivalent	    to
	    \x9a\x99\x99\x99\x99\x99\xf9\x3f.

       q    This  form	(mnemonically the mirror of d) is the same as d except │
	    that it stores the double-precision floating point numbers in lit‐ │
	    tle-endian order.  This conversion only produces meaningful output │
	    when used on machines which use the IEEE floating point  represen‐ │
	    tation (very common, but not universal.)

       Q    This form is the same as q except that it stores the double-preci‐ │
	    sion floating point numbers in big-endian order.

       x    Stores count null bytes in the output string.   If	count  is  not
	    specified,	stores	one  null  byte.   If count is *, generates an
	    error.  This type does not consume an argument.  For example,
		   binary format a3xa3x2a3 abc def ghi
	    will return a string equivalent to abc\000def\000\000ghi.

       X    Moves the cursor back count bytes in the output string.  If	 count
	    is	* or is larger than the current cursor position, then the cur‐
	    sor is positioned at location 0 so that the next byte stored  will
	    be	the first byte in the result string.  If count is omitted then
	    the cursor is moved back one byte.	This type does not consume  an
	    argument.  For example,
		   binary format a3X*a3X2a3 abc def ghi
	    will return dghi.

       @    Moves  the	cursor	to  the absolute location in the output string
	    specified by count.	 Position 0 refers to the first	 byte  in  the
	    output string.  If count refers to a position beyond the last byte
	    stored so far, then null bytes will be placed in the uninitialized
	    locations and the cursor will be placed at the specified location.
	    If count is *, then the cursor is moved to the current end of  the
	    output  string.  If count is omitted, then an error will be gener‐
	    ated.  This type does not consume an argument. For example,
		   binary format a5@2a1@*a3@10a1 abcde f ghi j
	    will return abfdeghi\000\000j.

BINARY SCAN
       The binary scan command parses fields from a binary  string,  returning
       the  number  of conversions performed.  String gives the input bytes to
       be parsed (one byte per character, and characters not representable  as
       a  byte have their high bits chopped) and formatString indicates how to
       parse it.  Each varName gives the name of a variable; when a  field  is
       scanned	from  string the result is assigned to the corresponding vari‐
       able.

       As with binary format, the formatString consists of a sequence of  zero
       or  more field specifiers separated by zero or more spaces.  Each field
       specifier is a single type character followed by an optional flag char‐
       acter  followed	by  an	optional numeric count.	 Most field specifiers
       consume one argument to obtain the variable into which the scanned val‐
       ues should be placed.  The type character specifies how the binary data
       is to be interpreted.  The count typically indicates how many items  of
       the specified type are taken from the data.  If present, the count is a
       non-negative decimal integer or *, which normally indicates that all of
       the  remaining  items  in  the  data  are to be used.  If there are not
       enough bytes left after the current cursor position to satisfy the cur‐
       rent field specifier, then the corresponding variable is left untouched
       and binary scan returns immediately with the number of  variables  that
       were  set.   If there are not enough arguments for all of the fields in
       the format string that consume arguments, then an error	is  generated.
       The  flag  character “u” may be given to cause some types to be read as
       unsigned values. The flag is  accepted  for  all	 field	types  but  is
       ignored for non-integer fields.

       A  similar  example  as	with binary format should explain the relation
       between field specifiers and arguments in case of the binary scan  sub‐
       command:
	      binary scan $bytes s3s first second

       This  command (provided the binary string in the variable bytes is long
       enough) assigns a list of three integers	 to  the  variable  first  and
       assigns a single value to the variable second.  If bytes contains fewer
       than 8 bytes (i.e. four 2-byte integers), no assignment to second  will
       be  made,  and  if bytes contains fewer than 6 bytes (i.e. three 2-byte
       integers), no assignment to first will be made.	Hence:
	      puts [binary scan abcdefg s3s first second]
	      puts $first
	      puts $second
       will print (assuming neither variable is set previously):
	      1
	      25185 25699 26213
	      can't read "second": no such variable

