How to use gdb

From ODF::Wiki

Using gdb can save coders a lot of time finding bugs. Here is a very short 5-minute-introduction to show you how to use it.

[edit] Getting started

[edit] Notice about compiling

You should note that it is recommended to compile your programs with the -g or (especially for the gdb compiler) with the -ggdb flag.

[edit] Starting it

Usually, you would start a program with parameters on the commandline. Using gdb, the programs name should

either not be passed when starting gdb (but passed when you will type run)
or be passed using gdb [gdb-options] --args program arguments

As an example, "./doh -f readme" would be written

either as gdb ./doh ... run -f readme
or as gdb --args ./doh -f readme

[edit] The prompt

After having started gdb, you will see a prompt, like

(gdb)

You will be able to type commands here. Whenever you will type quit, you will close gdb.

[edit] Commands

[edit] help

Type "help <command>" to show help about the desired command.

[edit] run

The probably most important command is run. It will start the program as you are used to, and it will only stop if the program stops usually - or if it hits a breakpoint. Breakpoints are points where the execution will be paused to let you display some information. See the break command for more information.

Note that you can run a program again after it has stopped without restarting gdb. Also note that, if you have not specified the arguments on starting gdb, you can do it now by typing

run <arguments>

For accessing files in the argument list, you can usually use your tab key as if you were in a shell:

./doh -f re<TAB> could result in ./doh -f readme

[edit] break

The break command lets you set a breakpoint at a specific line in the source code. Breakpoints can either be specified by giving a function name or a file and a line number.

(gdb) break tarfile.h:60
(gdb) break odf::TarFile::TarFile(char, char const*)

Those could also be achieved using the tab completion:

(gdb) break tarf<TAB>:60
(gdb) break odf::Ta<TAB>::T<TAB>

Note that you sometimes need to specify a namespace, though.

[edit] step

Once you arrived at a breakpoint, you can step every line of sourcecode. That way, you can easily watch which way your command goes. Watch this:

(gdb) run
Starting program: /home/johannes/cprogs/ODF/doh/doh -f readme

DOH - The better HOG-utility!
:::::::::::::::::::::::::::::
Version 1.0


Breakpoint 1, main (argc=3, argv=0xbfffefc4) at ./src/main.cpp:61
61              if(!strcmp(archive_name,"")) {
(gdb) step
67              switch( what )
(gdb) step
111                                     exit(0);
(gdb)

[edit] print

With the print command, you can print variables or even the results of functions. E.g. print what or print w<TAB> might print you the results of the variable what. Note that sometimes, you will need to prepend namespaces and classnames.

[edit] finish

If you are stepping and arriving in an stl function, like, e.g. vector::push_back(), you might not want to watch what's happening there. Try the finish command to skip that output.

[edit] continue

The continue command will let the program run on from the point you have stepped to. If you want to run from the begining, try the run command instead.

[edit] Backtrace

If your program is hanging somewhere, you can press Ctrl+C. Afterwards you can still debug.

In such situations, it is often useful to see where your program currently is. You can use the bt command to make a backtrace:

bt

If you have multiple threads, you can view a backtrace of each thread using

thread apply all bt

[edit] Discovering gdb

Most commands can be used with single letters:

help - h
quit - q
run - r
break - b
step - s
print - p
finish - f
continue - c

There are lots of interesting programs using gdb. Try, e.g.

watch
display
...

For more information, type "man gdb" on a linux shell, or visit The gdb homepage