path: root/doc/binary.n

'\"
'\" Copyright (c) 1997 by Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
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'\" RCS: @(#) $Id: binary.n,v 1.11.2.6 2004/10/27 09:35:38 dkf Exp $
'\" 
.so man.macros
.TH binary n 8.0 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
binary \- Insert and extract fields from binary strings
.SH SYNOPSIS
\fBbinary format \fIformatString \fR?\fIarg arg ...\fR?
.br
\fBbinary scan \fIstring formatString \fR?\fIvarName varName ...\fR?
.BE

.SH DESCRIPTION
.PP
This command provides facilities for manipulating binary data.  The
first form, \fBbinary format\fR, 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 command, \fBbinary scan\fR, does the opposite: it extracts data
from a binary string and returns it as ordinary Tcl string values.

.SH "BINARY FORMAT"
.PP
The \fBbinary format\fR command generates a binary string whose layout
is specified by the \fIformatString\fR and whose contents come from
the additional arguments.  The resulting binary value is returned.
.PP
The \fIformatString\fR 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 \fIcount\fR.
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 \fIcount\fR typically indicates how many items of the
specified type are taken from the value.  If present, the \fIcount\fR
is a non-negative decimal integer or \fB*\fR, 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.
.PP
Here is a small example to clarify the relation between the field
specifiers and the arguments:
.CS
\fBbinary format d3d {1.0 2.0 3.0 4.0} 0.1\fR
.CE
.PP
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.
.PP
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:
.IP \fBa\fR 5
Stores a character string of length \fIcount\fR 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 \fBencoding convertto\fR 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 \fIarg\fR has fewer than \fIcount\fR bytes, then additional zero
bytes are used to pad out the field.  If \fIarg\fR is longer than the
specified length, the extra characters will be ignored.  If
\fIcount\fR is \fB*\fR, then all of the bytes in \fIarg\fR will be
formatted.  If \fIcount\fR is omitted, then one character will be
formatted.  For example,
.RS
.CS
\fBbinary format a7a*a alpha bravo charlie\fR
.CE
will return a string equivalent to \fBalpha\\000\\000bravoc\fR.
.RE
.IP \fBA\fR 5
This form is the same as \fBa\fR except that spaces are used for
padding instead of nulls.  For example,
.RS
.CS
\fBbinary format A6A*A alpha bravo charlie\fR
.CE
will return \fBalpha bravoc\fR.
.RE
.IP \fBb\fR 5
Stores a string of \fIcount\fR binary digits in low-to-high order
within each byte in the output string.  \fIArg\fR must contain a
sequence of \fB1\fR and \fB0\fR 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 \fIarg\fR has fewer than
\fIcount\fR digits, then zeros will be used for the remaining bits.
If \fIarg\fR has more than the specified number of digits, the extra
digits will be ignored.  If \fIcount\fR is \fB*\fR, then all of the
digits in \fIarg\fR will be formatted.  If \fIcount\fR 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,
.RS
.CS
\fBbinary format b5b* 11100 111000011010\fR
.CE
will return a string equivalent to \fB\\x07\\x87\\x05\fR.
.RE
.IP \fBB\fR 5
This form is the same as \fBb\fR except that the bits are stored in
high-to-low order within each byte.  For example,
.RS
.CS
\fBbinary format B5B* 11100 111000011010\fR
.CE
will return a string equivalent to \fB\\xe0\\xe1\\xa0\fR.
.RE
.IP \fBh\fR 5
Stores a string of \fIcount\fR hexadecimal digits in low-to-high
within each byte in the output string.  \fIArg\fR 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 \fIarg\fR
has fewer than \fIcount\fR digits, then zeros will be used for the
remaining digits.  If \fIarg\fR has more than the specified number of
digits, the extra digits will be ignored.  If \fIcount\fR is
\fB*\fR, then all of the digits in \fIarg\fR will be formatted.  If
\fIcount\fR 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,
.RS
.CS
\fBbinary format h3h* AB def\fR
.CE
will return a string equivalent to \fB\\xba\\x00\\xed\\x0f\fR.
