On File Formats
108 comments
·May 21, 2025lifthrasiir
mort96
> Note that textual numbers are surprisingly complex to handle, so any formats with significant number uses should just use binary.
Especially true of floats!
With binary formats, it's usually enough to only support machines whose floating point representation conforms to IEEE 754, which means you can just memcpy a float variable to or from the file (maybe with some endianness conversion). But writing a floating point parser and serializer which correctly round-trips all floats and where the parser guarantees that it parses to the nearest possible float... That's incredibly tricky.
What I've sometimes done when I'm writing a parser for textual floats is, I parse the input into separate parts (so the integer part, the floating point part, the exponent part), then serialize those parts into some other format which I already have a parser for. So I may serialize them into a JSON-style number and use a JSON library to parse it if I have that handy, or if I don't, I serialize it into a format that's guaranteed to work with strtod regardless of locale. (The C standard does, surprisingly, quite significantly constrain how locales can affect strtod's number parsing.)
analog31
Here's a weird idea that has occurred me from time to time. What if your editor could recognize a binary float, display it in a readable format, and allow you to edit it, but leave it as binary when the file is saved.
Maybe it's discipline-specific, but with the reasonable care in handling floats that most people are taught, I've never had a consequential mishap.
mort96
I don't know how you would do that in practice, since every valid sequence of 4 or 8 bytes is a valid float. Maybe you could exclude some of the more unusual NaN representations but it still leaves you with most byte sequences being floats.
For example, the ASCII string "Morn", stored as the bytes '0b01001101 0b01101111 0b01110010 0b01101110', could be interpreted as the 32-bit float 0b01101110011100100110111101001101, representing the number 1.8757481691240478e+28.
So you couldn't really just have smart "float recognition" built in to an editor as a general feature, you would need some special format which the editor understands which communicates "the following 4 bytes is a single-precision float" or "the following 8-byte is a double-precision float".
chirsz
You can use hexadecimal floating-point literal format that introduced in the C language since C99[^1].
[^1]: https://cppreference.com/w/c/language/floating_constant.html
For example, `0x1.2p3` represents `9.0`.
HappMacDonald
To me this just sounds like an endianness nightmare waiting to happen.
mort96
Just add this to your serialiser and deserialiser:
if (nativeEndianness != littleEndian) swapBytes(&theFloat);
dwattttt
You do have to be aware of endianness, although it's really not hard to handle it: pick one. Your code always knows what endian the file format is, so it always knows how to read it.
hyperbolablabla
Couldn't you just write the hex bytes? That would be unambiguous, and it wouldn't lose precision.
mort96
That's still a binary format in my eyes, it's not human readable or writable. You've just written the binary format as hex.
lifthrasiir
At that point human-readability would suffer.
shakna
Spent the weekend with an untagged chunked format, and... I rather hate it.
A friend wanted a newer save viewer/editor for Dragonball Xenoverse 2, because there's about a total of two, and they're slow to update.
I thought it'd be fairly easy to spin up something to read it, because I've spun up a bunch of save editors before, and they're usually trivial.
XV2 save files change over versions. They're also just arrays of structs [0], that don't properly identify themselves, so some parts of them you're just guessing. Each chunk can also contain chunks - some of which are actually a network request to get more chunks from elsewhere in the codebase!
[0] Also encrypted before dumping to disk, but the keys have been known since about the second release, and they've never switched them.
InsideOutSanta
>Most extensions have three characters, which means the search space is pretty crowded. You may want to consider using four letters.
Is there a reason not to use a lot more characters? If your application's name is MustacheMingle, call the file foo.mustachemingle instead of foo.mumi?
This will decrease the probability of collision to almost zero. I am unaware of any operating systems that don't allow it, and it will be 100% clear to the user which application the file belongs to.
It will be less aesthetically pleasing than a shorter extension, but that's probably mainly a matter of habit. We're just not used to longer file name extensions.
Any reason why this is a bad idea?
Hackbraten
A 14-character extension might cause UX issues in desktop environments and file managers, where screen real estate per directory entry is usually very limited.
