GDB

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The GNU Debugger GDB is the most common debug tool for Linux. It features most used features one can think of, including server-client debug architecture (so you run the heavy debug part on your host/pc machine), but lack some bits as checkpoint-restart during execution.

Documentation

GDB ships with extensive documentation at http://www.gnu.org/software/gdb/documentation/, but there are some good quick reference cards as well.

Basic Usage

Documentation is so large that sometimes its hard to get started, so most simple tasks can be done with the following commands, but please read GDB docs as soon as possible!

Cross or not crosss

A common error while using gdb,is using the wrong gdb. In order to debug a program that runs on a target from your host you need a cross gdb,that is to say a GDB that runs on your computer but can debug the target architecture. Such version is normally included in your toolchain/SDK or buildable if you use a build system

If you want to debug use GDB on target you can too,it's easier but has a huge drawback(amongs other) : ram usage (you can easily use too much memory and have an OOM(out of memory) )

Cross GDB

Code sourcey Linux

Let's say that you downloaded arm-2009q3-67-arm-none-linux-gnueabi-i686-pc-linux-gnu.tar.bz2

You will need to unpack it,and to run the following command on your host computer (instead of just running gdb):

cd  arm-2009q3/bin
./arm-none-linux-gnueabi-gdb 
Openembedded

If your build system is also your debug workstation do:

bitbake gdb-cross gdbserver

And it will build a cross gdb for your host and gdbserver for your target The resulting binaries will be found in your TMPDIR here:

/home/embedded/tmpdir/cross/armv6/bin/

replace "/home/embedded/tmpdir" by your tmpdir and armv6 by your target architecture The binary name changes according to your distro settings,for me it was:

arm-angstrom-linux-gnueabi-gdb

So in order to run it do: ./arm-angstrom-linux-gnueabi-gdb

Howto

If you have networking,it's advised to choose it because it will be faster than serial,but if you only have serial choose serial

Networking

Fist start gdbserver on the target:

target/device$ gdbserver :2345 ./binary arg1 arg2

Note that will expose a gdb session to all networking interfaces,for instance if you have wifi on,someone could connect to it. So if you don't want that,make it listen only on your the ip of the target interface you want to use for debugging:

target/device$ gdbserver 192.168.0.202:2345 ./binary arg1 arg2

Then do on your debug workstation: host/pc$ ./arm-angstrom-linux-gnueabi-gdb

(gdb) target remote 192.168.0.123:2345
Serial

First set the serial port speed on the target(before you issue the gdbserver command):

 stty speed 115200 < /dev/ttyS1

Then start gdbserver on the target:

target/device$ gdbserver /dev/ttyS1 ./binary arg1 arg2

That will freeze the program until you connect with the cross gdb version and that you type "c" for continuing the execution of the program. Note that GDB will slow the program a lot. if you have a program that uses a lot of resources,it's advised to attach to it just before the crash instead like that:

target/device$ gdbserver /dev/ttyS1 --attach PID

Then do that on your workstation

host/pc$ ./arm-angstrom-linux-gnueabi-gdb
(gdb) target remote /dev/ttyS1

If you are using the serial method you need to make sure you have setup the serial speed correctly, On the host:

 (gdb) set remotebaud 115200

Non-cross GDB

Howto

To start a new application for debug, use:

 $ gdb ./binary
 $ gdb ./binary core.dump
 $ gdb --args ./binary arg1 arg2
 $ gdb --command=my-gdb-commands.txt --args ./binary arg1 arg2

and then run it with (args just required if no --args were used):

 (gdb) run arg1 arg2

If application is already running, find out its pid (ps, top, pidof, ...) and:

$ gdb --pid $PID

Tips

if you need to execute a series of commands every time, consider writing them on a file and use --command=file (or -x file).

Breakpoints

If you control-C (^C), it will break at that point, but you can also schedule a breakpoint with:

(gdb) break function
(gdb) break line
(gdb) break file:function
(gdb) break file:line

conditional breaks are in the form:

(gdb) break where if condition

where condition is some C expression that evaluates to 1 or 0, like *ptr == NULL

One can disable or remove breakpoints with:

(gdb) enable breakpoint-number
(gdb) disable breakpoint-number
(gdb) delete breakpoint-number
(gdb) clear       # removes all breakpoints

Examining

To list source code nearby position or specific places:

(gdb) list
(gdb) list line
(gdb) list function
(gdb) list file:line
(gdb) list file:function
(gdb) list *address

To list execution backtrace (or bt for short):

(gdb) backtrace

To change frame to operate on:

(gdb) frame frame-number

To change thread to operate on:

(gdb) thread thread-number

To print some value or expression:

(gdb) print $register
(gdb) print variable
(gdb) print *address
(gdb) print *(int *)address
(gdb) print *(char **)address
(gdb) print myfunc(p1, p2) # will actually execute it and return result!
(gdb) print *a = 123 # will actually change *a value and return 123!
(gdb) print file::variable
(gdb) print function::variable

To disassembly:

(gdb) disassembly
(gdb) disassembly file:line

Print function arguments:

(gdb) info args

Print locals:

(gdb) info locals

Print breakpoints:

(gdb) info breakpoints

Print threads:

(gdb) info threads

Stepping

To go to next instruction, possible entering a function (or s for short):

(gdb) step

To go to next instruction, but avoid entering new functions (or n for short):

(gdb) next

To continue until the function returns:

(gdb) finish

To continue execution (or c for short):

(gdb) continue

Manipulating Program

To set variable to some value:

(gdb) set var name=value

To force function to return:

(gdb) return value
(gdb) return expression

Changing Signal Handlers

(gdb) handle signal action

debugging applications with old libC, those pre-nptl, can be really annoying due SIG32 and SIG33, one can ignore those with:

(gdb) handle SIG32 nostop
(gdb) handle SIG32 noprint
(gdb) handle SIG33 nostop
(gdb) handle SIG33 noprint

Shared Object Paths

Often your cross compile root is not /, so you might have to add new paths to the search list.

Unset absolute prefix:

 (gdb) set solib-absolute-prefix null

Add paths to search paths:

 (gdb) set solib-search-path /path1:/path2

Alternatively you can choose to set the prefix to the root of your target file system. Specially if you are doing embedded development and already exporting your root file system from you host machine to your target machine it can be very rewarding so simply use that as root:

 (gdb) set solib-absolute-prefix /rootfs