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Handling binary data in Perl is easy!

Training in CambridgePerl can handle binary data just as easily as ASCII text - but YOU - if you're the programmer - must understand the format of the data that you'll be working with. With binary data it's every bit as important to get the right bytes in the right places as it is to get the appropriate separators between fields and lines in an ASCII file ... indeed, it's perhaps more important as you can't check your output file by pulling it into a text editor.

How does it work?

• Binary data can be held in a Perl scalar as a string. Unlike char array base string functions in C, there is no special character that's reserved for the end of string so you have total flexibility.

• You can read any number of bytes that you wish from a file (binary of otherwise) using Perl's read function.

• You can unpack the buffer that you've read the data into a list of variables, using all sorts of differet formats - see
  perldoc -f pack
for details and there are all sorts of other resources such as [this one]

• you can reassemble a new binary string with pack from a list of scalars and

• you can output the binary string with print.

Surprisingly, it reall is that easy!

Here's an example ...

Reading a binary file:

  open (FH,"talker.gif") or die;
  read (FH,$buffer,-s "talker.gif");
  close FH;


Getting data out of the binary file (you MUST know the format!)

  ($magic, $subtype, $widf, $ight) = unpack("A3A3vv",$buffer);
  print "Image was $widf by $ight and is type $subtype or $magic\n";


And writing a binary file:

  open (FHO,">shouter.gif") or die;
  print FHO $buffer;
  close FHO;


If you want to read and write your file through random access, you can uses seek - there's an example [here] on our website and another [here]. There's also a function called tell which tells you where you are in the file. Personally, I don't recommend its use - you should know where you are if your coding to a high standard, and it's one of the few things we don't have an example of on the site.

With binary files, you'll sometimes find that you require several programs to access them at the same time, and you can use the flock function to (cooperatively) lock them - there's an example of that [here]. And you can do clever things with sysread to read without buffering ... and sysread has "friends" such as syswrite and sysseek but - good on the Perl team - no systell. Example - [here].


Illustrattion - delegate on one of last week's Perl courses. Click on image to enlarge.
(written 2011-08-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P215 - Perl - More about Files
  [3839] Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15)
  [3320] Reading the nth line from a file (Perl and Tcl examples) - (2011-06-09)
  [2964] An introduction to file handling in programs - buffering, standard in and out, and file handles - (2010-09-21)
  [2405] But I am reading from a file - no need to prompt (Perl) - (2009-09-14)
  [1832] Processing all files in a directory - Perl - (2008-10-11)
  [1709] There is more that one way - Perl - (2008-07-14)
  [1225] Perl - functions for directory handling - (2007-06-09)

P223 - Perl - Interprocess Communication
  [3940] Run other processes from within your Perl program - (2012-12-03)
  [3011] What are .pid files? - (2010-10-23)
  [3010] Children, zombies, and reaping processes - (2010-10-23)
  [2970] Perl - doing several things at the same time - (2010-09-25)
  [2694] Multiple processes (forking) in Python - (2010-03-25)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [604] Perl - multiprocess applications - (2006-02-13)

P602 - Perl - Advanced File and Directory Handling
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [1861] Reactive (dynamic) formatting in Perl - (2008-10-31)
  [975] Answering ALL the delegate's Perl questions - (2006-12-09)
  [839] Reporting on the 10 largest files or 10 top scores - (2006-08-20)


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Some other Articles
Storing Tcl source code encoded, and running via your own C program
User defined sorting and other uses of callbacks in Tcl and Tk
Passing back multiple results in Tcl - upvar and uplevel
If its Sunday, must it be Weymouth?
Handling binary data in Perl is easy!
Single and double quotes strings in Perl - what is the difference?
A review of the Summer Sunday extra trains on the TransWilts line
When variables behave differently - Tie in Perl
Journey home by public transport for a Bank Holiday
Perl - a quick reminder and revision. Test yourself!
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