When under pixel pressure, a graphical file manager might choose to prioritize displaying the file extension and truncate only the base filename. This would help the user identify file formats. However, the longer the extension, the less space remains for the base name. So a low-entropy file extension with too many characters can contribute to poor UX.
delusional
> it will be 100% clear to the user which application the file belongs to.
The most popular operating system hides it from the user, so clarity would not improve in that case. At leat one other (Linux) doesn't really use "extensions" and instead relies on magic headers inside the files to determine the format.
Otherwise I think the decision is largely aestethic. If you value absolute clarity, then I don't see any reason it won't work, it'll just be a little "ugly"
hiAndrewQuinn
I don't even think it's ugly. I'm incredibly thankful every time I see someone make e.g. `db.sqlite`, it immediately sets me at ease to know I'm not accidentally dealing with a DuckDB file or something.
wvbdmp
Yes, oh my god. Stop using .db for Sqlite files!!! It’s too generic and it’s already used by Windows for those thumbnail system files.
whyoh
>The most popular operating system hides it from the user, so clarity would not improve in that case.
If you mean Windows, that's not entirely correct. It defaults to hiding only "known" file extensions, like txt, jpg and such. (Which IMO is even worse than hiding all of them; that would at least be consistent.)
EDIT: Actually, I just checked and apparently an extension, even an exotic one, becomes "known" when it's associated with a program, so your point still stands.
dist-epoch
> At leat one other (Linux) doesn't really use "extensions" and instead relies on magic headers inside the files to determine the format.
mostly for executable files.
I doubt many Linux apps look inside a .py file to see if it's actually a JPEG they should build a thumbnail for.
scrollaway
Your doubts are incorrect. There's a fairly standard way of extracting the file type out of files on linux, which relies on a mix of extensions and magic bytes. Here's where you can start to read about this:
https://wiki.archlinux.org/title/XDG_MIME_Applications
A lot of apps implement this (including most file managers)
layer8
It’s tedious to type when you want to do `ls *.mustachemingle` or similar.
It’s prone to get cut off in UIs with dedicated columns for file extensions.
As you say, it’s unconventional and therefore risks not being immediately recognized as a file extension.
On the other hand, Java uses .properties as a file extension, so there is some precedent.
dist-epoch
> call the file foo.mustachemingle
You could go the whole java way then foo.com.apache.mustachemingle
> Any reason why this is a bad idea
the focus should be on the name, not on the extension.
Rygian
Why should a file format be locked down to one specific application?
IAmBroom
Both species are needed:
Generic, standardized formats like "jpg" and "pdf", and
Application-specific formats like extension files or state files for your program, that you do not wish to share with competitors.
InsideOutSanta
I think the Mac got this right (before Mac OS X) and has since screwed it up. Every file had both a creator code and a type code. So, for every file, you would know which application created it and also which format it was.
So, double-clicking the file opened it in the application it was made in, but the Mac would also know which other applications could open that file.
thasso
For archive formats, or anything that has a table of contents or an index, consider putting the index at the end of the file so that you can append to it without moving a lot of data around. This also allows for easy concatenation.
zzo38computer
What probably allows for even more easier concatenation would be to store the header of each file immediately preceding the data of that file. You can make a index in memory when reading the file if that is helpful for your use.
HelloNurse
This would require a separate seek and read operation per archive member, each yielding only one directory entry, rather than very few read operation to load the whole directory at once.
charcircuit
Why not put it at the beginning so that it is available at the start of the filestream that way it is easier to get first so you know what other ranges of the file you may need?
>This also allows for easy concatenation.
How would it be easier than putting it at the front?
shakna
Files are... Flat streams. Sort of.
So if you rewrite an index at the head of the file, you may end up having to rewrite everything that comes afterwards, to push it further down in the file, if it overflows any padding offset. Which makes appending an extremely slow operation.
Whereas seeking to end, and then rewinding, is not nearly as costly.
PhilipRoman
You can do it via fallocate(2) FALLOC_FL_INSERT_RANGE and FALLOC_FL_COLLAPSE_RANGE but sadly these still have a lot of limitations and are not portable. Based on discussions I've read, it seems there is no real motivation for implementing support for it, since anyone who cares about the performance of doing this will use some DB format anyway.