.RE
.IP \fBH\fR 5
This form is the same as \fBh\fR except that the digits are stored in
high-to-low order within each byte.  For example,
.RS
.CS
\fBbinary format H3H* ab DEF\fR
.CE
will return a string equivalent to \fB\\xab\\x00\\xde\\xf0\fR.
.RE
.IP \fBc\fR 5
Stores one or more 8-bit integer values in the output string.  If no
\fIcount\fR is specified, then \fIarg\fR must consist of an integer
value; otherwise \fIarg\fR must consist of a list containing at least
\fIcount\fR integer elements.  The low-order 8 bits of each integer
are stored as a one-byte value at the cursor position.  If \fIcount\fR
is \fB*\fR, then all of the integers in the list are formatted.  If
the number of elements in the list is fewer than \fIcount\fR, then an
error is generated.  If the number of elements in the list is greater
than \fIcount\fR, then the extra elements are ignored.  For example,
.RS
.CS
\fBbinary format c3cc* {3 -3 128 1} 260 {2 5}\fR
.CE
will return a string equivalent to
\fB\\x03\\xfd\\x80\\x04\\x02\\x05\fR, whereas
.CS
\fBbinary format c {2 5}\fR
.CE
will generate an error.
.RE
.IP \fBs\fR 5
This form is the same as \fBc\fR 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,
.RS
.CS
\fBbinary format s3 {3 -3 258 1}\fR
.CE
will return a string equivalent to 
\fB\\x03\\x00\\xfd\\xff\\x02\\x01\fR.
.RE
.IP \fBS\fR 5
This form is the same as \fBs\fR except that it stores one or more
16-bit integers in big-endian byte order in the output string.  For
example,
.RS
.CS
\fBbinary format S3 {3 -3 258 1}\fR
.CE
will return a string equivalent to 
\fB\\x00\\x03\\xff\\xfd\\x01\\x02\fR.
.RE
.IP \fBi\fR 5
This form is the same as \fBc\fR 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,
.RS
.CS
\fBbinary format i3 {3 -3 65536 1}\fR
.CE
will return a string equivalent to 
\fB\\x03\\x00\\x00\\x00\\xfd\\xff\\xff\\xff\\x00\\x00\\x01\\x00\fR
.RE
.IP \fBI\fR 5
This form is the same as \fBi\fR except that it stores one or more one
or more 32-bit integers in big-endian byte order in the output string.
For example,
.RS
.CS
\fBbinary format I3 {3 -3 65536 1}\fR
.CE
will return a string equivalent to 
\fB\\x00\\x00\\x00\\x03\\xff\\xff\\xff\\xfd\\x00\\x01\\x00\\x00\fR
.RE
.IP \fBw\fR 5
.VS 8.4
This form is the same as \fBc\fR 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,
.RS
.CS
\fBbinary format w 7810179016327718216\fR
.CE
will return the string \fBHelloTcl\fR
.RE
.IP \fBW\fR 5
This form is the same as \fBw\fR except that it stores one or more one
or more 64-bit integers in big-endian byte order in the output string.
For example,
.RS
.CS
\fBbinary format Wc 4785469626960341345 110\fR
.CE
will return the string \fBBigEndian\fR
.VE
.RE
.IP \fBf\fR 5
This form is the same as \fBc\fR 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 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 precision in the conversion to single-precision.  For example,
on a Windows system running on an Intel Pentium processor,
.RS
.CS
\fBbinary format f2 {1.6 3.4}\fR
.CE
will return a string equivalent to 
\fB\\xcd\\xcc\\xcc\\x3f\\x9a\\x99\\x59\\x40\fR.
