how to use gdb to explore the stack/heap?

Question

Hello. 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.

Answer 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

Answer 2

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.

Answer 3

you can dump raw memory with the 'x' command

so if you want to look at bits of the stack of 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