Convert bit vector to binary in Perl - perl

Question

I'm not sure of the best way to describe this.
Essentially I am attempting to write to a buffer which requires a certain protocol. The first two bytes I would like to are "10000001" and "11111110" (bit by bit). How can I write these two bytes to a file handle in Perl?

Answer 1

To convert spelled-out binary to actual bytes, you want the pack function with either B or b (depending on the order you have the bits in):
print FILE pack('B*', '1000000111111110');
However, if the bytes are constant, it's probably better to convert them to hex values and use the \x escape with a string literal:
print FILE "\x81\xFE";

Answer 2

How about
# open my $fh, ...
print $fh "\x81\xFE"; # 10000001 and 11111110

Answer 3

Since version 5.6.0 (released in March 2000), perl has supported binary literals as documented in perldata:
Numeric literals are specified in any of the following floating point or integer formats:
12345
12345.67
.23E-10 # a very small number
3.14_15_92 # a very important number
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
0377 # octal (only numbers, begins with 0)
0b011011 # binary
You are allowed to use underscores (underbars) in numeric literals between digits for legibility. You could, for example, group binary digits by threes (as for a Unix-style mode argument such as 0b110_100_100) or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
You may be tempted to write
print $fh 0b10000001, 0b11111110;
but the output would be
129254
because 10000001₂ = 129₁₀ and 11111110₂ = 254₁₀.
You want a specific representation of the literals’ values, namely as two unsigned bytes. For that, use pack with a template of "C2", i.e., octet times two. Adding underscores for readability and wrapping it in a convenient subroutine gives
sub write_marker {
my($fh) = #_;
print $fh pack "C2", 0b1000_0001, 0b1111_1110;
}
As a quick demo, consider
binmode STDOUT or die "$0: binmode: $!\n"; # we'll send binary data
write_marker *STDOUT;
When run as
$ ./marker-demo | od -t x1
the output is
0000000 81 fe
0000002
In case it’s unfamiliar, the od utility is used here for presentational purposes because the output contains a control character and Þ (Latin small thorn) in my system’s encoding.
The invocation above commands od to render in hexadecimal each byte from its input, which is the output of marker-demo. Note that 10000001₂ = 81₁₆ and 11111110₂ = FE₁₆. The numbers in the left-hand column are offsets: the special marker bytes start at offset zero (that is, immediately), and there are exactly two of them.