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Re: [paho-dev] List * handles passed as NULL in MQTTAsync.c

Hi Jimmy,

can I have a snippet of your application code to show how you are using the client library?


On 04/24/2014 11:36 PM, Jimmy Johnson wrote:
Hi Ian,

I found another possible bug in the C code, and created a possible fix.  Here is what I found:

My client was making a call to MQTTClient_create() in MQTTClient.c which initalizes it's static instance of the "handles" pointer. When the client actually published a message it was calling an instance of Protocol_processPublication which is located in MQTTAsync.c and using the instance of the static "handles" pointer in that file which had never been initalized since I had originally call MQTTClient_create() in MQTTClient.c to set up the client.  This caused a segfault when it hit the ListFindItem method with handles set as NULL.

Here is my stack trace:

#0  0x4002e550 in ListFindItem (aList=0x0, content=0x3c68c,
     callback=0x40035b48 <clientStructCompare>) at src/LinkedList.c:154
#1  0x40035ff0 in Protocol_processPublication (publish=0x306bc,
     client=0x3c68c) at src/MQTTAsync.c:1809
#2  0x400403e8 in MQTTProtocol_handlePublishes (pack=0x306bc, sock=6)
     at src/MQTTProtocolClient.c:275
#3  0x400e5e2c in MQTTClient_cycle (sock=0x41616db4, timeout=1000,
     rc=0x41616db8) at src/MQTTClient.c:1508
#4  0x400e24c4 in MQTTClient_run (n=0x3c4fc) at src/MQTTClient.c:483
#5  0x40189910 in start_thread () from /lib/
#6  0x405913ec in clone () from /lib/

You can see that aList which is a List * is being passed in as NULL (0x0) to ListFindItem after being called from MQTTAsync.c

My fix was to add a parameter to Protocol_processPublication called handles for both MQTTAsync.c and MQTTClient.c files.


void Protocol_processPublication(Publish* publish, Clients* client)


void Protocol_processPublication(Publish* publish, Clients* client, List * handles)

I then re factored calling methods to include their instance of the handle when making this call.  It seemed to fix everything since the call is no longer relying on a global variable. Oddly enough I did not see this issue when I was compiling and testing under Ubuntu 12.04 on my laptop.  It only started happening only when I cross compiled to my arm platform.  Do you think it will cause any issues?  I am using the library with c++, I don't know if that would make a difference.  I know the docs say that a synchronous client becomes asynchronous when you create callbacks for it, but it doesn't seem to initialize the asynchronous part of the code.

Thanks for any insight you might have.


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Ian Craggs
icraggs@xxxxxxxxxx                 IBM United Kingdom
Committer on Paho, Mosquitto

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