Question

I have a problem understanding and using the ‘vec’ keyword.

I am reading a logpacket in which values are stored in little endian hexadecimal. In my code, I have to unpack the different bytes into scalars using the unpack keyword.

Here’s an example of my problem:

my @hexData1 = qw(50 65);
my $data = pack ('C*', @hexData1);
my $x = unpack("H4",$data);    # At which point the hexadecimal number became a number 
print $x."\n";
#my $foo = sprintf("%x", $foo);

print "$_-> " . vec("\x65\x50", $_, 1) . ", " for (0..15);    # This works.
print "\n";

But I want to use the above statement in the way below. I don’t want to send a string of hexadecimal in quotes. I want to use the scalar array of hex $x. But it won’t work. How do I convert my $x to a hexadecimal string. This is my requirement.

print "$_-> " . vec($x, $_, 1).", " for (0..15);    # This doesn't work.
print "\n";

My final objective is to read the third bit from the right of the two byte hexadecimal number.

How do I use the ‘vec’ command for that?

Answer 1

You are making the mistake of unpacking $data into $x before using it in a call to vec. vec expects a string, so if you supply a number it will be converted to a string before being used. Here’s your code

my @hexData1 = qw(50 65);
my $data= pack ('C*', @hexData1);

The C pack format uses each value in the source list as a character code. It is the same as calling chr on each value and concatenating them. Unfortunately your values look like decimal, so you are getting chr(50).chr(65) or "2A". Since your values are little-endian, what you want is chr(0x65).chr(0x50) or "\x65\x50", so you must write

my $data= pack ('(H2)*', reverse @hexData1);

which reverses the list of data (to account for it being little-endian) and packs it as if it was a list of two-digit hex strings (which, fortunately, it is).

Now you have done enough. As I say, vec expects a string so you can write

print join ' ', map vec($data, $_, 1), 0 .. 15;
print "\n";

and it will show you the bits you expect. To extract the the 3rd bit from the right (assuming you mean bit 13, where the last bit is bit 15) you want

print vec $data, 13, 1;