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Re: [libmicrohttpd] Accessing raw POST payload

From: Kenneth Mastro
Subject: Re: [libmicrohttpd] Accessing raw POST payload
Date: Thu, 16 Oct 2014 08:37:03 -0400

To further clarify what Christian is saying...

You have to process/parse the POST data yourself.  As you noted - the built-in one only works for form data (x-www-form-urlencoded or multipart/form-data).

In other words - when the access handler gets called the first time and you see that it's a POST request, you can create a post-processor to handle it (similar to the MHD one).  I.e., instead of creating one with the MHD-provided 'create post processor' function (sorry - don't have the code in front of me), you would create one of your own design.  Alternatively and perhaps easier, you could not write a separate 'post processor' and just process the POST data as it comes in on the subsequent 'access handler callback' calls.  (I assume that will work, anyway.)

However you parse/gather the POST data... In your case, I'm guessing you'd just keep it simple and have it collect all the data from your JSON request into a string (remember - post data can appear in chunks, not necessarily all in one shot).  After all POST data has come in (indicated by an upload data size of 0 ...?), you can do whatever you want with the data/string.  If there is some kind of encoding applied to the string, you'll have to parse it into regular text, of course.

If you want to do something really clean/slick, you could have your 'post processor code' parse the JSON into some kind of mapping that you can just ask for key/value pairs.  That way, the code handling the request doesn't even need to know that it's JSON.  Completely optional from your web server's perspective - it could be perfectly happy to just hand the string off to somebody else.  You can hide the complexity wherever you want, but it's going to be there somewhere. :)

Side note: I don't know what you're using to parse your JSON... If memory is an issue, some libraries have 'sax-like' JSON parsing (yajl comes to mind).  Maybe you don't have to collect all the data before parsing some of it?  Depends on what the library supports.  I probably wouldn't bother with that unless memory/performance is an issue, though.

Fair warning - I've never done any of this (I've only used the built-in post processor), so it may not be quite as simple as I'm describing.  I can't be too far off, though.


On Thu, Oct 16, 2014 at 8:08 AM, Christian Grothoff <address@hidden> wrote:
Ah, yes, the MHD PostProcessor does not support 'all' possible
encoding types. If your client sends an unsupported encoding,
you need to parse "*upload_data" yourself.  Note that the
MHD_PostProcessor only exists for convenience, you could write that
entire logic as a client.  So please simply look at what the PP does for
the supported encodings, and then write a 'custom' PP for JSON.

Happy hacking!


On 10/16/2014 02:02 PM, Matej Svejda wrote:
> Hi Christian, thank you for your reply. It seems to me that the problem
> is that post_iterator works only for data submitted by HTML forms and
> not for custom POST data. For example, if I send a request with
> Content-Type application/json to the post_example.c I get:
> Failed to setup post processor for `/2'
> Internal application error, closing connection.
> If i set the Content-Type to x-www-form-urlencoded it works but still
> tries to parse it as key-value pairs, which in my case it shouldn't. I
> am looking for a way to directly access the data submitted by post,
> regardles of the Content-Type.
> Regards,
> Matej

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