CS 537 Notes, Debugging

What Is A Debugger?

A debugger is a program that can control other programs. A programmer uses it to:

Find bugs!
Start up a program
Stop the program at a desired point (example: "when I enter function foo")
Examine (and change) the state of the program
Wait for the program to do something bad.

A debugger helps to answer the following questions:

What statement made my program crash?
If a function returns an error, who called it (and what were the arguments?)
What is the value of a variable at a point in execution?
What is the result of a particular expression in the program?

Knowledge and familiarity with a debugger is critical to your success as a programmer.

What Is GDB?

GDB is the GNU Project's debugger, and is the most commonly used UNIX debugger. Google for "gdb tutorial" and you'll see what we mean. In these notes we will briefly describe how GDB works; the tutorials available on-line go into much more detail. GDB can:

Start a program running and stop it at any point in execution.
Single-step through a program.
Print source or disassembly surrounding the current instruction.
Print out the contents of registers and memory, or change them.
Execute functions within the program.
Stop at a particular function or line in a function.
Stop the program when the value of a variable changes.
Print the call chain to the current location in the program.

We'll go over these here.

First, you should compile your program with full debugging information; this means that GDB can return more useful information to you. This is done by putting "-g" on the command line:

gcc -g -o simulation simulation.c

When using multiple source files, each file gets the "-g", as well as the link line:

gcc -g -c process.c
gcc -g -c unix.c
gcc -g -c binary.c
gcc -g -o program process.o unix.o binary.o

In general, there is no reason not to always use "-g".

Once you have a compiled program, you run it under GDB:

gdb program

This will get you a screen like so:

GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb)

The (gdb) is the prompt, much like a command prompt. Here are some commands:

help
Gives help about GDB.
run or run arg1 arg2 arg3 ...
This starts the program running. If you just type "run" it will run the program with no arguments; any command line arguments are specified after the run command.
break location
Creates a breakpoint: the program will halt when the point is reached and GDB will ask you what to do next. The most common is to break at the entry of a function:
```
(gdb) break main
Breakpoint 1 at 0x80483a0: file foo.c, line 3.
(gdb)
```
but you can also break at a particular line (or address):
```
(gdb)break foo.c:4
Breakpoint 2 at 0x80483ac: file foo.c, line 4.
(gdb)
```
When you run the program, it will automatically stop when a breakpoint is reached, and report that to you:
```
(gdb) run
Starting program: /afs/cs.wisc.edu/u/b/e/bernat/foo 

Breakpoint 1, main () at foo.c:3
3         printf("42");
```
Note that GDB tells you which breakpoint was reached (1), the file and line (foo.c, line 3), and then prints out the statement that was going to be executed (printf).
delete
Deletes a breakpoint. For example, delete 2.
step, next and finish
Step: executes a single statement and then stops again. Next: as with step, but if the next statement is a function call, it executes the entire call. These are very useful for fine-grained control over the program, and are equivalent to setting breakpoints at each statement. Finish runs until the end of the current function, then stops.
continue
Continues a process that was stopped via a breakpoint or single-stepping. It will run until you interrupt it or it hits a breakpoint.
where or backtrace
Print the call chain - the list of called functions - that brought the program to its current location, and print their arguments as well.

print E

Print the value of expression E from the point of view of the program, where E is some C statement. For example, take the following simple program:

brie(62)% cat foo.c
int main() {
    int i = 32;
    printf("42");
    i = 42;
    return i;
}

which we compile and start under GDB:

(gdb) break main
(gdb) break foo.c:3
(gdb) break foo.c:5
(gdb) run
...
Breakpoint 1, main() at foo.c:2
(gdb) print i
$1 = 9989280    <--- Note - i isn't initialized yet, and so is garbage
(gdb) continue
Breakpoint 2, main () at foo.c:3
(gdb) print i
$2 = 32
(gdb) continue
Breakpoint 3, main () at foo.c:5
(gdb) print i
$3 = 42
(gdb) continue
Program exited with code 052. <--- the return value of the program

We can use breakpoints and print to track the value of a variable through the execution of the program.

quit