llvm dev - Jan 2013 - [LLVMdev] LLVM seg fault : dereference an uninitialized pointer Instruction*

Hello everyone,

I encounter a segfault problem in my LLVM function pass. I think the
problem is that I dereference an uninitialized pointer from an array of
Instruction* elements which results in undefined behavior. The array is :

Instruction** ifsInstrArray = new Instruction*[100];

The problem appears only sometimes (during runOnFunction && visiting
BasicBlocks and Instructions). Sometimes means, for e.g. :

errs()<<*ifsInstrArray[0]<<"\n"<<*ifsInstrArray[1];

only for *ifsInstrArray[1] the segfault appears. I already checked if
ifsInstrArray[1] is not NULL. Please note that the above is only a test for
my primary segfault problem. The primary segfault problem is : More
precisely, I am trying to check if two instructions are equivalent by using

if ( CC->operEquiv(ifsInstrArray[i], ifsInstrArray[j]) )

where CC and operEquiv are defined by me. If extra code is needed, please
let me know. The seg fault appears here. I think that the problem is
related with the "test problem" defined at the beginning of the
question.
Can you please tell me how can I initialize the ifsInstrArray array? And
for its elements I should use the constructor of Instruction* (I cannot
find it in Instruction.h)? I cannot initialize the elements by assigning
NULL.

Thank you for your answer !

PS: i,j are boundary-ok in my for loops; ifChecker* CC=new ifChecker(); in
ifChecker class there is: virtual bool operEquiv(Instruction *I1,
Instruction *I2);


-- 
Best regards,
Alexandru Ionut Diaconescu
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On 1/8/2013 6:23 AM, Alexandru Ionut Diaconescu wrote:
> Can you please tell me how can I initialize the ifsInstrArray
> array? And for its elements I should use the constructor of Instruction*
> (I cannot find it in Instruction.h)? I cannot initialize the elements by
> assigning NULL.
You need to make sure that the pointers in the array actually point to 
instructions.  I suspect that your code tries to dereference pointers 
that have indeterminate values (i.e. are uninitialized).

-Krzysztof


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