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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 con-
vertto 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 truncating 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 equiva-
lent 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 equiva-
lent 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 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 digits. 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 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 Hel-
loTcl
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 equiva-
lent 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 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 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 unmod-
ified, 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 and 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 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 converted 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 converted 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 converted 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)
binary 8.5.4 - Generated Sun Aug 17 16:17:16 CDT 2008
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