.RE
.IP \fBd\fR 5
This form is the same as \fBf\fR 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,
.RS
.CS
\fBbinary format d1 {1.6}\fR
.CE
will return a string equivalent to 
\fB\\x9a\\x99\\x99\\x99\\x99\\x99\\xf9\\x3f\fR.
.RE
.IP \fBx\fR 5
Stores \fIcount\fR null bytes in the output string.  If \fIcount\fR is
not specified, stores one null byte.  If \fIcount\fR is \fB*\fR,
generates an error.  This type does not consume an argument.  For
example,
.RS
.CS
\fBbinary format a3xa3x2a3 abc def ghi\fR
.CE
will return a string equivalent to \fBabc\\000def\\000\\000ghi\fR.
.RE
.IP \fBX\fR 5
Moves the cursor back \fIcount\fR bytes in the output string.  If
\fIcount\fR is \fB*\fR 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 \fIcount\fR is
omitted then the cursor is moved back one byte.  This type does not
consume an argument.  For example,
.RS
.CS
\fBbinary format a3X*a3X2a3 abc def ghi\fR
.CE
will return \fBdghi\fR.
.RE
.IP \fB@\fR 5
Moves the cursor to the absolute location in the output string
specified by \fIcount\fR.  Position 0 refers to the first byte in the
output string.  If \fIcount\fR 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
\fIcount\fR is \fB*\fR, then the cursor is moved to the current end of
the output string.  If \fIcount\fR is omitted, then an error will be
generated.  This type does not consume an argument. For example,
.RS
.CS
\fBbinary format a5@2a1@*a3@10a1 abcde f ghi j\fR
.CE
will return \fBabfdeghi\\000\\000j\fR.
.RE

.SH "BINARY SCAN"
.PP
The \fBbinary scan\fR command parses fields from a binary string,
returning the number of conversions performed.  \fIString\fR gives the
input to be parsed and \fIformatString\fR indicates how to parse it.
Each \fIvarName\fR gives the name of a variable; when a field is
scanned from \fIstring\fR the result is assigned to the corresponding
variable.
.PP
As with \fBbinary format\fR, the \fIformatString\fR 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 \fIcount\fR.  Most field specifiers consume one
argument to obtain the variable into which the scanned values should
be placed.  The type character specifies how the binary data is to be
interpreted.  The \fIcount\fR typically indicates how many items of
the specified type are taken from the data.  If present, the
\fIcount\fR is a non-negative decimal integer or \fB*\fR, 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 \fBbinary scan\fR 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.
.PP
A similar example as with \fBbinary format\fR should explain the
relation between field specifiers and arguments in case of the binary
scan subcommand:
.CS
\fBbinary scan $bytes s3s first second\fR
.CE
.PP
This command (provided the binary string in the variable \fIbytes\fR
is long enough) assigns a list of three integers to the variable
\fIfirst\fR and assigns a single value to the variable \fIsecond\fR.
If \fIbytes\fR contains fewer than 8 bytes (i.e. four 2-byte
integers), no assignment to \fIsecond\fR will be made, and if
\fIbytes\fR contains fewer than 6 bytes (i.e. three 2-byte integers),
no assignment to \fIfirst\fR will be made.  Hence:
.CS
\fBputs [binary scan abcdefg s3s first second]\fR
\fBputs $first\fR
\fBputs $second\fR
.CE
will print (assuming neither variable is set previously):
.CS
\fB1\fR
\fB25185 25699 26213\fR
\fIcan't read "second": no such variable\fR
.CE
.PP
It is \fBimportant\fR to note that the \fBc\fR, \fBs\fR, and \fBS\fR
(and \fBi\fR and \fBI\fR 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:
.CS
\fBset signShort [binary format s1 0x8000]\fR
\fBbinary scan $signShort s1 val; \fI# val == 0xFFFF8000\fR
.CE
If you want to produce an unsigned value, then you can mask the return 
value to the desired size.  For example, to produce an unsigned short 
value:
.CS
\fBset val [expr {$val & 0xFFFF}]; \fI# val == 0x8000\fR
.CE
.PP
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:
.IP \fBa\fR 5
The data is a character string of length \fIcount\fR.  If \fIcount\fR
is \fB*\fR, then all of the remaining bytes in \fIstring\fR will be
scanned into the variable.  If \fIcount\fR is omitted, then one
character will be scanned.
