Memory corruption related to signal emission?

[blooglet]

I suspect I'm dealing with memory corruption somewhere in my program. I'm getting a SIGSEGV on a call to a function from a trusted third-party library. With some trial and error I've discovered that this segfault happens when the lines

emit newStreamFound(_streams[streamOneSSRC]);
emit newStreamFound(_streams[streamTwoSSRC]);

emit newStreamFound(_streams[streamOneSSRC]);
emit newStreamFound(_streams[streamTwoSSRC]);

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are not commented. This happens even when no slots are connected to this signal. As far as I know there are no thread safety issues. I ran a Valgrind memory analysis with these lines disabled and this is what came up:



I can't make heads or tails of this output. The test condition in the do while loop depends on one variable that is, in fact, initialized at the top of the function. Does any of this point to issues that should be investigated? If not, what are the next steps I can take?

[amleto]

right, so have you debugged the values of streamOneSSRC, and the array value at that index?

[blooglet]

I should probably mention that _streams is a QMap. streamOneSSRC and streamTwoSSRC are uint32_t with values similar to 738621152 and 3435571404. I've asserted that

assert(_streams.find(streamOneSSRC) != _streams.end());
        assert(_streams.find(streamTwoSSRC) != _streams.end());

assert(_streams.find(streamOneSSRC) != _streams.end());
        assert(_streams.find(streamTwoSSRC) != _streams.end());

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right before the signal emission happens.

[stampede]

Why do you think this is related to signal emission, and not to accessing the QMap ?
Does it crash if you just print the values ?

// emit newStreamFound(_streams[streamOneSSRC]);
// emit newStreamFound(_streams[streamTwoSSRC]);
qDebug() << _streams[streamOneSSRC];
qDebug() << _streams[streamTwoSSRC];

// emit newStreamFound(_streams[streamOneSSRC]);
// emit newStreamFound(_streams[streamTwoSSRC]);
qDebug() << _streams[streamOneSSRC];
qDebug() << _streams[streamTwoSSRC];

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[blooglet]

Nope, that seems to work just fine.

VideoStream(0x7fffe4110e20) 
VideoStream(0x7fffe4110e90)

VideoStream(0x7fffe4110e20) 
VideoStream(0x7fffe4110e90)

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But then again if I'm dealing with a memory corruption bug this behavior could be undefined.

[amleto]

how are you pasing the data through the signaal/slot? const ref?

[blooglet]

By pointer.

void newStreamFound(VideoStream*);

void newStreamFound(VideoStream*);

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_streams is a QMap<uint32_t, VideoStream*>.

[amleto]

you've got a problem somewhere else then. emitting a signal with a pointer as an argument, and with no slots connected, does not cause a seg fault.

[blooglet]

Right... so it's likely to be memory corruption like I thought then? Can you conclude anything from the Valgrind memory analysis about where the error might be?

[amleto]

seg fault is always doing something dodgy to memory isnt it?

valgrind doesnt play too nicely with qt from what little I have read about it so not sure you can conclude much from it.

[stampede]

Build debug version, launch with debugger and examine (post here) backtrace after crash.