Converting hexadecimal numbers in strings to negative numbers, in Perl

Question

I have a bunch of numbers represented as hexadecimal strings in log files that are being parsed by a Perl script, and I'm relatively inexperienced with Perl.

Some of these numbers are actually signed negative numbers, i.e. 0xFFFE == -2 when represented as a 16-bit signed integer.

Can somebody please tell me the canonical way of getting the signed representation of this number from the string FFFE in Perl, or otherwise point me to a tutorial or other resource?

Just FWIW, at least in 2's complement, FFFE is -2 -- FFFF is -1. — Jerry Coffin, Jan 21 '10 at 18:37

score 21 · Accepted Answer · edited Apr 25 '12 at 08:19

21

You can use the hex() function to convert from hexadecimal to decimal, but it interprets the input as an unsigned value. To compensate for that, pack the decimal value as an unsigned quantity and unpack it as a signed one:

my $num = unpack('s', pack('S', hex('FFFE')));

The 's' and 'S' templates are for signed and unsigned 16-bit quantities, respectively. See the documentation for the pack function for other templates and usage information.

edited Apr 25 '12 at 08:19

Peter Mortensen

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answered Jan 21 '10 at 18:41

Michael Carman

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I wish more people knew about pack. It's such a wonderful tool. :) – brian d foy Jan 21 '10 at 18:52
At least one more does now. This was exactly what I needed, thank you so much Michael. – Alex Marshall Jan 21 '10 at 18:56

score 1 · Answer 2 · answered Apr 25 '12 at 09:12

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print unpack('s>', pack('H4', 'FFFE'));
-2

answered Apr 25 '12 at 09:12

Oleg V. Volkov

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Converting hexadecimal numbers in strings to negative numbers, in Perl

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