| |
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 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 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 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 character
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 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 low-to-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 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* 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 consist 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 gener-
ated. If the number of elements in the list is greater than
count, then the extra elements 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.
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 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
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 repre-
sentation in the output string. This representation is not porta-
ble 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 overflows 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 pre-
cision in the conversion to single-precision. For example, on a
Windows system running on an Intel Pentium processor,
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 repre-
sentation 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 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 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.
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 numeric
count. Most field specifiers consume one argument to obtain the vari-
able into which the scanned values should be placed. The type charac-
ter specifies how the binary data is to be interpreted. The count typ-
ically 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 corre-
sponding 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 argu-
ments, 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 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 $signShort
s1 val; # val == 0xFFFF8000 If you want to produce an unsigned value,
then you can mask the return value to the desired size. For example,
to produce an unsigned short value: set val [expr {$val & 0xFFFF}]; #
val == 0x8000
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 omitted, then one character will be
scanned. All characters 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 unmod-
ified.
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'' charac-
ters. 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 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 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: 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: 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 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,
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 }
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 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,
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 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.
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.
x Moves the cursor forward count bytes in string. If 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 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.
PLATFORM ISSUES
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 tcl_platform array to decide which type character to use when for-
matting 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 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)
|