GDB: Difference between revisions

 

While debugging system code is not its intended use, it does a fine job of it; it can be used directly with some emulators, without modifying your code at all, or it can be used over a serial line. The latter option involves implementing a Remote Serial Protocol stub in your operating system.

==Remote Debugging==

We are interested in the remote debugging facility of GDB. From the [http://sourceware.org/gdb/current/onlinedocs/gdb/Remote-Debugging.html GDB manual], “If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.”

 

For remote debugging, a remote stub should be implemented in the remote program that needs to be debugged. That means the kernel should contain the remote stub to talk to the host GDB during the debug session. This requires a kernel source change and it is a must if you are running your kernel on a test(real) machine and debugging from another machine. If you are using an emulator (Bochs or QEMU), you can use the GDB stub compiled into the emulator.

===Implementing GDB Stub===

To debug (using GDB) a kernel running on a real machine, the kernel needs to contain a GDB stub. For the i386 platform, GDB source includes a reference implementation of gdb-stub.c. It requires the following three functions from the kernel to read/write from a serial port and to set up the exception handlers.

getDebugChar(); // to read a character from serial port

putDebugChar(int); // to write a character to serial port

exceptionHandler(int exception_number, void *exception_address); // to set exception handler

Implement the above functions in your kernel, include the gdb-stub.c in the compilation, and also call the following functions during kernel start to connect to the gdb host.

InitSerialPort(sys_gdb_port, 9600, UART_DATA_BIT_8, UART_PARITY_NONE, UART_STOP_BIT_1); /*set up the serial port*/

set_debug_traps(); /*setup exception handlers*/

kprintf("Waiting for GDB(0x%X) : ", sys_gdb_port );

__asm__("int3"); /*break point exception to sync with GDB host*/

Now start your test machine and your kernel will wait for a GDB host connection.

===Using Emulator Stubs===

'''QEMU''' - GDB can be connected to [[QEMU]] by simply starting qemu with the <tt>-s -S</tt> command line switches.

'''Bochs''' - For GDB to be able to interface with [[Bochs]], Bochs will need to be configured with the <tt>--enable-gdb-stub</tt> argument.

The <tt>bochssrc</tt> (or whatever configuration script is being used) will need to have the <tt>gdbstub</tt> line set to something like ''gdbstub: enabled=1, port=1234, text_base=0, data_base=0, bss_base=0''.

==Starting GDB==

After an emulator/kernel is configured it will then wait for a connection from GDB. GDB on the host machine can be started like this:

0x0000fff0 in ?? ()

(gdb)

If you are debugging a kernel running on a real machine then use ''target remote /dev/tty2'' instead of the network port.

 

 

If you are using a 64-bit kernel, you may need to set up the address size using set command.

===gdbinit files===

file kernel.bin

target remote localhost:1234

This will connect to GDB-stub whenever you start GDB and will load symbols from kernel.bin.

===GDB in Emacs===

===Virtual Serial Ports===

 

[http://com0com.sourceforge.net/ com0com] can be used in Windows and a pseudo terminal might be used in Linux to create virtual serial port pairs.

== See Also ==

=== External Links ===

* [http://stackoverflow.com/questions/1471226/most-tricky-useful-commands-for-gdb-debugger StackOverflow: Most tricky / useful commands for GDB debugger]

[[Category:Tools]]

[[de:GDB]]