All characters scanned will be interpreted as being in the
range \\u0000-\\u00ff so the \fBencoding convertfrom\fR command might be
needed if the string is not an ISO 8859\-1 string.
For example,
.RS
.CS
\fBbinary scan abcde\\000fghi a6a10 var1 var2\fR
.CE
will return \fB1\fR with the string equivalent to \fBabcde\\000\fR
stored in \fBvar1\fR and \fBvar2\fR left unmodified.
.RE
.IP \fBA\fR 5
This form is the same as \fBa\fR, except trailing blanks and nulls are stripped from
the scanned value before it is stored in the variable.  For example,
.RS
.CS
\fBbinary scan "abc efghi  \\000" A* var1\fR
.CE
will return \fB1\fR with \fBabc efghi\fR stored in \fBvar1\fR.
.RE
.IP \fBb\fR 5
The data is turned into a string of \fIcount\fR 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 \fIcount\fR is \fB*\fR, then
all of the remaining bits in \fBstring\fR will be scanned.  If
\fIcount\fR is omitted, then one bit will be scanned.  For example,
.RS
.CS
\fBbinary scan \\x07\\x87\\x05 b5b* var1 var2\fR
.CE
will return \fB2\fR with \fB11100\fR stored in \fBvar1\fR and
\fB1110000110100000\fR stored in \fBvar2\fR.
.RE
.IP \fBB\fR 5
This form is the same as \fBb\fR, except the bits are taken in
high-to-low order within each byte.  For example,
.RS
.CS
\fBbinary scan \\x70\\x87\\x05 B5B* var1 var2\fR
.CE
will return \fB2\fR with \fB01110\fR stored in \fBvar1\fR and
\fB1000011100000101\fR stored in \fBvar2\fR.
.RE
.IP \fBh\fR 5
The data is turned into a string of \fIcount\fR 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 \fIcount\fR
is \fB*\fR, then all of the remaining hex digits in \fBstring\fR will be
scanned.  If \fIcount\fR is omitted, then one hex digit will be
scanned.  For example,
.RS
.CS
\fBbinary scan \\x07\\x86\\x05 h3h* var1 var2\fR
.CE
will return \fB2\fR with \fB706\fR stored in \fBvar1\fR and
\fB50\fR stored in \fBvar2\fR.
.RE
.IP \fBH\fR 5
This form is the same as \fBh\fR, except the digits are taken in
high-to-low order within each byte.  For example,
.RS
.CS
\fBbinary scan \\x07\\x86\\x05 H3H* var1 var2\fR
.CE
will return \fB2\fR with \fB078\fR stored in \fBvar1\fR and
\fB05\fR stored in \fBvar2\fR.
