Finding the pointer to a singleton postmortem in GDB (C++)

Question

I am doing a postmortem analysis of a crashed program. I am on Linux (Ubuntu 12.04, x86), the code is written in C++. The Program is using some singletons that may contain valuable information. Is it possible to find the pointer to the instance of a singleton if it was created like this:

SingletonType& SingletonType::getInstance(){
    static SingletonType* instance = new SingletonType();
    return *instance;
}

And if its is possible, how is it done in GDB?

Nope, unfortunately you cannot do this whitout a running process. — techshack, Jul 16 '13 at 14:48

score 4 · Accepted Answer · edited Sep 10 '15 at 10:14

4

Run gdb with the core file, and run the command

disassemble  SingletonType::getInstance

On my test-program I found a mov 0x<addr>, %eax instruction near the end of the method. A print *(*(SingletonType**) <0xaddr>) should print the contents of your singleton structure.

edited Sep 10 '15 at 10:14

Vivek Jain

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answered Jul 16 '13 at 15:41

tfk

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I don't know why you wouldn't `dissassemble SingletonType::getInstance` directly? For one thing, depending on the base address, `disassemble ` might not even do the correct thing. – sehe Jul 16 '13 at 20:03
You are right. The given procedure gives me what I want but it is absolutely possible to disassemble by using the name of the function and it is also more elegant :) – techshack Jul 17 '13 at 07:50
Good point sehe. I removed the objdump part and updated the dissassemble command in my answer. – tfk Jul 17 '13 at 07:59

score 3 · Answer 2 · answered Jul 16 '13 at 14:52

show modules¹ should probably tell you the base addresses, and instance, being statically allocated, should be visible in some kind of objdump/nm report. Yeah hairy maths.

The alternative would be to disassemble SingletonType::getInstance() and see what effective address gets loaded in the initialization/return path.

¹ Mmm can't find the exact match I was remembering. info sharedlibrary would get you most info.

ellysisland · Answer 3 · 2017-05-09T09:20:50.903

1

this is what I do, while inside the core with gdb:

(gdb) info var instance

this will list all the addresses of all the singletons instances, among which you will find the one of SingletonType

0x86aa960 SingletonType::getInstance()::instance

Now that I have the address you can print the your instance' pointed memory:

(gdb) p *((SingletonType*)0x86aa960)

edited May 09 '17 at 09:20

answered May 09 '17 at 09:13

ellysisland

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Finding the pointer to a singleton postmortem in GDB (C++)

3 Answers3