       It is important to note that the c, s, and S (and i and I on 64bit sys‐
       tems)  will be scanned into long data size values.  In doing this, val‐
       ues that have their high bit set (0x80 for chars,  0x8000  for  shorts,
       0x80000000  for	ints), will be sign extended.  Thus the following will
       occur:
	      set signShort [binary format s1 0x8000]
	      binary scan $signShort s1 val; # val == 0xFFFF8000
       If you require unsigned values you can include the “u”  flag  character
       following the field type. For example, to read an unsigned short value:
	      set signShort [binary format s1 0x8000]
	      binary scan $signShort su1 val; # val == 0x00008000

       Each type-count pair moves an imaginary cursor through the binary data,
       reading bytes from the current position.	 The cursor  is	 initially  at
       position	 0  at	the beginning of the data.  The type may be any one of
       the following characters:

       a    The data is a byte string of length count.	If count  is  *,  then
	    all	 of  the  remaining  bytes  in string will be scanned into the
	    variable.  If count is omitted, then one  byte  will  be  scanned.
	    All	 bytes	scanned will be interpreted as being characters in the
	    range \u0000-\u00ff so the encoding convertfrom  command  will  be
	    needed if the string is not a binary string or a string encoded in
	    ISO 8859-1.	 For example,
		   binary scan abcde\000fghi a6a10 var1 var2
	    will return 1 with the string equivalent to	 abcde\000  stored  in
	    var1 and var2 left unmodified, and
		   binary scan \342\202\254 a* var1
		   set var2 [encoding convertfrom utf-8 $var1]
	    will store a Euro-currency character in var2.

       A    This  form	is the same as a, except trailing blanks and nulls are
	    stripped from the scanned value before it is stored in  the	 vari‐
	    able.  For example,
		   binary scan "abc efghi  \000" A* var1
	    will return 1 with abc efghi stored in var1.

       b    The data is turned into a string of count binary digits in low-to-
	    high order represented as a sequence of “1”	 and  “0”  characters.
	    The	 data  bytes  are scanned in first to last order with the bits
	    being taken in low-to-high order within each byte.	Any extra bits
	    in	the  last  byte	 are  ignored.	If count is *, then all of the
	    remaining bits in string will be scanned.  If  count  is  omitted,
	    then one bit will be scanned.  For example,
		   binary scan \x07\x87\x05 b5b* var1 var2
	    will  return  2  with  11100  stored  in var1 and 1110000110100000
	    stored in var2.

       B    This form is the same as b, except the bits are taken in  high-to-
	    low order within each byte.	 For example,
		   binary scan \x70\x87\x05 B5B* var1 var2
	    will  return  2  with  01110  stored  in var1 and 1000011100000101
	    stored in var2.

       H    The data is turned into a string of count  hexadecimal  digits  in
	    high-to-low	 order	represented as a sequence of characters in the
	    set “0123456789abcdef”.  The data bytes are scanned	 in  first  to
	    last  order	 with  the hex digits being taken in high-to-low order
	    within each byte. Any extra bits in the last byte are ignored.  If
	    count is *, then all of the remaining hex digits in string will be
	    scanned. If count is omitted, then one hex digit will be  scanned.
	    For example,
		   binary scan \x07\xC6\x05\x1f\x34 H3H* var1 var2
	    will return 2 with 07c stored in var1 and 051f34 stored in var2.

       h    This form is the same as H, except the digits are taken in reverse
	    (low-to-high) order within each byte. For example,
		   binary scan \x07\x86\x05\x12\x34 h3h* var1 var2
	    will return 2 with 706 stored in var1 and 502143 stored in var2.
       Note that most code that wishes to parse the  hexadecimal  digits  from
       multiple bytes in order should use the H format.

       c    The	 data is turned into count 8-bit signed integers and stored in
	    the corresponding variable as a list. If count is *, then  all  of
	    the	 remaining bytes in string will be scanned.  If count is omit‐
	    ted, then one 8-bit integer will be scanned.  For example,
		   binary scan \x07\x86\x05 c2c* var1 var2
	    will return 2 with 7 -122 stored in var1 and  5  stored  in	 var2.
	    Note  that	the integers returned are signed, but they can be con‐
	    verted to unsigned 8-bit quantities using an expression like:
		   set num [expr { $num & 0xff }]

       s    The data is interpreted as count  16-bit  signed  integers	repre‐
	    sented  in	little-endian  byte order.  The integers are stored in
	    the corresponding variable as a list.  If count is *, then all  of
	    the	 remaining bytes in string will be scanned.  If count is omit‐
	    ted, then one 16-bit integer will be scanned.  For example,
		   binary scan \x05\x00\x07\x00\xf0\xff s2s* var1 var2
	    will return 2 with 5 7 stored in var1  and	-16  stored  in	 var2.
	    Note  that	the integers returned are signed, but they can be con‐
	    verted to unsigned 16-bit quantities using an expression like:
		   set num [expr { $num & 0xffff }]