In theory, files should be just unrolled linked lists (or trees) of bytes, but I guess a lot of internal code still assumes full, aligned blocks.
charcircuit
Most workflows do not modify files in place but rather create new files as its safer and allows you to go back to the original if you made a mistake.
lifthrasiir
If the archive is being updated in place, turning ABC# into ABCD#' (where # and #' are indices) is easier than turning #ABC into #'ABCD. The actual position of indices doesn't matter much if the stream is seekable. I don't think the concatenation is a good argument though.
MattPalmer1086
Imagine you have a 12Gb zip file, and you want to add one more file to it. Very easy and quick if the index is at the end, very slow if it's at the start (assuming your index now needs more space than is available currently).
Reading the index from the end of the file is also quick; where you read next depends on what you are trying to find in it, which may not be the start.
orphea
Some formats are meant to be streamable. And if the stream is not seekable, then you have to read all 12 Gb before you get to the index.
The point is, not all is black and white. Where to put the index is just another trade off.
McGlockenshire
> How would it be easier than putting it at the front?
Have you ever wondered why `tar` is the Tape Archive? Tape. Magnetic recording tape. You stream data to it, and rewinding is Hard, so you put the list of files you just dealt with at the very end. This now-obsolete hardware expectation touches us decades later.
jclulow
tar streams don't have an index at all, actually, they're just a series of header blocks and data blocks. Some backup software built on top may include a catalog of some kind inside the tar stream itself, of course, and may choose to do so as the last entry.
charcircuit
But new file formats being developed are most likely not going to be designed to be used with tapes. If you want to avoid rewinds you can write a new concatenated version of the files. This also allows you to keep the original in case you need it.
leiserfg
If binary, consider just using SQLite.
paulddraper
Did you read the article?
That wouldn’t support partial parsing.
HelloNurse
On the contrary, loading everything from a database is the limit case of "partial parsing" with queries that read only a few pages of a few tables and indices.
From the point of view of the article, a SQLite file is similar to a chunked file format: the compact directory of what tables etc. it contains is more heavyweight than listing chunk names and lengths/offsets, but at least as fast, and loading only needed portions of the file is automatically managed.
lifthrasiir
Using SQLite as a container format is only beneficial when the file format itself is a composite, like word processor files which will include both the textual data and any attachments. SQLite is just a hinderance otherwise, like image file formats or archival/compressed file formats [1].
[1] SQLite's own sqlar format is a bad idea for this reason.
SyrupThinker
From my own experience SQLite works just fine as the container for an archive format.
It ends up having some overhead compared to established ones, but the ability to query over the attributes of 10000s of files is pretty nice, and definitely faster than the worst case of tar.
My archiver could even keep up with 7z in some cases (for size and access speed).
Implementing it is also not particularly tricky, and SQLite even allows streaming the blobs.
Making readers for such a format seems more accessible to me.
lifthrasiir
SQLite format itself is not very simple, because it is a database file format in its heart. By using SQLite you are unknowingly constraining your use case; for example you can indeed stream BLOBs, but you can't randomly access BLOBs because the SQLite format puts a large BLOB into pages in a linked list, at least when I checked last. And BLOBs are limited in size anyway (4GB AFAIK) so streaming itself might not be that useful. The use of SQLite also means that you have to bring SQLite into your code base, and SQLite is not very small if you are just using it as a container.
> My archiver could even keep up with 7z in some cases (for size and access speed).
7z might feel slow because it enables solid compression by default, which trades decompression speed with compression ratio. I can't imagine 7z having a similar compression ratio with correct options though, was your input incompressible?
duskwuff
One gotcha to be aware of is that SQLite blobs can't exceed 1* GB. Don't use SQLite archives for large monolithic data.
*: A few bytes less, actually; the 1 GB limit is on the total size of a row, including its ID and any other columns you've included.
InsideOutSanta
The Mac image editor Acorn uses SQLite as its file format. It's described here:
https://shapeof.com/archives/2025/4/acorn_file_format.html
The author notes that an advantage is that other programs can easily read the file format and extract information from it.
lifthrasiir
It is clearly a composite file format [1]:
> Acorn’s native file format is used to losslessly store layer data, editable text, layer filters, an optional composite of the image, and various metadata. Its advantage over other common formats such as PNG or JPEG is that it preserves all this native information without flattening the layer data or vector graphics.
