GDB Debugger

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This guide was written for Ubuntu 10.04. For the windows page, see Windows GDB Debugger.

GDB, the GNU Project Debugger is a debugging tool provided with the GNU Compiler Collection (GCC). GDB allows you to stop and start a running program, examine its functioning, and make changes.


GDB Support in OpenOCD

The configure script provided with OpenOCD 0.5.0 already compiles OpenOCD to support the GDB debugger. The OpenOCD Ubuntu Package also includes GDB Support. If you have installed OpenOCD according to the guides on the Compiling OpenOCD page, your version of OpenOCD already supports GDB.

The -g flag tells the gcc compiler to build with GDB support. OpenOCD's configure script already includes the -g flag.

GDB Debugger is provided by default on Ubuntu 10.04. However, this version of GDB is only built to debug programs running on a Linux PC. You will need to download or compile a version of GDB that supports embedded devices. One such GDB build is provided with the CodeSourcery ARM toolchain.


Installing the CodeSourcery ARM Toolchain

CodeSourcery provides development tools for use with embedded devices, including a GCC cross-compiler and a GDB build for ARM targets. Download the latest version of the .bin installer from http://www.codesourcery.com/sgpp/lite/arm/portal/release1592. Direct link is here. Save this file anywhere on your computer, then navigate to it in a terminal window.

Run the installer like this:

sh arm-2010.09-51-arm-none-eabi.bin

You should see a loading bar followed by an installer GUI. However, you may see an error stating that the installer doesn't support the DASH shell.

Arm-none-eabi dash err.png

If you see this error, disable DASH by typing

sudo dpkg-reconfigure -plow dash

This will give you the option to enable DASH by choosing <YES> or disable it by choosing <NO>. Choose no and run the installer again.

 sh arm-2010.09-51-arm-none-eabi.bin

You should see the installer GUI now. The installer is fairly straightforward. This guide will assume that you choose the default option on each page.

Sourcery-install.png

The installer installs the CodeSourcery toolchain to your home directory. Add it to your PATH by editing your environment variables:

sudo cp /etc/environment /etc/backup_environment
sudo gedit /etc/environment

This backs up your environment file and then opens it a text editor. (The backup is just in case something goes wrong; unless you make a mistake, you can delete backup_environment later.) Find the line "Path=..." (probably the only line). Add the following to the end of that line, inside the quotes:

:~/CodeSourcery/Sourcery_G++_Lite/bin

That line should now read something like this:

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:~/CodeSourcery/Sourcery_G++_Lite/bin"

Restart your computer.

Make sure everything installed correctly by running arm-none-eabi-gcc from anyhere, with no arguments. If everything installed correctly, you will see:

arm-none-eabi-gcc: no input files


Starting GDB

In the terminal window, run arm-none-eabi-gdb from any directory.

arm-none-eabi-gdb

Your terminal window should look like the image below, with a prompt reading (gdb) in place of the normal > command prompt.

Eabi-gdb startup.png


Connecting to OpenOCD

Run OpenOCD as normal, as described in Running OpenOCD on Linux. To connect to OpenOCD, start GDB as above:

arm-none-eabi-gdb

OpenOCD listens for GDB connections on port 3333. In GDB, connect to OpenOCD by typing target remote localhost:3333. (In this guide if you see (gdb) at the beginning of a command, that means enter that line into the GDB command prompt. Don't actually type the characters (gdb).)

(gdb) target remote localhost:3333

Before doing anything else, run reset init on the target. Use the monitor command to tell GDB to send the command to OpenOCD, like this:

(gdb) monitor reset init

Monitor reset init.png

This is important. You need to do this while GDB is connected to the OpenOCD, or you won't be able to halt or reset the target. If you don't run monitor reset init, you will encounter errors like this:

Gdb packet err.png


Sending Commands to OpenOCD

You can send commands to OpenOCD through GDB just like you can through a telnet connection. Type monitor, then the command, then enter. You can see a list of common OpenOCD commands here.

Gdb openocd commands.png

You can use the Linux command cd to change the current working directory in GDB. This changes the current directory only for GDB, not for Linux; when you exit GDB, you will be back in the directory where you started.

To quit GDB, type quit.


.gdbinit

Instead of typing commands yourself every time you start GDB, you can create a script to always start GDB with the same series of commands. This script file is named .gdbinit. GDB looks for it in the current working directory.

You can use the script below to have GDB automatically connect to OpenOCD and run reset init on the target. Create a new folder in your home directory called GDB_OpenOCD_init, and a new text file in that folder. Copy the code below into the file:

echo Executing GDB with .gdbinit to connect to OpenOCD.\n
echo .gdbinit is a hidden file. Press Ctrl-H in the current working directory to see it.\n
# Connect to OpenOCD
target remote localhost:3333
# Reset the target and call its init script
monitor reset init
# Halt the target. The init script should halt the target, but just in case
monitor halt

Save the file as .gdbinit and close it. Like any file whose name begins with a period, .gdbinit is a hidden file. To see it, open the folder and press CTRL-H.