       S    This form is the same as s except that the data is interpreted  as
	    count 16-bit signed integers represented in big-endian byte order.
	    For example,
		   binary scan \x00\x05\x00\x07\xff\xf0 S2S* var1 var2
	    will return 2 with 5 7 stored in var1 and -16 stored in var2.

       t    The data is interpreted as count  16-bit  signed  integers	repre‐ │
	    sented  in	the  native  byte order of the machine running the Tcl │
	    script.  It is otherwise identical to s and S.  To determine  what │
	    the	 native	 byte  order of the machine is, refer to the byteOrder │
	    element of the tcl_platform array.

       i    The data is interpreted as count  32-bit  signed  integers	repre‐
	    sented  in	little-endian  byte order.  The integers are stored in
	    the corresponding variable as a list.  If count is *, then all  of
	    the	 remaining bytes in string will be scanned.  If count is omit‐
	    ted, then one 32-bit integer will be scanned.  For example,
		   set str \x05\x00\x00\x00\x07\x00\x00\x00\xf0\xff\xff\xff
		   binary scan $str i2i* var1 var2
	    will return 2 with 5 7 stored in var1  and	-16  stored  in	 var2.
	    Note  that	the integers returned are signed, but they can be con‐
	    verted to unsigned 32-bit quantities using an expression like:
		   set num [expr { $num & 0xffffffff }]

       I    This form is the same as I except that the data is interpreted  as
	    count 32-bit signed integers represented in big-endian byte order.
	    For example,
		   set str \x00\x00\x00\x05\x00\x00\x00\x07\xff\xff\xff\xf0
		   binary scan $str I2I* var1 var2
	    will return 2 with 5 7 stored in var1 and -16 stored in var2.

       n    The data is interpreted as count  32-bit  signed  integers	repre‐ │
	    sented  in	the  native  byte order of the machine running the Tcl │
	    script.  It is otherwise identical to i and I.  To determine  what │
	    the	 native	 byte  order of the machine is, refer to the byteOrder │
	    element of the tcl_platform array.

       w    The data is interpreted as count  64-bit  signed  integers	repre‐
	    sented  in	little-endian  byte order.  The integers are stored in
	    the corresponding variable as a list.  If count is *, then all  of
	    the	 remaining bytes in string will be scanned.  If count is omit‐
	    ted, then one 64-bit integer will be scanned.  For example,
		   set str \x05\x00\x00\x00\x07\x00\x00\x00\xf0\xff\xff\xff
		   binary scan $str wi* var1 var2
	    will return 2 with 30064771077 stored in var1 and  -16  stored  in
	    var2.   Note  that	the integers returned are signed and cannot be
	    represented by Tcl as unsigned values.

       W    This form is the same as w except that the data is interpreted  as
	    count 64-bit signed integers represented in big-endian byte order.
	    For example,
		   set str \x00\x00\x00\x05\x00\x00\x00\x07\xff\xff\xff\xf0
		   binary scan $str WI* var1 var2
	    will return 2 with 21474836487 stored in var1 and  -16  stored  in
	    var2.

       m    The	 data  is  interpreted	as count 64-bit signed integers repre‐ │
	    sented in the native byte order of the  machine  running  the  Tcl │
	    script.   It is otherwise identical to w and W.  To determine what │
	    the native byte order of the machine is, refer  to	the  byteOrder │
	    element of the tcl_platform array.

