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Apr 18, by David Farrell. Parsing binary data is one of those tasks that seems to come up rarely, but is useful to know. Many common file types like images, music, timestamps, network packets and auth logs all come in binary flavors. The good news though is parsing binary data with Perl is easy using the unpack function. This is a suitably Modern Perlish beginning.
I start by importing autodie which ensures the code will die if any function call fails. Next I use the: This will avoid newline translation issues. No need for binmode here. All binary files have a specific format that they follow. Now comes the fun part.
The tzfile man page defines the header format:. The unpack function takes a template of the binary data to read this is defined in the pack documentation and returns Perl variables. The template code a4 matches this. Next is the version, this is a single ASCII character matched by a the strings are not space or null terminated, I could have use A instead.
The next 15 bytes are reserved and can be ignored, so I use x15 to skip over them. Finally there are 6 numbers of type long. Each one is separate variable so I must write N 6 times instead of N6. This code passes my template to unpack and it returns the variables we asked for.
In the case of a tzfile, the header defines the length of the body of the file, so I can use these variables to calculate how much more data to read from the file. The first thing you can do is print the binary data to the terminal with hexdump. This gives you a chance to inspect the data byte by byte and see if it matches your template.
To create a template to match binary data, take it one value at a time. Get the right bit length and for numbers, be sure to know if it is signed or unsigned. The other thing to be aware of is endianness of the data. This means big endian.
Tzfiles have several 32 bit signed integers in big endian order. There is no unpack template code which matches that type. This article was originally posted on PerlTricks. David is the editor of Perl. Something wrong with this article? Help us out by opening an issue or pull request on GitHub.
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How to parse binary data with Perl Apr 18, by David Farrell Parsing binary data is one of those tasks that seems to come up rarely, but is useful to know. Open a binary filehandle Start things off right by opening a filehandle to binary file: David Farrell David is the editor of Perl.
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