Getting to Know gdb

It's worth making friends with a good C debugger.
Executing a Program

To run the program you are debugging, use the run command. This may be followed by any arguments you want to pass to the program, including the standard input and output specifiers < and >, and shell wildcards (*, ?, [, ]). You can't use C-shell history (!) or pipes (|).

For example, consider running the program exp through gdb. The following gdb command runs exp with the argument -b, taking the standard input to exp from invalues and redirecting standard output to the file outtable:

$ gdb exp
(gdb) run -b < invalues > outtable

That is, this command runs exp -b < invalues > outtable. If you have not set any breakpoints or used any other gdb debugging features, exp will run until it terminates, either correctly or incorrectly.

If the program you're debugging terminates abnormally, control returns to gdb. You can then use gdb commands to find out why the program terminated. The backtrace command gives a stack backtrace showing exactly what the program was doing when it bombed out:

$ gdb badref
(gdb) run
Starting program: /home/los/mikel/cuser/badref
0x22c8 in march_to_infinity () at badref.c:16
16               h |= *p;
(gdb) backtrace
#0  0x22c8 in march_to_infinity () at badref.c:16
#1  0x2324 in setup () at badref.c:25
#2  0x2340 in main () at badref.c:30

backtrace (which is usually abbreviated back) produces a list of all active procedures and the arguments with which they were called, starting with the most recent. So this display shows that the program died in a function named march_to_infinity(); this function was called by the function setup(), which in turn was called by the function main(). The only thing left is to figure out exactly what wrong in march_to_infinity().

Printing Data

You can inspect the variable values by using the print command. Let's use it to see exactly what happened in the previous program. First, we'll list some code to see what we're dealing with:

(gdb) list
9            p=&j;
10           /* march off the end of the world*/
11           for ( i = 0; i < VERYBIG; i++)
12           {
13                h |= *p;
14                p++;
15           }
16      printf("h: %d\en",h);

It should already be pretty clear what's happening. p is some kind of a pointer; we can test that by using the whatis command, which shows us its declaration:

(gdb) whatis p
type = int *
(gdb) print p
$1 = (int *) 0xf8000000
(gdb) print *p
$2 = Cannot access memory at address 0xf8000000.
(gdb) print h
$3 = -1

When we look at p, we see that it's pointing somewhere up in the stratosphere. Of course, there's no ad hoc way to know whether this value for p is legitimate or not. But we can see if we can read the the data p points to, just as our program did—and when we give the command print *p, we see that it's pointing to inaccessible data.

print is one of gdb's true power features. You can use it to print the value of any expression that's valid in the language you're debugging. In additions to variables from your program, expressions may include:

  • Calls to functions within your program; these function calls may have “side-effects” (i.e., they can do things like modify global variables that will be visible when you continue program execution).

    (gdb) print find_entry(1.0)
    $1 = 3
  • Data structures and other complex objects.

    (gdb) print *table_start
    $8 = {e_reference = '\e000' <repeats 79 times>,
    location = 0x0, next = 0x0}