What is gdb?
“GNU Debugger”
A debugger for several languages, including C and C++
It allows you to inspect what the program is doing at a certain
point during execution.
Errors like segmentation faults may be easier to find with the
help of gdb.
http://sourceware.org/gdb/current/onlinedocs/gdb toc.html
online manual
GDB Tutorial
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Additional step when compiling program
Normally, you would compile a program like:
gcc [flags] -o
For example:
gcc -Wall -Werror -ansi -pedantic-errors prog1.c -o prog1.x
Now you add a -g option to enable built-in debugging support
(which gdb needs):
gcc [other flags] -g -o
For example:
gcc -Wall -Werror -ansi -pedantic-errors -g prog1.c -o prog1.x
GDB Tutorial
Starting up gdb
Just try “gdb” or “gdb prog1.x.” You’ll get a prompt that looks
like this:
(gdb)
If you didn’t specify a program to debug, you’ll have to load it in
now:
(gdb) file prog1.x
Here, prog1.x is the program you want to load, and “file” is the
command to load it.
GDB Tutorial
Before we go any further
gdb has an interactive shell, much like the one you use as soon as
you log into the linux grace machines. It can recall history with the
arrow keys, auto-complete words (most of the time) with the TAB
key, and has other nice features.
Tip
If you’re ever confused about a command or just want more
information, use the “help” command, with or without an
argument:
(gdb) help [command]
You should get a nice description and maybe some more useful
tidbits. . .
GDB Tutorial
Running the program
To run the program, just use:
(gdb) run
This runs the program.
If it has no serious problems (i.e. the normal program didn’t
get a segmentation fault, etc.), the program should run fine
here too.
If the program did have issues, then you (should) get some
useful information like the line number where it crashed, and
parameters to the function that caused the error:
Program received signal SIGSEGV, Segmentation fault.
0×0000000000400524 in sum array region (arr=0x7fffc902a270, r1=2, c1=5,
r2=4, c2=6) at sum-array-region2.c:12
GDB Tutorial
So what if I have bugs?
Okay, so you’ve run it successfully. But you don’t need gdb for
that. What if the program isn’t working?
Basic idea
Chances are if this is the case, you don’t want to run the program
without any stopping, breaking, etc. Otherwise, you’ll just rush past the
error and never find the root of the issue. So, you’ll want to step through
your code a bit at a time, until you arrive upon the error.
This brings us to the next set of commands. . .
GDB Tutorial
Setting breakpoints
Breakpoints can be used to stop the program run in the middle, at
a designated point. The simplest way is the command “break.”
This sets a breakpoint at a specified file-line pair:
(gdb) break file1.c:6
This sets a breakpoint at line 6, of file1.c. Now, if the program
ever reaches that location when running, the program will pause
and prompt you for another command.
Tip
You can set as many breakpoints as you want, and the program
should stop execution if it reaches any of them.
GDB Tutorial
More fun with breakpoints
You can also tell gdb to break at a particular function. Suppose
you have a function my func:
int my func(int a, char *b);
You can break anytime this function is called:
(gdb) break my func
GDB Tutorial
Now what?
Once you’ve set a breakpoint, you can try using the run
command again. This time, it should stop where you tell it to
(unless a fatal error occurs before reaching that point).
You can proceed onto the next breakpoint by typing
“continue” (Typing run again would restart the program
from the beginning, which isn’t very useful.)
(gdb) continue
You can single-step (execute just the next line of code) by
typing “step.” This gives you really fine-grained control over
how the program proceeds. You can do this a lot…
(gdb) step
GDB Tutorial
Now what? (even more!)
Similar to “step,” the “next” command single-steps as well,
except this one doesn’t execute each line of a sub-routine, it
just treats it as one instruction.
(gdb) next
Tip
Typing “step” or “next” a lot of times can be tedious. If you just
press ENTER, gdb will repeat the same command you just gave it.
You can do this a bunch of times.
GDB Tutorial
Querying other aspects of the program
So far you’ve learned how to interrupt program flow at fixed,
specified points, and how to continue stepping line-by-line.
However, sooner or later you’re going to want to see things
like the values of variables, etc. This might be useful in
debugging. 
The print command prints the value of the variable
specified, and print/x prints the value in hexadecimal:
(gdb) print my var
(gdb) print/x my var
GDB Tutorial
Setting watchpoints
Whereas breakpoints interrupt the program at a particular line or
function, watchpoints act on variables. They pause the program
whenever a watched variable’s value is modified. For example, the
following watch command:
(gdb) watch my var
Now, whenever my var’s value is modified, the program will
interrupt and print out the old and new values.
Tip
You may wonder how gdb determines which variable named my var to watch if there
is more than one declared in your program. The answer (perhaps unfortunately) is
that it relies upon the variable’s scope, relative to where you are in the program at the
time of the watch. This just means that you have to remember the tricky nuances of
scope and extent :(.
GDB Tutorial
Example programs
Some example files are found in
~/212public/gdb-examples/broken.c on the linux grace
machines.
Contains several functions that each should cause a
segmentation fault. (Try commenting out calls to all but one
in main())
The errors may be easy, but try using gdb to inspect the code.
GDB Tutorial
Other useful commands
backtrace – produces a stack trace of the function calls that
lead to a seg fault (should remind you of Java exceptions)
where – same as backtrace; you can think of this version as
working even when you’re still in the middle of the program
finish – runs until the current function is finished
delete – deletes a specified breakpoint
info breakpoints – shows information about all declared
breakpoints
Look at sections 5 and 9 of the manual mentioned at the beginning
of this tutorial to find other useful commands, or just try help.
GDB Tutorial
gdb with Emacs
Emacs also has built-in support for gdb. To learn about it, go here:
http://tedlab.mit.edu/~dr/gdbintro.html
GDB Tutorial
More about breakpoints
Breakpoints by themselves may seem too tedious. You have to
keep stepping, and stepping, and stepping. . .
Basic idea
Once we develop an idea for what the error could be (like dereferencing a
NULL pointer, or going past the bounds of an array), we probably only
care if such an event happens; we don’t want to break at each iteration
regardless.
So ideally, we’d like to condition on a particular requirement (or set
of requirements). Using conditional breakpoints allow us to
accomplish this goal. . .
GDB Tutorial
Conditional breakpoints
Just like regular breakpoints, except that you get to specify some
criterion that must be met for the breakpoint to trigger. We use
the same break command as before:
(gdb) break file1.c:6 if i >= ARRAYSIZE
This command sets a breakpoint at line 6 of file file1.c, which
triggers only if the variable i is greater than or equal to the size of
the array (which probably is bad if line 6 does something like
arr[i]). Conditional breakpoints can most likely avoid all the
unnecessary stepping, etc.
GDB
