WASM example not working with Pdfium release V5668 due to malloc() function change

[inzanez]

I just tried running the WASM example and I always get Initialization of pdfium-render failed!. I used to build everything manually so far with emscripten which is quite a pain. Now it feels more like magic, but...can't tell why that wouldn't work.

Addition: I do get an error about malloc not being defined:
pdfium-render::initialize_pdfium_render(): Module._malloc() not defined. I guess that will be related to the actual wasm build?

[ajrcarey]

Hi @inzanez , thank you for reporting the problem. Can I check the following:

• whether you see any other errors in your browser's debug console
• which version of Pdfium you're using and where you acquired it from
• whether the problem persists if you use the WASM build of Pdfium V5407 from https://github.com/paulocoutinhox/pdfium-lib/releases/tag/5407

[inzanez]

@ajrcarey hm,...I have been using the latest build of Pdfium (V5668)

Switching to the version that you mentioned fixed the issue. Is there any versioning hint or similar that would point out what version works? Not sure, maybe I missed something here. But great solution anyway! :-)

[ajrcarey]

No, there isn't any versioning hint, and annoyingly Pdfium itself doesn't provide any function to return the library version :/

I wouldn't describe this as a "solution" exactly, more a work-around. The problem is that the emscripten-provided malloc() function has changed in some way, although glancing at the function table for the latest Pdfium WASM build I don't immediately see any obvious problem.

Probably something subtle changed in the emscripten packaging, and it's just a question of tracking down what it was and figuring out how to work around it in pdfium-render. I'll leave this issue open to work on that. But for now you have a work-around: use an older library version!

[inzanez]

Perfect, many thanks for that insight! I hope I can join you soon on digging into that issue. Sounds really interesting, but not that easy, remembering the whole emscripten around :-)

[ajrcarey]

The emscripten wrappers are in the pdfium.js file. Best guess is that the definition of Module._malloc() has changed in there in a recent WASM build of Pdfium compared to older releases. I'll try to take a look at it over the weekend.

[ajrcarey]

The pdfium.js file from V5407 contains the following export:

var _malloc=Module["_malloc"]=createExportWrapper("malloc")

But the pdfium.js file from V5668 has only the following:

var _malloc=createExportWrapper("malloc")

The missing Module["_malloc"]= is the source of the problem. Changing this to match V5407's pdfium.js resolves the problem. The same adjustment needs to be made for the free function.

[inzanez]

@ajrcarey I would assume that must be updated in src/wasm.rs, right? The link to _malloc and _link...

[ajrcarey]

The createExportWrapper() function in emscripten's pdfium.js looks like it returns a reference to Module["asm"][<function name>]. It might be possible to use that internally in src/wasm.rs instead of Module["_malloc"], etc.

[ajrcarey]

Adjusted PdfiumRenderWasmState::bind_to_pdfium() to use Module["asm"]["malloc"] and Module["asm"]["free"] in the event Module["_malloc"] and Module["_free"] are not available directly. Updated README. Ready to release as part of crate version 0.8.8.

[ajrcarey]

As there have been no further comments and I believe the issue is now resolved, I am closing the issue and publishing release 0.8.8. Feel free to re-open if you do not believe the issue is resolved in release 0.8.8.