.RE
.IP \fBc\fR 5
The data is turned into \fIcount\fR 8-bit signed integers and stored
in the corresponding variable as a list. If \fIcount\fR is \fB*\fR,
then all of the remaining bytes in \fBstring\fR will be scanned.  If
\fIcount\fR is omitted, then one 8-bit integer will be scanned.  For
example,
.RS
.CS
\fBbinary scan \\x07\\x86\\x05 c2c* var1 var2\fR
.CE
will return \fB2\fR with \fB7 -122\fR stored in \fBvar1\fR and \fB5\fR
stored in \fBvar2\fR.  Note that the integers returned are signed, but
they can be converted to unsigned 8-bit quantities using an expression
like:
.CS
\fBexpr ( $num + 0x100 ) % 0x100\fR
.CE
.RE
.IP \fBs\fR 5
The data is interpreted as \fIcount\fR 16-bit signed integers
represented in little-endian byte order.  The integers are stored in
the corresponding variable as a list.  If \fIcount\fR is \fB*\fR, then
all of the remaining bytes in \fBstring\fR will be scanned.  If
\fIcount\fR is omitted, then one 16-bit integer will be scanned.  For
example,
.RS
.CS
\fBbinary scan \\x05\\x00\\x07\\x00\\xf0\\xff s2s* var1 var2\fR
.CE
will return \fB2\fR with \fB5 7\fR stored in \fBvar1\fR and \fB-16\fR
stored in \fBvar2\fR.  Note that the integers returned are signed, but
they can be converted to unsigned 16-bit quantities using an expression
like:
.CS
\fBexpr ( $num + 0x10000 ) % 0x10000\fR
.CE
.RE
.IP \fBS\fR 5
This form is the same as \fBs\fR except that the data is interpreted
as \fIcount\fR 16-bit signed integers represented in big-endian byte
order.  For example,
.RS
.CS
\fBbinary scan \\x00\\x05\\x00\\x07\\xff\\xf0 S2S* var1 var2\fR
.CE
will return \fB2\fR with \fB5 7\fR stored in \fBvar1\fR and \fB-16\fR
stored in \fBvar2\fR. 
.RE
.IP \fBi\fR 5
The data is interpreted as \fIcount\fR 32-bit signed integers
represented in little-endian byte order.  The integers are stored in
the corresponding variable as a list.  If \fIcount\fR is \fB*\fR, then
all of the remaining bytes in \fBstring\fR will be scanned.  If
\fIcount\fR is omitted, then one 32-bit integer will be scanned.  For
example,
.RS
.CS
\fBbinary scan \\x05\\x00\\x00\\x00\\x07\\x00\\x00\\x00\\xf0\\xff\\xff\\xff i2i* var1 var2\fR
.CE
will return \fB2\fR with \fB5 7\fR stored in \fBvar1\fR and \fB-16\fR
stored in \fBvar2\fR.  Note that the integers returned are signed and
cannot be represented by Tcl as unsigned values.
.RE
.IP \fBI\fR 5
This form is the same as \fBI\fR except that the data is interpreted
as \fIcount\fR 32-bit signed integers represented in big-endian byte
order.  For example,
.RS
.CS
\fBbinary scan \\x00\\x00\\x00\\x05\\x00\\x00\\x00\\x07\\xff\\xff\\xff\\xf0 I2I* var1 var2\fR
.CE
will return \fB2\fR with \fB5 7\fR stored in \fBvar1\fR and \fB-16\fR
stored in \fBvar2\fR.
.RE
.IP \fBw\fR 5
.VS 8.4
The data is interpreted as \fIcount\fR 64-bit signed integers
represented in little-endian byte order.  The integers are stored in
the corresponding variable as a list.  If \fIcount\fR is \fB*\fR, then
all of the remaining bytes in \fBstring\fR will be scanned.  If
\fIcount\fR is omitted, then one 64-bit integer will be scanned.  For
example,
.RS
.CS
\fBbinary scan \\x05\\x00\\x00\\x00\\x07\\x00\\x00\\x00\\xf0\\xff\\xff\\xff wi* var1 var2\fR
.CE
will return \fB2\fR with \fB30064771077\fR stored in \fBvar1\fR and
\fB-16\fR stored in \fBvar2\fR.  Note that the integers returned are
signed and cannot be represented by Tcl as unsigned values.
.RE
.IP \fBW\fR 5
This form is the same as \fBw\fR except that the data is interpreted
as \fIcount\fR 64-bit signed integers represented in big-endian byte
order.  For example,
.RS
.CS
\fBbinary scan \\x00\\x00\\x00\\x05\\x00\\x00\\x00\\x07\\xff\\xff\\xff\\xf0 WI* var1 var2\fR
.CE
will return \fB2\fR with \fB21474836487\fR stored in \fBvar1\fR and \fB-16\fR
stored in \fBvar2\fR.