To test the init script, start OpenOCD as normal. Then navigate to GDB_OpenOCD_init/ and run CodeSourcery GDB.

cd ~/GDB_OpenOCD_init
arm-none-eabi-gdb

You should see something like the image below. (In this image the target device is the Beagleboard. With different hardware the output of reset init will be different.)

Gdbinit.png


Debugging a Program on the Beagleboard

This part of the guide will demonstrate how to run and debug a simple program on the Beagleboard. The program will only work on the Beagleboard, but the commands are the same on other target devices.


Installing Packages

For this section you will need the Ubuntu packages git and make. Install the following packages by opening a new terminal window and typing:

sudo apt-get install git make

You may see a message stating that these pachages are already installed. That's fine.


Compiling LEDblink

Navigate to your home directory and use git to clone into https://github.com/mlu/cortal_dendrites.git.

cd ~
git clone https://github.com/mlu/cortal_dendrites.git

You should now have a folder in your home directory called cortal_dendrites/. Navigate to /home/USERNAME/cortal_dendrites/cortex_a8/standalone/LEDblink. This directory contains the source for LEDblink. Run make to compile it.

cd ~/cortal_dendrites/cortex_a8/standalone/LEDblink
make

The folder should now contain a binary file called LEDblink. It also contains a hidden file called .gdbinit. This script should start GDB and load LEDblink, but the file is outdated and no longer works. Delete it so that you can't accidentally run it later.

rm .gdbinit


Loading LEDblink

Start OpenOCD as normal, then open a new terminal window. Run GDB, connect it to OpenOCD, and reset the Beagleboard. You can do this manually:

arm-none-eabi-gdb
(gdb) target remote localhost:3333
(gdb) monitor reset init

...or using the .gdbinit script you created above:

cd ~/GDB_OpenOCD_init
arm-none-eabi-gdb

Beagle usrleds.png

The USR0 and USR1 LEDs on the Beagleboard should now be off. The Beagleboard has just been reset and is halted. Tell the Beagleboard to resume to let it boot. You should see USR0 and USR1 come on.

(gdb) monitor resume

Wait for the lights to come on, and then halt the Beagleboard again.

(gdb) monitor halt

Navigate to the directory containing LEDblink and load it:

(gdb) cd ~/cortal_dendrites/cortex_a8/standalone/LEDblink
(gdb) load LEDblink
(gdb) symbol-file LEDblink

Ledblink load.png

Now run the program by typing:

(gdb) cont

You should see the USR0 and USR1 LEDs pulse on and off in sequence. LEDblink will run forever if you let it. When you're ready to stop it, press CTRL-C in the GDB terminal. To start it again, type cont again.


Breakpoints

When debugging a program it's often useful to stop it in the middle to see what it's doing. You can do that with GDB using breakpoints. When you ran the command symbol-file LEDblink above, you loaded a file that allows GDB to map lines and functions in the source code to instructions in the executable. You can now instruct GDB to stop the program when it reaches a particular line or function.

Add a breakpoint associated with line 22 of LEDblink.c, then let LEDblink continue, like this:

(gdb) break 22
(gdb) cont

GDB will run until it hits line 22, then return you to the command prompt. Line 22 is inside a loop that never terminates, so GDB will hit this breakpoint over and over. Each time GDB hits the breakpoint, you can tell it to proceed by typing cont again.

You can have multiple breakpoints at a time. Type:

(gdb) break 30

...to create a second breakpoint at line 30. You can get a list of all breakpoints by typing:

(gdb) info breakpoints

info breakpoints lists your breakpoints by number. You can delete a breakpoint by entering delete followed by a number.

(gdb) delete 1

...deletes Breakpoint 1.

Ledblink breakpoints.png


Reading Memory

You can use GDB to read the value stored in a memory address with the x command. Type x followed by a memory address to output the value at that address. GDB assumes that the address is in decimal format unless you preface it with 0x to indicate hexadecimal. Memory addresses are usually expressed in hexadecimal, so remember to type 0x.

(gdb) x 0x49056090

Gdb x.png

The values at 0x49056090 and 0x49056094 control the USR0 and USR1 LEDs. The 22nd bit of each (0x00400000, in hex) controls USR0, and the 21st bit (0x00200000) controls USR1. Writing to 0x49056090 turns a light off, and writing to 0x49056094 turns a light on.


address 0x49056090 = 0x00400000: turn USR0 off

address 0x49056090 = 0x00200000: turn USR1 off

address 0x49056090 = 0x00600000: turn both LEDs off


address 0x49056094 = 0x00400000: turn USR0 on

address 0x49056094 = 0x00200000: turn USR1 on

address 0x49056094 = 0x00600000: turn both LEDs on


You can type x 0x49056090 and x 0x49056094 when LEDblink reaches a breakpoint to see what the program is doing.


External Links

GDB man page