CODEWITHSUNDEEP
cdebugginggdbcompiler-optimization
avd
avd

Reputation: 14451

Value optimized out in GDB: Can gdb handle decoding it automatically?

1) First I want to know, how to decode such variables ?

I know the solutions to this problem, remove optimization flag, make it volatile, I dont want to do all that. Is there any solution which can be done without compiling the source again ? The problem is whenever i make any changes, it takes ages to compile, so I dont want to compile it with different optimization flags, also I had tried once changing the optimization flag, but it crashed just because of change in compilation flags, for reasons I cant fathom.

Also I am not able to find documentation about understanding various registers when I do "info reg". i was expecting some variable ( whose value I knew, what would it be ) but info reg is showing me all different values. I am missing something here. The architecture I am working on is x86_64

2) I want to know what are the restrictions faced by gdb to decode such register variables ? Or is this problem already tackled by someone. I have read at many places that going through the assembly code, you can find out which variable is in that register. If thats true, why it cant be build into gdb. Please point me to relevant pages if there are solutions to this problem

Upvotes: 0

Views: 4254

Answers (2)

Dave Rager
Dave Rager

Reputation: 8150

If you don't have the source and compile with debug/no optimizations (i.e. 3rd party code.) the best you can do would be to disassemble the code and try to determine how the variables are stored.

In gdb the disassemble instruction will dump the assembly for the given function:

disassemble <function name>

Or if symbols have been stripped

disassemble <address>

where <address> is the entry point to the function.

You may also have to inspect where the function is called to determine the calling conventions used.

Once you've figured out the structure of the functions and variable layout (stack variables or registers), when debugging you can step through each instruction with nexti and stepi and watch how the values in the variables change by dumping the contents of the registers or memory locations.

I don't know any good primers or tutorials myself but this question and its answers may be of use to you. Personally I find myself referencing the Intel manuals the most. They can be downloaded in pdf from Intel's website. I don't have a link handy at the moment. If someone else does perhaps they can update my answer.

Upvotes: 2

abelenky
abelenky

Reputation: 64682

Have you looked at compiling your code un-optimized?

Try one of these in your gcc options:

-Og
    Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. 


-O0
    Reduce compilation time and make debugging produce the expected results. This is the default.

Upvotes: 0

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