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Re: [libmicrohttpd] Memory leak and global variables

From: Nicolas Mora
Subject: Re: [libmicrohttpd] Memory leak and global variables
Date: Thu, 28 Aug 2014 14:54:15 -0400
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.0

Hi Kenneth, thanks a lot for your answer, you helped me moving on with my problems.

I fixed a lot of small problems thanks to your answers, but the memory leaks are still here with the same conditions: about 20kb is lost each time I call the 6 main webservices simultaneously with jquery, but no leak at all when I call each webservice on by one.

I'm sure I'll find the solution, I'm so close.

Here are my feedbacks.

* I'm far from an expert, but are you sure you can use
thread-per-connection and use-select-internally together?  (You very
well may be able to, I just thought they weren't normally used
together.  Could be completely wrong about that, though.)
Not really, maybe it's not recommended, but the program never yelled at me for that. I skipped select-internally flag, to keep just thread-per-connection, but no obvious change since then.

* There's a minor bug in the last release (0.9.37 and earlier) of MHD
where it doesn't properly cycle through the existing connections when in
thread-per-connection mode.  The fix was simple (essentially deleting
one character), but if you're detecting leaks by looking at the result
just before your program terminates, there's a remote chance that has
something to do with it.
Actually, I'm using raspian repository, which provides a 0.9.20-1 release. I tried yesterday using the last release by downloading, compiling and installing it, but not change then too.

* I haven't noticed any leaks in MHD.  I'm not saying there aren't any,
but I haven't seen anything like what you're describing.  (Granted - I'm
not hammering it like you would be in a web-service-based solution.)
I'm pretty sure the leaks come from my code somewhere, but most likely because of my misuse of MHD, that's why I'm asking the questions here.

* You have a lot going on in that code.  If it were me, I'd strip it way
down to a simple test case to try and isolate the leak(s).  You're doing
a lot of mallocs in general - any chance you could (at least
temporarily) remove them and reply with fake data?  The point is to try
and see if the problem is memory that MHD is allocating or if it's
something you're doing.
You're right about the useless mallocs. I changed all the fixed size mallocs to local variables, there is one (the page variable) that can't be change to a local variable since its size may vary.
I'll try with fake data as you suggest it.

* You're doing a few mallocs at the start of the request - are you sure
that's necessary (e.g., for the 'page' and 'urlcopy')?  Also, are you
sure they're all getting cleaned up?  For testing, maybe you could just
create a stack-based character string (i.e., no malloc) and tell MHD to
copy the response string instead of de-allocating the memory later.  I'm
not sure that would be any less efficient, anyway - malloc is kind of
slow, and your strings are really short.  (I.e., instead of 'MUST_FREE'
use the one that tells MHD to copy the buffer).  I mean - you're
returning in the middle of the code all the time (MHD_YES, MHD_NO), but
your cleanup code executes at the end of the function.  That seems very
very suspicious to me.  In fact - I bet that's what your problem is (or
at least one of them).  That connection request function gets called
multiple times - but you'll return early some times and the cleanup code
at the end never gets executed.  That leaves your 'urlcopy' (and
possibly other things) not freed, right?
You were totally right, I forgot about those return statements, and the 'free' calls that were forgotten there.

* I see you have a 'request completed' callback that frees the memory
you allocated for/in your connection info.  I assume you've verified
that it's getting called?
Yes, the request_completed function is called at the end of each angharad_rest_webservice function.

* As for the globals - I didn't analyze your code in any kind of depth,
but you probably already know that MHD can take a pointer to any object
you want and keep giving you the pointer through the connection cycle
(typically a structure).  At a glance, it looks like you're taking
advantage of that.  For things that you need across connections.. I'm
not sure what the suggested usage pattern is.  I'm using C++, so I just
use class data members.

I finally found it, indeed the parameter dh_cls in MHD_start_daemon is designed for that ! :)

That's all I've got.  Hope it helps.

That helped, thanks


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