Re: [libmicrohttpd] get all post data in a map

[Kenneth Mastro]

I haven't seen the question on SO so I don't know what it says, but to be fair to others who might be reading this... Using POST with MHD isn't necessarily HARD - it's just not EASY.  :)

I don't know what Rahul came up with as a solution, but I strongly suspect his problem was more MHD-flow related than POST-specific.  I mean - he was getting a seg fault in his code and didn't seem to know what to do with that.  My suggestion for using SO was more along the lines of getting help for learning how to debug a seg fault.  (It didn't even occur to me that a new question on SO was appropriate - I was trying to suggest searching for help of existing topics.)  I'm certainly not against having more MHD-specific help available or anything like that (so please clarify things on SO - example code for others is always useful) - I just don't think the problem was MHD-POST specific/related and was really more just not understanding what his own code was doing.

For the record - I'd have helped him more here if he could have narrowed things down rather than just saying "I have a seg fault".  I.e., I wasn't trying to pawn him off on SO, it just didn't seem like the MHD mailing list was an appropriate place to help somebody deal with a more or less run-of-the-mill C/C++ issue.

Doesn't really matter, of course - I'm just clarifying. :)

Rahul - if you're reading this - I mean no offense.  It took me a while to get the kinks worked out of my MHD-based web server, so I understand that there are a few tricky parts to doing it.  I had my own fair share of seg faults and problems that I needed to work through, too.

To anybody else who happens to be reading this - for what it's worth - I'm very pleased with what I was able to do with MHD.  Don't let a few snags/complications dissuade you from trying it.

Ken

On Thu, Jan 28, 2016 at 11:35 AM, Martin Bonner <address@hidden> wrote:

Yes, you should definitely edit the question and post an answer.  libmicrohttpd support for POST is hard to use; good questions and answers on SO are useful.

 

Von: libmicrohttpd-bounces+martin.bonner=address@hidden [mailto:libmicrohttpd-bounces+martin.bonner=address@hidden] Im Auftrag von rahul bhola
Gesendet: Mittwoch, 27. Januar 2016 17:12

An: libmicrohttpd development and user mailinglist <address@hidden>
Betreff: Re: [libmicrohttpd] get all post data in a map

 

I really appreciate all the help and advice that i got from this group. The problem on post was resolved although took me some time. About the stack overflow question i am not sure i can delete that . Any advise ? Should i edit the question with proper codes and post an answer for future programmers also?

Also martin thanks for the blog links i will be sure to check them out.

 

Regards

Rahul

 

On Wed, Jan 27, 2016 at 10:56 AM Martin Bonner <address@hidden> wrote:

Before sending a question to Stack Overflow, take the time to read:

 

http://stackoverflow.com/help/how-to-ask

 

In particular, you *must* create a *complete* program that shows the problem.  See http://stackoverflow.com/help/mcve

 

However, before you think about asking a question on Stack Overflow, please read

 

http://ericlippert.com/2014/03/05/how-to-debug-small-programs

 

and learn how to debug your code (I appreciate you are a step up from the target audience for

that article, but there’s lots of good advice for hoary old-timers like me.)

 

Von: libmicrohttpd-bounces+martin.bonner=address@hidden [mailto:libmicrohttpd-bounces+martin.bonner=address@hidden] Im Auftrag von Kenneth Mastro
Gesendet: Montag, 25. Januar 2016 17:44
An: libmicrohttpd development and user mailinglist <address@hidden>
Betreff: Re: [libmicrohttpd] get all post data in a map

 

Sorry.. I don't have time to debug your code for you.  If you have a seg fault, do you have a core dump?  If so, why not look at it in gdb or some other debugger?  Failing that, just use print statements to figure out where the problem is.  If you need programming help in general, check out Stack Overflow (as the purpose of this mailing list is to help with MHD).  The code you posted really isn't very extensive.  It shouldn't be too hard to debug it.  If you don't know how to do that, I can pretty much guarantee that you're going to want to step away from MHD and get some more experience in general first.