.VE
.RE
.IP \fBf\fR 5
The data is interpreted as \fIcount\fR 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
\fIcount\fR is \fB*\fR, then all of the remaining bytes in
\fBstring\fR will be scanned.  If \fIcount\fR 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 dependent.  For example, on a Windows system running on an
Intel Pentium processor,
.RS
.CS
\fBbinary scan \\x3f\\xcc\\xcc\\xcd f var1\fR
.CE
will return \fB1\fR with \fB1.6000000238418579\fR stored in
\fBvar1\fR.
.RE
.IP \fBd\fR 5
This form is the same as \fBf\fR except that the data is interpreted
as \fIcount\fR double-precision floating point numbers in the
machine's native representation. For example, on a Windows system
running on an Intel Pentium processor,
.RS
.CS
\fBbinary scan \\x9a\\x99\\x99\\x99\\x99\\x99\\xf9\\x3f d var1\fR
.CE
will return \fB1\fR with \fB1.6000000000000001\fR
stored in \fBvar1\fR.
.RE
.IP \fBx\fR 5
Moves the cursor forward \fIcount\fR bytes in \fIstring\fR.  If
\fIcount\fR is \fB*\fR or is larger than the number of bytes after the
current cursor cursor position, then the cursor is positioned after
the last byte in \fIstring\fR.  If \fIcount\fR is omitted, then the
cursor is moved forward one byte.  Note that this type does not
consume an argument.  For example,
.RS
.CS
\fBbinary scan \\x01\\x02\\x03\\x04 x2H* var1\fR
.CE
will return \fB1\fR with \fB0304\fR stored in \fBvar1\fR.
.RE
.IP \fBX\fR 5
Moves the cursor back \fIcount\fR bytes in \fIstring\fR.  If
\fIcount\fR is \fB*\fR 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 \fIstring\fR.  If \fIcount\fR
is omitted then the cursor is moved back one byte.  Note that this
type does not consume an argument.  For example,
.RS
.CS
\fBbinary scan \\x01\\x02\\x03\\x04 c2XH* var1 var2\fR
.CE
will return \fB2\fR with \fB1 2\fR stored in \fBvar1\fR and \fB020304\fR
stored in \fBvar2\fR.
.RE
.IP \fB@\fR 5
Moves the cursor to the absolute location in the data string specified
by \fIcount\fR.  Note that position 0 refers to the first byte in
\fIstring\fR.  If \fIcount\fR refers to a position beyond the end of
\fIstring\fR, then the cursor is positioned after the last byte.  If
\fIcount\fR is omitted, then an error will be generated.  For example,
.RS
.CS
\fBbinary scan \\x01\\x02\\x03\\x04 c2@1H* var1 var2\fR
.CE
will return \fB2\fR with \fB1 2\fR stored in \fBvar1\fR and \fB020304\fR
stored in \fBvar2\fR.
.RE
.SH "PLATFORM ISSUES"
Sometimes it is desirable to format or scan integer values in the
native byte order for the machine.  Refer to the \fBbyteOrder\fR
element of the \fBtcl_platform\fR array to decide which type character
to use when formatting or scanning integers.
.SH EXAMPLES
This is a procedure to write a Tcl string to a binary-encoded channel as
UTF-8 data preceded by a length word:
.CS
proc writeString {channel string} {
    set data [encoding convertto utf-8 $string]
    puts -nonewline [\fBbinary\fR format Ia* \e
            [string length $data] $data]
}
.CE
.PP
This procedure reads a string from a channel that was written by the
previously presented \fBwriteString\fR procedure:
.CS
proc readString {channel} {
    if {![\fBbinary\fR scan [read $channel 4] I length]} {
        error "missing length"
    }
    set data [read $channel $length]
    return [encoding convertfrom utf-8 $data]
}
.CE

.SH "SEE ALSO"
format(n), scan(n), tclvars(n)

.SH KEYWORDS
binary, format, scan