As I've mentioned, this is a good use case for SQLite as a container. But ZIP would work equally well here.
[1] https://flyingmeat.com/acorn/docs/technotes/ACTN002.html
sureglymop
I think it's fine as an image format. I've used the mbtiles format which is basically just a table filled with map tiles. Sqlite makes it super easy to deal with it, e.g. to dump individual blobs and save them as image files.
It just may not always be the most performant option. For example, for map tiles there is alternatively the pmtiles binary format which is optimized for http range requests.
aidenn0
Except image formats and archival formats are composites (data+metadata). We have Exif for images, and you might be surprised by how much metadata the USTar format has.
lifthrasiir
With that reasoning almost every format is a composite, which doesn't sound like a useful distinction. Such metadata should be fine as long as the metadata itself is isolated and can be updated without the parent format.
frainfreeze
sqlar proved a great solution in the past for me. Where does it fall short in your experience?
lifthrasiir
Unless you are using the container file as a database too, sqlar is strictly inferior to ZIP in terms of pretty much everything [1]. I'm actually more interested in the context sqlar did prove useful for you.
Dwedit
Sometimes, you'll need to pack multiple files inside of a single file. Those files will need to grow, and be able to be deleted.
At that point, you're asking for a filesystem inside of a file. And you can literally do exactly that with a filesystem library (FAT32, etc).
zzo38computer
Consider DER format. Partial parsing is possible; you can easily ignore any part of the file that you do not care about, since the framing is consistent. Additionally, it works like the "chunked" formats mentioned in the article, and one of the bits of the header indicates whether it includes other chunks or includes data. (Furthermore, I made up a text-based format called TER which is intended to be converted to DER. TER is not intended to be used directly; it is only intended to be converted to DER for then use in other programs. I had also made up some additional data types, and one of these (called ASN1_IDENTIFIED_DATA) can be used for identifying the format of a file (which might conform to multiple formats, and it allows this too).)
I dislike JSON and some other modern formats (even binary formats); they often are just not as good in my opinion. One problem is they tend to insist on using Unicode, and/or on other things (e.g. 32-bit integers where you might need 64-bits). When using a text-based format where binary would do better, it can also be inefficient especially if binary data is included within the text as well, especially if the format does not indicate that it is meant to represent binary data.
However, even if you use an existing format, you should avoid using the existing format badly; using existing formats badly seems to be common. There is also the issue of if the existing format is actually good or not; many formats are not good, for various reasons (some of which I mentioned above, but there are others, depending on the application).
About target hardware, not all software is intended for a specific target hardware, although some is.
For compression, another consideration is: there are general compression schemes as well as being able to make up a compression scheme that is specific for the kind of data that is being compressed.
They also mention file names. However, this can also depend on the target system; e.g. for DOS files you will need to be limited to three characters after the dot. Also, some programs would not need to care about file names in some or all cases (many programs I write don't care about file names).
aidenn0
Maybe it's just because I've never needed the complexity, but ASN.1 seems a bit much for any of the formats I've created.
jeroenhd
The ASN.1 format itself is pretty well-suited for generic file types. Unfortunately, there are very few good, open source/free ASN.1 (de)serializers out there.
In theory you could use ASN.1 DER files the same way you would JSON for human-readable formats. In practice, you're better off picking a different format.
Modern evolutions of ASN.1 like ProtoBuf or Cap'n Proto designed for transmitting data across the network might fit this purpose pretty well, too.
On the other hand, using ASN.1 may be a good way to make people trying to reverse engineer your format give up in despair, especially if you start using the quirks ASN.1 DER comes with and change the identifiers.
zzo38computer
> Unfortunately, there are very few good, open source/free ASN.1 (de)serializers out there.
I wrote a library to read/write DER, which I have found suitable for my uses. (Although, I might change or add some things later, and possibly also some things might be removed too if I think they are unnecessary or cause problems.)
https://github.com/zzo38/scorpion/blob/trunk/asn1/asn1.c https://github.com/zzo38/scorpion/blob/trunk/asn1/asn1.h
(You can complain about it if there is something that you don't like.)
