how to unpack a big-endian signed short integer

robinbowes has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I'm using unpack(..) to decode a mixed binary/ascii record. It's mostly working well, but a couple of the values are two-byte, signed short integer. How do I unpack these?

Here's my current code:

my $packet_template = "B16 s2 A4 A8 A2 A A A3 C2";

# read in next record from STDIN and store in $buffer

($Flags,$x,$y,$Mode3,$Callsign,
    $Route,$Spare1,$Spare2,$Flight_Level,
    $Terminator,$Checksum) =
  unpack ( $packet_template, $buffer );
[download]

This all works except the $x and $y values which are big-endian. Any offers?

R.

Comment on how to unpack a big-endian signed short integer Download Code

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Re: how to unpack a big-endian signed short integer by BrowserUk (Patriarch) on Oct 15, 2003 at 19:45 UTC
Instead of 's', use 'n'. 's' is unsigned short in your platform specific/c-compiler format. 'n' is unsigned short in "network" (big endian) format. Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail Hooray!	[reply]
Re: Re: how to unpack a big-endian signed short integer by robinbowes (Beadle) on Oct 15, 2003 at 19:52 UTC
Thanks for the suggestion. However the values I'm reading are signed, i.e. signed, big-endian 16-bit integers. There doesn't seem to be a an appropriate format character for this. Bummer! -- Robin Bowes \| http://robinbowes.com	[reply]
Re: Re: Re: how to unpack a big-endian signed short integer by BrowserUk (Patriarch) on Oct 15, 2003 at 20:06 UTC
Sorry. I misread that. Try this... `print unpack 's', pack 'S', unpack 'n', $n;` [download] Unpacking it with 'n' will handle the big-endianess without dropping bits. Repacking it with 'S' should likewise not drop anything, but will get it into a format that 's' will correctly unpack. I would use 'n' in the composite unpack and then `sub utos16{ unpack 's', pack 'S', $_[0] }` [download] to retrieve the signedness afterwards. (Note: Please check I got that right:) Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail Hooray!	[reply] [d/l] [select]
Re: how to unpack a big-endian signed short integer by jmcnamara (Monsignor) on Oct 15, 2003 at 22:10 UTC
You can usually change endianess by calling reverse on the packed data: `$x = unpack 's', reverse pack 's', $x;` [download] The following should also work although it may be architecture dependent: `$x = unpack 's', pack 'n', $x;` [download] -- John.	[reply] [d/l] [select]
Re: how to unpack a big-endian signed short integer by etcshadow (Priest) on Oct 15, 2003 at 20:10 UTC
You can use the vec builtin function in combination with pack/unpack to do whatever bitwise craziness you desire. ------------ :Wq Not an editor command: Wq	[reply]
Re: how to unpack a big-endian signed short integer by robinbowes (Beadle) on Oct 15, 2003 at 19:30 UTC
Ooops. I thought I was logged in when I posted this question. Anyway to assign the node to me? -- Robin Bowes \| http://robinbowes.com	[reply]