Debuggers

Debuggers In General

never depend on a debugger to find errors
you can waste a lot of time stepping through
many debuggers will show you the wrong data
- e.g., ints represented as longs
- e.g., memory (pointers) may be initialized differently

Very important: An ounce of analysis is worth a pound of debugging!

Debugging is often best when done with a print out away from your terminal
Be sure to get enough sleep

Using the `dbx` Debugger

Nota Bene

Be sure to compile with the -g switch and without any -O switches (Can you say `Makefile'?) [The advice about the -O switch does not apply anymore. My notes about using gcc at UWO are more current. -- 19 May 1996]

Invocation

dbx program
dbx program core

Slide 9: Using `dbx`

Essential Commands

where: print current call stack (can be used to discover where the program stopped after a core dump)
stop in func: set a breakpoint at first executable line of function func, e.g. main
stop at line: set a breakpoint at line line
run args: run your program using any (optional) command line arguments you supply
print expr: display the value of the expression expr (e.g., value of a variable, result of executing a function)

Some Other Commands

cont

continue executing after a breakpoint

step

execute the current line of your program

next

same as step but will not enter functions

status

shows breakpoints, etc. you have set

list

displays source lines. Use depends on dbx version.

In GAUL, list start:num: shows lines start to start + num.
Other versions use list start,stop.
Some versions use list func: to display the source lines of function func.

alias