Jsmn, a minimalistic JSON parser in C

http://zserge.bitbucket.org/jsmn.html

by jasonmoo • 14 years ago

89 35 14 years ago

zserge.bitbucket.org

nwjsmith• 14 years ago

Looks good. For those of us shopping around, what advantages does Jsmn have over yajl?

pjscott2• 14 years ago

It does no runtime memory allocation, and since it doesn't depend on libc and has a very small size, it can be used on highly resource-constrained embedded processors.

revelation3• 14 years ago

Sorry, but no. Shuffling around strings is all fine when you're writing Javascript and have GHz and GB of RAM at your disposal, but that stuff just doesn't fly when you need to conserve RAM and especially need to have easily determined time and space constraints.

JSON just doesn't fit that profile. Neat implementation, but don't give the impression that any of this would help with embedded.

rmk• 14 years ago

How is this different from jansson? I have used jansson in the past, and it has served me very well.

hendi_2• 14 years ago

jsmn is only one .h and one .c file, jannson is more.

jsmn only parses the JSON into tokens, you handle all the rest.

jannson provides its own hashmap etc., jsmn doesn't.

I've just used jsmn for a current project of mine where I've already had implemented e.g. a custom hashmap, and I didn't want to link two into my code. So the choice for the lean (if not to say "minimally") jsmn came naturally. And I don't regret it :)

popee• 14 years ago

Hey. What do you think about this?

https://github.com/popee/libjason/blob/master/jason.rl

It is implemented in ragel state machine compiler, with no other dependencies. It is modified version of libejson but simplified to use only standard JSON. Also small. Btw ragel is great utility ;-)

gilgad13• 14 years ago

If we're golfing:

https://github.com/quartzjer/js0n/blob/master/js0n.c

Appears to work the same way, though it doesn't bubble back type information.

(Also, the `goto * go[ * cur];` trick was pretty crazy the first time I saw it)

judofyr• 14 years ago

This is called computed (or assigned) goto and is a GCC extension (that is, not in the C standard): http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html

binarycrusader• 14 years ago

Note that other compilers (such as Oracle Solaris Studio) support computed gotos as well:

  http://docs.oracle.com/cd/E19205-01/820-7598/bjabt/index.html

And LLVM:

  http://blog.llvm.org/2010/01/address-of-label-and-indirect-branches.html

droithomme2• 14 years ago

Arg! "[0 ... 255] = &&l_bad" compiles as valid C, totally confused by this code. Is this some weird gcc specific extension? Hm, probably part of the C99 array extensions. Checking...

Answer: C99, borrowed from a GCC extension. http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Designated-Inits...

tedunangst• 14 years ago

There are two not in C99 gcc extensions in that snippet. && to take the address of a label, and ... to produce a range of indices. It may also happen to be a designated initializer, but the whole piece is far from standard.

benatkin• 14 years ago

Direct link to the source: https://bitbucket.org/zserge/jsmn/src/1caee52d37e3/jsmn.c

This looks good to me. It isn't going to be the fastest or the shortest (no we aren't golfing) but it's simple and easy to understand.

alisdair• 14 years ago

I wrote some simple jsmn examples, since it doesn't ship with any: https://github.com/alisdair/jsmn-example/

And then I wrote about writing the examples: http://alisdair.mcdiarmid.org/2012/08/14/jsmn-example.html

hendi_2• 14 years ago

This is really nice and I'm sure it will help new users a lot. I've myself only used the provided test.c file to see how to use the API. While this was surely doable, your example (and its explanation) has improved the situation a lot. Thank you!

udp• 14 years ago

The code is certainly very short, but what about \u escape sequences in strings, parsing different representations of numbers, etc.? Since those things are part of the JSON standard, you're not a JSON parser if you just leave them to the application to handle.

Since this skimps out on half of the work, it won't even be able to tell you with certainty what's valid JSON and what isn't.

(disclaimer: I also wrote a popular ANSI C JSON parser)

stevelaz• 14 years ago

I saw json-parser. Looks good. I'm currently using YAJL for a project, but I'm open to switching to something faster/easier.

Have you done any performance tests against other C json parsers?

Thanks,

duaneb• 14 years ago

It fails to parse basic unicode escapes - not a JSON parser.

Xion• 14 years ago

This is not a JSON parser, it's a tokenizer. From a parser I'd expect at least acknowledgement of the basic key-value association.

It doesn't say that it's not useful. It's just that for anything non-trivial, you'd need to supplement this library with quite a bit of your own code, e.g. a stack for tracking nesting levels.

alexgoldstone• 14 years ago

For those interested in other lightweight implementations, I have not yet had a chance to compare this but have been very happy with cJSON by @dave_gamble for extremely resource-constrained embedded microcontrollers.

http://sourceforge.net/projects/cjson/

forgotAgain• 14 years ago

I've used cJson for a couple of projects as well. No complaints here.

mturmon• 14 years ago

Ditto. Found it easy to use and integrate. I needed something lightweight for a highly nested numerical model containing numbers and descriptive strings. I didn't want the Json bit to have a big footprint because the numerical computations make the software complex enough as it is.

lpgauth• 14 years ago

Out of curiosity, what's the fastest JSON parser written in C out there?

Is it still YAJL?

mpd• 14 years ago

Oj[1] benchmarks significantly faster, but I'm not clear if it's usable outside of a Ruby environment, as I am not aware of any non-gem distributions.

[1] http://www.ohler.com/oj/

ijobs-ly• 14 years ago

What is the purpose of JSON?

Does it have to do with efficiency?

Because if so, now we find ourselves discussing the resource requirements just to scan/tokenise and parse it to get it back into a human readable form. Why did we translate it to a non-readable form in the first place? What were we trying to achieve?

Maybe we should let JSON be something the receiver translates text to (if they want that sort of format), not the sender. The receiver knows what resources she has to work with, the sender has to guess. The same principle applies to XML. By all means, play around with these machin-readable formats to your heart's content. But do it on the receiver side. No need to impose some particular format on everyone.

The "universal format" is plain text. The UNIX people realised this long ago. People read data as plain text, not JSON and not XML, not even HTML. No matter how many times you translate it into something else, using a machine to help you, it will, if humans are to read it, be translated back to plain text.

As for the "plain text haters", let us be reminded that UNIX can do typesetting. Professional quality typesetting. But that's the receiver's job.[1] There's a learning curve, sure, but what the receiver can produce using typesetting utilities on her own machine is world's better than what a silly web browser can produce from markup.

1. I am so tired of dumping PDF's to text and images. PDF makes it seemingly impossble to scan through a large number of documents quickly. Ever been tasked with reading through 100 documents all in PDF format (i.e., scanned images from a photocopier)? What could be accmplished in minutes with BRE takes hours or even days to accomplish. This is a problem that persists year after year. OCR is a hack. In most cases, the text should never have been scanned to an image in the first place. The documents are being created on computers, not typewriters!

So, as I see it, if you were a plain text hater, and you were really sincere about making things look nice, then you would be a proponent of educating people how to do typesetting and sending them plain text, the universal format, that they can easily work with.

My solution to JSON and XML is sed. It works in all resource conditions and most times is just as fast as any RAM hungry parser. If I need to do complex things, that's what lex and yacc are there for. Pipes and filters; small buffers. 'Nuf said.

Jsmn, a minimalistic JSON parser in C

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