Example gdb session to see variable values

Preparing to use gdb:

Overview of simple gdb commands

list
list source code
break n
set break point at line number n
set args arg1 arg2 arg3 ... argn
set the command line arguments to arg1 arg2 arg3 ... argn
run
run the program. (Whatever break points and command line arguments you previous set will be used.)
print expr
print the value of the expression expr.
expr can be a variable name, or most any other C expression that doesn't require a function call (e.g. x + 1, a[3], a * b)
quit
quit the gdb program

A sample gdb session

Script started on Mon 19 Oct 2009 11:36:03 AM PDT
bash-3.2$ ls
cmdlinedemo.c  
bash-3.2$ gcc -g cmdlinedemo.c -o cmdlinedemo
bash-3.2$ gdb cmdlinedemo
GNU gdb Fedora (6.8-32.fc10)
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...
(gdb) list
7	
8	#include <stdio.h>
9	
10	int main(int argc, char *argv[])
11	{
12	  int i;
13	
14	  // echo back argc and argv
15	
16	  printf("argc=%d\n",argc);
(gdb) break 15
Breakpoint 1 at 0x80483d8: file cmdlinedemo.c, line 15.
(gdb) set args foo bar fum
(gdb) run
Starting program: /cs/faculty/pconrad/public_html/cs16/09F/lectures/10.19/cmdlinedemo foo bar fum

Breakpoint 1, main (argc=4, argv=0xbffff624) at cmdlinedemo.c:16
16	  printf("argc=%d\n",argc);
(gdb) print argc
$1 = 4
(gdb) print argv[0]
$2 = 0xbffff771 "/cs/faculty/pconrad/public_html/cs16/09F/lectures/10.19/cmdlinedemo"
(gdb) print argv[1]
$3 = 0xbffff7b5 "foo"
(gdb) print argv[2]
$4 = 0xbffff7b9 "bar"
(gdb) print argv[1][0]
$5 = 102 'f'
(gdb) print argv[1][1]
$6 = 111 'o'
(gdb) print argv[1][2]
$7 = 111 'o'
(gdb) print argv[1][3]
$8 = 0 '\0'
(gdb) print argv[2][0]
$9 = 98 'b'
(gdb) print argv[2][1]
$10 = 97 'a'
(gdb) quit
The program is running.  Exit anyway? (y or n) y
bash-3.2$ gdb cmdlinedemo
GNU gdb Fedora (6.8-32.fc10)
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...
(gdb) set args foo bar fum
(gdb) run
Starting program: /cs/faculty/pconrad/public_html/cs16/09F/lectures/10.19/cmdlinedemo foo bar fum
argc=4
argv[0]=/cs/faculty/pconrad/public_html/cs16/09F/lectures/10.19/cmdlinedemo
argv[1]=foo
argv[2]=bar
argv[3]=fum

Program exited normally.
(gdb) quit
bash-3.2$ exit
exit

Script done on Mon 19 Oct 2009 11:38:56 AM PDT

Learning more about gdb

There are lots more gdb commands. The six commands covered in this page—i.e., list, break, set args, run, print, quit—are just the tip of the iceberg, but they are enough to make gdb a very useful tool. As we need more, we'll learn more.

To learn more about these six commands, you can use help from inside gdb followed by a command name—for example, help list will give information about the list command:

(gdb) help list
List specified function or line.
With no argument, lists ten more lines after or around previous listing.
"list -" lists the ten lines before a previous ten-line listing.
One argument specifies a line, and ten lines are listed around that line.
Two arguments with comma between specify starting and ending lines to list.
Lines can be specified in these ways:
  LINENUM, to list around that line in current file,
  FILE:LINENUM, to list around that line in that file,
  FUNCTION, to list around beginning of that function,
  FILE:FUNCTION, to distinguish among like-named static functions.
  *ADDRESS, to list around the line containing that address.
With two args if one is empty it stands for ten lines away from the other arg.
(gdb)

If you cant wait to learn more though, here are some links you may visit. They have more information about gdb than you can possibly make use of in CS16: