Tcl Built-In Commands binary(1T)
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 values. For example, given the
values 16 and 22, on a 32 bit architecture, 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 com-
mand, 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 additional arguments. The resulting binary
value is returned.
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 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.
Here is a small example to clarify the relation between the
field specifiers 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 specifier. The resulting binary string
contains the four numbers 1.0, 2.0, 3.0 and 0.1.
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Tcl Built-In Commands binary(1T)
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 charac-
ters:
a Stores a character string of length count in the output
string. Every character is taken as modulo 256 (i.e.
the low byte of every character is used, and the high
byte discarded) so when storing character strings not
wholly expressible using the characters \u0000-\u00ff,
the encoding convertto command should be used first if
this truncation 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 formatted. If count is omitted, then one char-
acter will be formatted. For example,
binary format a7a*a alpha bravo charlie
will return a string equivalent to alpha\000\000bravoc.
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 result-
ing 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 remain-
ing 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 low-to-
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Tcl Built-In Commands binary(1T)
high 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 low-to-high 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 for-
matted does not end at a byte boundary, the remaining
bits of the last byte will be zeros. For example,
binary format h3h* AB def
will return a string equivalent to \xba\x00\xed\x0f.
H This form is the same as h except that the digits are
stored in high-to-low order within each byte. For
example,
binary format H3H* ab DEF
will return a string equivalent to \xab\x00\xde\xf0.
c Stores one or more 8-bit integer values in the output
string. If no count is specified, then arg must con-
sist of an integer value; otherwise arg must consist of
a list containing at least count integer elements. 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 fewer than count,
then an error is generated. 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,
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Tcl Built-In Commands binary(1T)
binary format S3 {3 -3 258 1}
will return a string equivalent to
\x00\x03\xff\xfd\x01\x02.
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
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 posi-
tion 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
f This form is the same as c except that it stores one or
more one or more single-precision floating in the
machine's native representation in the output string.
This representation is not portable across architec-
tures, so it should not be used to communicate floating
point numbers across the network. The size of a float-
ing point number may vary across architectures, so the
number of bytes that are generated may vary. If the
value overflows the machine's native representation,
then the value of FLTMAX 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-
precision. For example, on a Windows system running on
an Intel Pentium processor,
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Tcl Built-In Commands binary(1T)
binary format f2 {1.6 3.4}
will return a string equivalent to
\xcd\xcc\xcc\x3f\x9a\x99\x59\x40.
d This form is the same as f except that it stores one or
more one or more double-precision floating 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.
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 cursor 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 generated. 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 to be parsed 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 variable.
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Tcl Built-In Commands binary(1T)
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 char-
acter followed by an optional numeric count. Most field
specifiers consume one argument to obtain the variable into
which the scanned values should be placed. The type charac-
ter 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 current 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.
A similar example as with binary format should explain the
relation between field specifiers and arguments in case of
the binary scan subcommand:
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 vari-
able 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 systems) will be scanned into long data size values.
In doing this, values 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 $signShort s1 val; # val == 0xF8000
If you want to produce an unsigned value, then you can mask
the return value to the desired size. For example, to pro-
duce an unsigned short value:
set val [expr {$val & 0xF}]; # val == 0x8000
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Tcl Built-In Commands binary(1T)
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 character string of length count. If
count is *, then all of the remaining bytes in string
will be scanned into the variable. If count is omit-
ted, then one character will be scanned. All charac-
ters scanned will be interpreted as being in the range
\u0000-\u00ff so the encoding convertfrom command might
be needed if the string is not an ISO 8859-1 string.
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.
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 variable. 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 low-to-high 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 low-to-high 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\x86\x05 h3h* var1 var2
will return 2 with 706 stored in var1 and 50 stored in
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Tcl Built-In Commands binary(1T)
var2.
H This form is the same as h, except the digits are taken
in high-to-low order within each byte. For example,
binary scan \x07\x86\x05 H3H* var1 var2
will return 2 with 078 stored in var1 and 05 stored in
var2.
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 omitted, 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:
expr { $num & 0xff }
s The data is interpreted as count 16-bit signed integers
represented 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 omitted, 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:
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.
i The data is interpreted as count 32-bit signed integers
represented 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 omitted, then one 32-bit
integer will be scanned. For example,
binary scan \x05\x00\x00\x00\x07\x00\x00\x00\xf0\xff\xff\xff 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:
expr { $num & 0xffffffff }
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Tcl Built-In Commands binary(1T)
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,
binary scan \x00\x00\x00\x05\x00\x00\x00\x07\xff\xff\xff\xf0 I2I* var1 var2
will return 2 with 5 7 stored in var1 and -16 stored in
var2.
w The data is interpreted as count 64-bit signed integers
represented 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 omitted, then one 64-bit
integer will be scanned. For example,
binary scan \x05\x00\x00\x00\x07\x00\x00\x00\xf0\xff\xff\xff 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,
binary scan \x00\x00\x00\x05\x00\x00\x00\x07\xff\xff\xff\xf0 WI* var1 var2
will return 2 with 21474836487 stored in var1 and -16
stored in var2.
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 archi-
tectures, 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 com-
piler dependent. For example, on a Windows system run-
ning on an Intel Pentium processor,
binary scan \x3f\xcc\xcc\xcd f var1
will return 1 with 1.6000000238418579 stored in var1.
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 Pen-
tium processor,
binary scan \x9a\x99\x99\x99\x99\x99\xf9\x3f d var1
will return 1 with 1.6000000000000001 stored in var1.
x Moves the cursor forward count bytes in string. If
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Tcl Built-In Commands binary(1T)
count is * or is larger than the number of bytes after
the current cursor cursor position, 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 positioned 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 argu-
ment. 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 specified 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.
PLATFORM ISUES
Sometimes it is desirable to format or scan integer values
in the native byte order for the machine. Refer to the
byteOrder element of the tclplatform array to decide which
type character to use when formatting or scanning integers.
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 writ-
ten 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]
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Tcl Built-In Commands binary(1T)
return [encoding convertfrom utf-8 $data]
}
SEE ALSO
format(1T), scan(1T), tclvars(1T)
KEYWORDS
binary, format, scan
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
Tcl Last change: 8.0 11
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