 

That said, segmentation faults are usually the result of trying to access memory that isn't what you think it is - with an uninitialized pointer or something that got deleted/freed that you didn't intend.

I'm not asking the following questions to get the answers to them - like I said, you're going to need to debug your own code - but some things for you to think about:

* Where are you storing the map of key/value pairs?  Is it in some type of request object you created?  (I didn't see where that was created in the code you posted.)

* Is the post processor callback being called at all?  (Have you verified that?)

* If so, do the values ever get inserted into your map?  (Have you checked that when you insert them?)

* Do you truly understand MHD's 'connection callback' flow - are you creating and destroying objects and assigning pointers when you're supposed to?  (If not, make sure you read MHD's documentation and understand what's going on in the tutorials.)

 

Like I said earlier - MHD is great, but POST processing with it is not trivial/easy/simple.  You have to understand what's going on in order to use it correctly.  Maybe you're trying to run before you can walk...

I can promise you that MHD's code works and will not seg-fault internally if you're using it correctly (and it might not even if you screw up).  MHD is very solid - the problem is definitely in your code.  You need to figure out where the seg fault is - figure out which pointer is messed up and why.

 

That's probably not much help or what you were looking for, but that's the most I can offer you.  Hope it helped.

 

Ken

 

 

On Mon, Jan 25, 2016 at 11:17 AM, rahul bhola <address@hidden> wrote:

Hi,

I followed what you told me in last mail and the browser started to get response even to post messages but i am still on able to access the map data. It shows me seg fault. Most of the codes are almost similar to what i found in example for the site. Can you please look over them once and guide me in the right direction.

Here is the link to the same

https://gist.github.com/georoot/aff2825e0177cb3cc1ae

 

thanks for helping me out.

 

On Mon, Jan 25, 2016 at 1:30 PM Kenneth Mastro <address@hidden> wrote:

I'm not sure what the problem is, then.. I thought you said the request 'hangs on'?  So that's not the case..?  Your server IS responding to the request?

You don't need to 'terminate' the post-iterator callback function yourself from inside the callback.  It will stop being called when there is no more data to parse.

I do basically the same thing you're trying to do - put the post parameters into a map.  The callback doesn't get to decide when the post data has been completely processed - that's decided by your server (probably in your connection callback).  Maybe you're missing the step where you set the 'data size' pointer to 0 to tell MHD you've absorbed all the data from that chunk of the POST request?

The second-to-last parameter in the connection callback is a size_t* that represents the 'upload data size'.  You need to set the value to 0 if you absorbed all the data - very probably after you call 'MHD_post_process'.  Something like this:

if(*uploadDataSize != 0) {

    // process the post data using MHD's default post-processor - this will call your post-processor-callback function

    result = MHD_post_process(pointerToYourPostProcessorForThisConnection, uploadData, *uploadDataSize);

    // tell MHD you're done with all that data

    *uploadDataSize = 0;
}

POST data comes in chunks to MHD - so your 'connection callback' will be called repeatedly until the data is complete (or probably if you return MHD_NO, but I'm not sure).  So - If you put something like the above snippet in your connection callback, will ultimately be called repeatedly until MHD tells you that 'uploadDataSize' is 0 - that there is no more data to process.

After that point, MHD will stop sending you the POST data and your server can do whatever it wants with all the data you've collected.

In the snippet you sent, I don't see anything in there that actually CALLS the post processor (I see the create, but not the call to process the data).  I'm guessing that's what you're missing.

As a side note: MHD is really really great, but using it to process POST data is not trivial.  It took me a while to get right.  You should at least start with the example in the tutorial and go from there.

Hope that helps.

Ken

 

 

 

 

 

On Mon, Jan 25, 2016 at 8:06 AM, rahul bhola <address@hidden> wrote:

yes every response is just the url. I am still not doing anything with the post data. Just trying to store it in a map. Let me attach gist files for the same

 

here is the code snippets for the same

https://gist.github.com/georoot/89394b3a792e9e2d34b2