how to use gdb to explore the stack/heap?

Question

Could anyone please give me a quick overview/point me to documentation of a way to inspect the stack (and heap?) of a C program? I thought this should be done with GDB, but if there are other more straighforward alternatives, then that should be fine as well.

Thanks.

Solution 1

My first approach to using GDB for debugging is to setup breakpoints. This is done like so:

prompt> gdb ./x_bstree.c
(gdb) #prompt
(gdb) b 123 #break at line 123
(gdb) r #start program

Now your program halts at line 123 of your program. Now you can examine variables in stack or heap using print. For stack variables just use print <varname>. For heap variables (pointers) use print <*varname>. Not sure there is anything special to do for examining stack/heap variables?

Of course to debug multi-threaded applications you would need to make it run in single-threaded mode & then dubug Otherwise it becomes difficult to predict what's happening.

For anything else there is extensive documentation of gdb & many sites also provide gdb cheat sheets.

Solution 2

you can dump raw memory with the 'x' command

so if you want to look at bits of the stack or heap try things like

x/10b &stackvar
x/200b &heapvar-20

The last one will show you 200 bytes starting from 20 bytes before heapvar. So if you just malloced that you can see the heap header too

Solution 3

View stack: gdb> backtrace

View current stack frame: gdb> info frame

View arguments of current stack frame: gdb> info args

View local variable of current stack frame: gdb> info locals

Navigate to parent stack frame: gdb> frame 1

Examining the Stack

Solution 4

Try using ddd. ddd manual

Ok. Maybe I elaborate a little. I use it like this.

compile my program with debug symbols:

gcc -g program.c -o program

run ddd:

ddd program

In gui you can do all sorts of things, view machine code, view memory, etc. . Look around. In manual there is also a section of examining stack. ddd provides good interface for you to examine C program.

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Comments

[Dervin Thunk]

i thought ddd was just a front end for gdb, no?

[Casual Coder]

I've found it more convenient than gdb. In gui you can look at several things at once. In gdb I am scrolling back all the time.

[rc0r]

I like your approach! Is there any good gdb plugin that allows to examine the heap showing it's chunk data, headers, in-use bits and the like? I just found gdb-heap. Unfortunately it seems pretty outdated and unmaintained.

[starturtle]

It IS just a front end for gdb, see updated manual location. It can, however, be a lot more straightforward for certain types of workflow than a console-only tool.

[Max Power]

None of that displays raw memory content.