3. How do I use the call stack?

The call stack is where we find the stack frames that control program flow. When a function is called, it creates a stack frame that tells the computer how to return control to its caller after it has finished executing. Stack frames are also where local variables and function arguments are 'stored'. We can look at these stack frames to determine how our program is running. Finding the list of stack frames below the current frame is called a backtrace.

How do I ...?

1. get a backtrace?
2. change stack frames?
3. examine stack frames?

3.1 How do I get a backtrace? [top]   [toc]

Use the gdb command backtrace. In the backtrace below, we can see that we are currently inside func2(), which was called bu func1(), which was called from main()

(gdb) backtrace
#0  func2 (x=30) at test.c:5
#1  0x80483e6 in func1 (a=30) at test.c:10
#2  0x8048414 in main (argc=1, argv=0xbffffaf4) at test.c:19
#3  0x40037f5c in __libc_start_main () from /lib/libc.so.6
(gdb) 

3.2 How do I change stack frames? [top]   [toc]

Use the gdb command frame. Notice in the backtrace above that each frame has a number beside it. Pass the number of the frame you want as an argument to the command.

(gdb) frame 2
#2  0x8048414 in main (argc=1, argv=0xbffffaf4) at test.c:19
19        x = func1(x);
(gdb) 

3.3 How do I examine stack frames? [top]   [toc]

To look at the contents of the current frame, there are 3 useful gdb commands. info frame displays information about the current stack frame. info locals displays the list of local variables and their values for the current stack frame, and info args displays the list of arguments.

(gdb) info frame
Stack level 2, frame at 0xbffffa8c:
 eip = 0x8048414 in main (test.c:19); saved eip 0x40037f5c
 called by frame at 0xbffffac8, caller of frame at 0xbffffa5c
 source language c.
 Arglist at 0xbffffa8c, args: argc=1, argv=0xbffffaf4
 Locals at 0xbffffa8c, Previous frame's sp is 0x0
 Saved registers:
  ebp at 0xbffffa8c, eip at 0xbffffa90

(gdb) info locals
x = 30
s = 0x8048484 "Hello World!\n"

(gdb) info args
argc = 1
argv = (char **) 0xbffffaf4