HPR4767: pseudoformats

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2026/11/10再生時間は不明です

概要

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This podcast explores the concept of pseudo formats, which are files that use existing standards like XML or Zip but are rebranded with unique extensions to serve specific software needs. It discusses the two primary ways computers identify files: extensions and magic numbers.

要点 1File extensions are text labels after the last dot in a file name that tell operating systems which default program to use.

要点 2Magic numbers are unique byte sequences at the start of a file that provide a more corruption-resistant way to identify file types.

要点 3Pseudo formats are created by modifying file extensions outside of their common uses, often for semantic or programming purposes.

要点 4XML and Zip are popular bases for pseudo formats because they easily represent data hierarchies and can bundle multiple files.

要点 5Common file types like JAR, EPUB, and office suite documents are technically Zip files in disguise.

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Adapted from “HPR4767: pseudoformats” by Hacker Public Radio, licensed under CC BY-SA 4.0.

This transcript was generated by speech recognition and the summary and chapters by AI, so they can contain errors. BestPodcast is not affiliated with the creator. These adaptations are available under the same license.

This is Hacker Public Radio episode 4,767 for Tuesday the 10th of November 2026. Today's show is entitled pseudo formats. It is hosted by Candy Canarder and is about 5 minutes long. It carries a clean flag. The summary is a small episode kind of talking about formats that don't really exist.

Hi again. It's been a while, I know. I'm sorry. All the stuff about recovering from the forum and then like school happening. Uh but I'm back to make another episode and get back into the swing of things hopefully. Also, I'm trying to talk quieter because be the orange study room instead of like my old room in my old house that we moved from. And if that all over the way, let's talk about some pseudo formats, file formats. So first of all, what is a file format?

There are two main ways to think of what format a file is, that being its magic number and its extension. A file extension is the text that appears after a dot in the file name. Most systems will only care about the last dot in the file name and ignore all others. But the other dots can be used for semantic meaning, such as the tar.gz file extension, which is technically two separate file extensions put together. It is still a .gz file, but the tar extension also tells you what type of file the compressed file is.

On Windows, the file extension is the main way for the operating system to tell what type of file it is. It is usually hidden from the user and trying to edit it manually will cause a giant error to appear. However, changing the file extension does not change the content of the file any more than renaming it. It merely changes what program the operating system Windows will use to open it by default. However, there is another way for a file type to identify itself and that is with its magic number.

A magic number is merely the first couple of bytes of a file, which is often a consistent string for each file format. These bytes will get often contain both printing characters in order to identify the file type visually in hex editor, as well as the non-printing bytes in order to prevent misidentification by normal text files. For example, the magic bytes for PNG is the percent P N G followed by 0D 0A 1A 0A. Any file with those bytes in the beginning will be treated as a PNG regardless of the other content of the file.

Both types of identification have the use of as file extensions are easy to tell at a glance for both a computer and a human, whereas magic numbers are more resistant to corruption and can be communicated without the use of additional metadata. Pseudo file format come about when the file extension is modified in a way outside of the common uses. Since the file extension is part of the name, there is nothing stopping you from taking a PNG file and renaming the extension to whatever you want. Nearly every type of source code for programs uses this type of modification in order to identify their own programming language.

There is absolutely nothing stopping you from writing code in Notepad besides your own sanity. The next type are what I would call extended file formats or one that take another file format and use its optional features as well as renaming it. For example, APNG or animated PNG is technically a PNG in all ways except for that it has multiple frames. Anything that could read a PNG file can read an APNG file, though obviously it would not be animated and would simply show the first frame. Another example is a DMI or Dream Maker image file format.

This file format is a PNG file with extra sprite sheet data stored in the zed.txt field inside of the PNG. The two most common formats that are uses bases for these pseudo formats are XML and zip for text and binary respectively. This is likely due to how easy it is to represent a hierarchy within these file formats, as well as putting lots of different types of files inside of one file. With XML, you get free representation of hierarchy, arbitrary text fields, and attributes for any type of object.

It's flexible enough for many use cases and nearly every single programming language in existence can read and parse XML files. Zip files have the benefit of letting you hide an entire folder hierarchy inside of one file and treat it as a single embedded file format. A renamed zip file can easily hold all of the files you might need to represent a certain object. Many popular file extensions are simply zip files in disguise, such as JAR files, office suite files, and EPUB files. So the next time you're working on your own project, try making your own file extension.

It'll help make your file stand out among others. And happy file extension renaming. Yeah. You have been listening to the Hacker Public Radio podcast at hackerpublicradio.org. Today's show was contributed by a HPR listener like yourself. If you ever thought of recording a podcast, then visit the HPR site to find out how easy it really is. Hosting for HPR has been kindly provided by an honesthost.com, the Internet Archive rsync.net, and the HPR community content delivery network. Unless otherwise stated, today's show is released under a Creative Commons Attribution Share Alike 4.0 International license.