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Handling Binary data (.gif file example) in Perl

Perl is very good for handling binary data - it can do things you can't do with other utilities and scripting languages, and things that are very much harder to do in C - that's because C's strings are null terminated and in the case on binary strings, there may be an embedded null anywhere.

Finding good examples is a bit tricky. And that's because binary data tends to come with long and involved specifications. However, a .gif image file has the height and width of the image encoded into the 7th to 10th bytes of the file, so that does make a reasonable example

# Find all files ending in ".gif" in current directory
@files = glob("*.gif");
print ("@files\n");
 
# Handle each of them in turn
foreach $picture(@files) {
        # If we can't open a file, PANIC!
        open (FH,$picture) or die ("couldn't open $picture\n");
 
        # Read first 10 bytes into $stuff
        read (FH,$stuff,10) ;
 
# Use "v" as the most significant byte is last - little endian
# Not "n" which is the other way round - big endian
        # skip 6 bytes, the pick up 2 x 2-byte integers
        # (see the manual for unpack - v means 2 byte integer ;-) )
        ($wide,$high) = unpack("x6vv",$stuff);
 
        # With a .gif file, these two numbers are the image size!
        print "$picture - $wide x $high\n\n";
 
        }


I've put plenty of comments into that code ... good practice ... and so there's not much need for extra detailed description here. But I should add that there's a pack function that's the opposite of unpack if you want to reform binary data, and you can output binary data using the regular print function - nothing special at all is needed.

Yes - it's Perl, yes the code is that short, and when you know the language really well you can write it REALLY quickly!
(written 2008-01-17, updated 2008-01-19)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P212 - Perl - More on Character Strings
  [4452] Binary data handling - Python and Perl - (2015-03-09)
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [608] Don't expose your regular expressions - (2006-02-15)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [583] Remember to process blank lines - (2006-01-31)
  [453] Commenting Perl regular expressions - (2005-09-30)


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Autovivification - the magic appearance of variables in Perl
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