Compiling with debugger flag -g fixes SEGfault (fortran90)


 
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# 1  
Old 12-17-2010
Compiling with debugger flag -g fixes SEGfault (fortran90)

So the title kinda says it all. I was getting a SEGfault, so I decided to compile with the -g option to find where, and low and behold the SEGfault doesn't occur.

I suppose the answer is "Problem solved! You fixed yet another SEGfault." But I am very curious how this could have happened.

Some info that may help:
1 At first the executable instantly killed itself. After removing some unnecessary variables from the header file, this was fixed. (stack overflow i assume)
2 This is the full compilation command:
ifort -r8 -i4 -mcmodel=large -i-dynamic -align all -g -o code code.f90
3 When compiling without debugger flag messages informing me that loops have been vectorized are printed to screen. Nothing is printed to screen when I compile WITH '-g'.

Thanks!
# 2  
Old 12-17-2010
Quote:
Originally Posted by drbones
So the title kinda says it all. I was getting a SEGfault, so I decided to compile with the -g option to find where, and low and behold the SEGfault doesn't occur.

I suppose the answer is "Problem solved! You fixed yet another SEGfault." But I am very curious how this could have happened.
Enabling debugging likely disabled some optimizations, altering the way the code was generated and perhaps the pattern of memory use.

Imagine you're using an uninitialized variable. It may just happen to always be zero when the code isn't optimized since it wasn't used before. But optimize it and it decides it doesn't need a variable at all and puts it in a register! Suddenly you're using an uninitialized register, which could be anything, and it crashes...

Or you're overflowing the end of an array, and the arrangement of memory is different when the executable's optimized. When it's not optimized, you might be corrupting empty space and not care. When it's optimized, you could be munging something important, like pointers to somewhere else, or even your stack frame...

Can you debug it when it's not compiled for debugging? The information you get might be limited, but limited's better than nothing.
# 3  
Old 12-17-2010
Thanks for quick and useful reply!!

I actually just figured out the issue by rigorously debugging with print* commands.

I was searching for x within a list x1, x2, x3...xn, but did not account for x<x1. D'oh! So despite executing without an issue, the result would have been garbage. I think you hit the nail on the head, I had no idea this could happen!
# 4  
Old 12-17-2010
Yep, that's why it's best to remain wary of segfaults that just "go away". Something'd probably still be wrong somewhere. Smilie
 
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