> In theory you could use ASN.1 DER files the same way you would JSON for human-readable formats. In practice, you're better off picking a different format.
I do use ASN.1 DER for some things, because, in my opinion it is (generally) better than JSON, XML, etc.
> Modern evolutions of ASN.1 like ProtoBuf or Cap'n Proto designed for transmitting data across the network might fit this purpose pretty well, too.
I have found them to be unsuitable, with many problems, and that ASN.1 does them better in my experience.
> On the other hand, using ASN.1 may be a good way to make people trying to reverse engineer your format give up in despair, especially if you start using the quirks ASN.1 DER comes with and change the identifiers.
I am not so sure of this.
zzo38computer
For me too, although you only need to use (and implement) the parts which are relevant for your application and not all of them, so it is not really the problem. (I also never needed to write ASN.1 schemas, and a full implementation of ASN.1 is not necessary for my purpose.) (This is also a reason I use DER instead of BER, even if canonical form is not required; DER is simpler to handle than all of the possibilities of BER.)
mjevans
Most of that's pretty good.
Compression: For anything that ends up large it's probably desired. Though consider both algorithm and 'strength' based on the use case carefully. Even a simple algorithm might make things faster when it comes time to transfer or write to permanent storage. A high cost search to squeeze out yet more redundancy is probably worth it if something will be copied and/or decompressed many times, but might not be worth it for that locally compiled kernel you'll boot at most 10 times before replacing it with another.
adelpozo
I would add to make it streamable or at least allow to be read remotely efficiently.
flowerthoughts
Agreed on that one. With a nice file format, streamable is hopefully just a matter of ordering things appropriately once you know the sizes of the individual chunks. You want to write the index last, but you want to read it first. Perhaps you want the most influential values first if you're building something progressive (level-of-detail split.)
Similar is the discussion of delimited fields vs. length prefix. Delimited fields are nicer to write, but length prefixed fields are nicer to read. I think most new formats use length prefixes, so I'd start there. I wrote a blog post about combining the value and length into a VLI that also handles floating point and bit/byte strings: https://tommie.github.io/a/2024/06/small-encoding
lifthrasiir
I don't think a single encoding is generally useful. A good encoding for given application would depend on the value distribution and neighboring data. For example any variable-length scalar encoding would make vectorization much harder.
flowerthoughts
Depends if you're optimizing for storage size or code size, and in-memory vs transfer. This encoding was meant to optimize transfer (and perhaps storage.)
teddyh
Designing your file (and data) formats well is important.
“Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won’t usually need your flowcharts; they’ll be obvious.”
— Fred Brooks
Const-me
Good article, but I’d add one more paragraph.
If your data format contains multiple streams inside, consider ZIP for the container. Enables standard tools, and libraries available in all languages. The compression support is built-in but optional, can be enabled selectively for different entries.
The approach is widely used in practice. MS office files, Java binaries, iOS app store binaries, Android binaries, epub books, chm documentation are all using ZIP container format.
nikeee
Depending on the use-case, it might be good to have a file format "diffable", especially when it's going to be checked in in some version control.
Generally good points. Unfortunately existing file formats are rarely following these rules. In fact these rules should form naturally when you are dealing with many different file formats anyway. Specific points follow:
- Agreed that human-readable formats have to be dead simple, otherwise binary formats should be used. Note that textual numbers are surprisingly complex to handle, so any formats with significant number uses should just use binary.
- Chunking is generally good for structuring and incremental parsing, but do not expect it to provide reorderability or back/forward compatibility somehow. Unless explicitly designed, they do not exist. Consider PNG for example; PNG chunks were designed to be quite robust, but nowadays some exceptions [1] do exist. Versioning is much more crucial for that.
[1] https://www.w3.org/TR/png/#animation-information
- Making a new file format from scratch is always difficult. Already mentioned, but you should really consider using existing file formats as a container first. Some formats are even explicitly designed for this purpose, like sBOX [2] or RFC 9277 CBOR-labeled data tags [3].
[2] https://nothings.org/computer/sbox/sbox.html
[3] https://www.rfc-editor.org/rfc/rfc9277.html