Help with infrequent segmentation fault in accessing boost::unordered_multimap or struct?

Correct me if I'm wrong but I would bet that erasing an item in an unordered multimap invalidates all iterators pointing into it. Try "it = carriage. Erase(it)".

You'll have to do something about ret as well.

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I'm having trouble debugging a segmentation fault. I'd appreciate tips on how to go about narrowing in on the problem. The error appears when an iterator tries to access an element of a struct Infection, defined as: struct Infection { public: explicit Infection( double it, double rt ) : infT( it ), recT( rt ) {} double infT; // infection start time double recT; // scheduled recovery time }; These structs are kept in a special structure, InfectionMap: typedef boost::unordered_multimap InfectionMap; Every member of class Host has an InfectionMap carriage.

Recovery times and associated host identifiers are kept in a priority queue. When a scheduled recovery event arises in the simulation for a particular strain s in a particular host, the program searches through carriage of that host to find the Infection whose recT matches the recovery time (double recoverTime). (For reasons that aren't worth going into, it's not as expedient for me to use recT as the key to InfectionMap; the strain s is more useful, and coinfections with the same strain are possible.

) assert( carriage.size() > 0 ); pair ret = carriage. Equal_range( s ); InfectionMap::iterator it; for ( it = ret. First; it!

= ret. Second; it++ ) { if ( ((*it). Second).

RecT == recoverTime ) { // produces seg fault carriage. Erase( it ); } } I get a "Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address..." on the line specified above.

The recoverTime is fine, and the assert(...) in the code is not tripped. As I said, this seg fault appears 'randomly' after thousands of successful recovery events. How would you go about figuring out what's going on?

I'd love ideas about what could be wrong and how I can further investigate the problem. Update I added a new assert and a check just inside the for loop: assert( carriage.size() > 0 ); assert( carriage. Count( s ) > 0 ); pair ret = carriage.

Equal_range( s ); InfectionMap::iterator it; cout First; it! = ret. Second; it++ ) { cout First=" RecT == recoverTime ) { carriage.

Erase( it ); } } The EXC_BAD_ACCESS error now appears at the (*it). First call, again after many thousands of successful recoveries. Can anyone give me tips on how to figure out how this problem arises?

I'm trying to use gdb. Frame 0 from the backtrace reads "#0 0x0000000100001d50 in Host::recover (this=0x100530d80, s=0, recoverTime=635.91148029170529) at Host. Cpp:317" I'm not sure what useful information I can extract here.

Update 2 I added a break; after the carriage. Erase(it). This works.

C++ struct segmentation-fault segmentation link|improve this question edited May 19 '10 at 20:08 asked May 18 '10 at 22:15Sarah3741213 91% accept rate.

Eclipse is often good at halting execution at the time of the fault so you can check the call stack. Have you tried printf debugging by printing some value associated with the iterator each time through the loop? Have you checked the values of ret.

First and ret. Last for sanity? – Bruce May 18 '10 at 22:20 I'm using gdb but obviously not well enough!

Will investigate ret. First and ret.last. – Sarah May 18 '10 at 22:25.

Correct me if I'm wrong but I would bet that erasing an item in an unordered multimap invalidates all iterators pointing into it. Try "it = carriage. Erase(it)".

You'll have to do something about ret as well. Update in reply to your latest update: The reason breaking out of the loop after calling "carriage. Erase(it)" fixed the bug is because you stopped trying to access an erased iterator.

3 +1, except that erase does not return the next iterator, you have to keep track of that yourself. – avakar May 18 '10 at 22:23 That and don't keep incrementing it in the for loop. – SB.

May 18 '10 at 22:24 @Noah and @avakar: Does your advice hold if there's always only one Infection with the specific recovery time? (I've confirmed that this is the case when the seg fault appears. ) Using "it=carriage.

Erase(it)" creates a new seg fault that appears immediately in the simulation. – Sarah May 18 '10 at 22:32 @SB: Could you explain why I shouldn't increment it in the for loop? – Sarah May 18 '10 at 22:32 @sarah - you need to increment it only when you don't erase.

Otherwise you're already at the next one. I've never used the unordered_xxx pieces of boost so I was mostly guessing based on how Meyers described the implementation. Unordered_xxx are vectors of vectors or something similar so I expect them to behave as such.

For example: if you're iterating to a destination and you delete objects between here and there...there moves and your pointer to there is no longer pointing at there. – Crazy Eddie May 18 '10 at 22:42.

Compile the program with gcc -g and run it under gdb. When you get an EXC_BAD_ACCESS crash you'll drop into the gdb command line. At that point you can type bt to get a backtrace, which will show you how you got to the point where the crash occurred.

I've been staring at the backtrace for an hour. It points to the line I noted above. I'll read more about how to decipher the addresses.

– Sarah May 18 '10 at 22:22.

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