Department of Computer Science

gdb - the C/C++ symbolic debugger.

GDB is a symbolic debugger that works well with code compiled with g++. It allows you to step through your program line by line and see the contents of variables. It can be quite complex but you only need to know a few commands for it to be useful.

Compiling

You must add a flag when compiling source code (-g). It is a good idea to always include the flag when compiling so that you can debug whenever you want. This is also important when you have several object files. Each one must be compiled with the -g oprtion or else you will not be able to debug any functions within that object file.

g++ -o program source.c -g

Finding core dumps

One major benefit of using gdb is when you get a core dump. Start gdb by including the program name and core file on the command line. GDB will show you what line the program aborted on!

gdb lab1 core

Looking at variables

The other benefits come from stepping thru the program line by line and seeing the contents of the variables. This is more convenient than placing print statements throughout the code. If you have a local variable called temp you would type: p temp to see the current value. However, if temp is a data member in a member function then you must type: p this->temp to print the contents of the variable.

Essential Commands

gdb program [core] start gdb with your program and optional core

b function or line# set a breakpoint at specified location

l [-] # of lines display source code for next several lines

p expr display the value of an expression such as a variable

n next line, stepping over function calls

s next line, stepping into function calls

c continue running the program

run [arglist] run the current program with optional command line arguments

quit exit from gdb

Finding Help

help list classes of commands

help class descriptions of commands in class

help command describe command

gdb program [core]	start gdb with your program and optional core
b function or line#	set a breakpoint at specified location
l [-] # of lines	display source code for next several lines
p expr	display the value of an expression such as a variable
n	next line, stepping over function calls
s	next line, stepping into function calls
c	continue running the program
run [arglist]	run the current program with optional command line arguments
quit	exit from gdb

help	list classes of commands
help class	descriptions of commands in class
help command	describe command