       f    The	 data  is interpreted as count single-precision floating point
	    numbers in the  machine's  native  representation.	 The  floating
	    point  numbers are stored in the corresponding variable as a list.
	    If count is *, then all of the remaining bytes in string  will  be
	    scanned.   If count is omitted, then one single-precision floating
	    point number will be scanned.  The size of a floating point number
	    may	 vary  across  architectures,  so the number of bytes that are
	    scanned may vary.  If the data does not represent a valid floating
	    point number, the resulting value is undefined and compiler depen‐
	    dent.  For example, on a Windows system running on an  Intel  Pen‐
	    tium processor,
		   binary scan \x3f\xcc\xcc\xcd f var1
	    will return 1 with 1.6000000238418579 stored in var1.

       r    This  form is the same as f except that the data is interpreted as │
	    count single-precision  floating  point  number  in	 little-endian │
	    order.  This conversion is not portable to the minority of systems │
	    not using IEEE floating point representations.

       R    This form is the same as f except that the data is interpreted  as │
	    count  single-precision floating point number in big-endian order. │
	    This conversion is not portable to the  minority  of  systems  not │
	    using IEEE floating point representations.

       d    This  form is the same as f except that the data is interpreted as
	    count double-precision floating point  numbers  in	the  machine's
	    native representation. For example, on a Windows system running on
	    an Intel Pentium processor,
		   binary scan \x9a\x99\x99\x99\x99\x99\xf9\x3f d var1
	    will return 1 with 1.6000000000000001 stored in var1.

       q    This form is the same as d except that the data is interpreted  as │
	    count  double-precision  floating  point  number  in little-endian │
	    order.  This conversion is not portable to the minority of systems │
	    not using IEEE floating point representations.

       Q    This  form is the same as d except that the data is interpreted as │
	    count double-precision floating point number in big-endian	order. │
	    This  conversion  is  not  portable to the minority of systems not │
	    using IEEE floating point representations.

       x    Moves the cursor forward count bytes in string.  If count is *  or
	    is	larger than the number of bytes after the current cursor posi‐
	    tion, then the cursor is positioned after the last byte in string.
	    If	count  is  omitted, then the cursor is moved forward one byte.
	    Note that this type does not consume an argument.  For example,
		   binary scan \x01\x02\x03\x04 x2H* var1
	    will return 1 with 0304 stored in var1.

       X    Moves the cursor back count bytes in string.  If count is * or  is
	    larger  than the current cursor position, then the cursor is posi‐
	    tioned at location 0 so that the next byte	scanned	 will  be  the
	    first  byte	 in  string.   If  count is omitted then the cursor is
	    moved back one byte.  Note that this  type	does  not  consume  an
	    argument.  For example,
		   binary scan \x01\x02\x03\x04 c2XH* var1 var2
	    will return 2 with 1 2 stored in var1 and 020304 stored in var2.

       @    Moves the cursor to the absolute location in the data string spec‐
	    ified by count.  Note that position 0 refers to the first byte  in
	    string.   If  count refers to a position beyond the end of string,
	    then the cursor is positioned after the last byte.	 If  count  is
	    omitted, then an error will be generated.  For example,
		   binary scan \x01\x02\x03\x04 c2@1H* var1 var2
	    will return 2 with 1 2 stored in var1 and 020304 stored in var2.

PORTABILITY ISSUES
       The  r, R, q and Q conversions will only work reliably for transferring
       data between computers which are all using IEEE floating	 point	repre‐
       sentations.   This  is  very  common,  but  not universal.  To transfer
       floating-point numbers portably between all  architectures,  use	 their
       textual representation (as produced by format) instead.

EXAMPLES
       This  is	 a procedure to write a Tcl string to a binary-encoded channel
       as UTF-8 data preceded by a length word:
	      proc writeString {channel string} {
		  set data [encoding convertto utf-8 $string]
		  puts -nonewline [binary format Ia* \
			  [string length $data] $data]
	      }

       This procedure reads a string from a channel that was  written  by  the
       previously presented writeString procedure:
	      proc readString {channel} {
		  if {![binary scan [read $channel 4] I length]} {
		      error "missing length"
		  }
		  set data [read $channel $length]
		  return [encoding convertfrom utf-8 $data]
	      }

SEE ALSO
       format(n), scan(n), tclvars(n)

KEYWORDS
       binary, format, scan

Tcl				      8.0			     binary(n)
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