The gdbstub feature provides an implementation of the GDB Remote Serial Protocol (RSP) that allows you to remotely debug Zephyr using GDB.
The protocol supports different connection types: serial, UDP/IP and TCP/IP. Zephyr currently supports only serial device communication.
The GDB program acts as a client while the Zephyr gdbstub acts as a server. When this feature is enabled, Zephyr stops its execution after :c:func:`gdb_init` starts gdbstub service and waits for a GDB connection. Once a connection is established it is possible to synchronously interact with Zephyr. Note that currently it is not possible to asynchronously send commands to the target.
The following features are supported:
- Add and remove breakpoints
- Continue and step the target
- Print backtrace
- Read or write general registers
- Read or write the memory
GDB stub can be enabled with the :kconfig:option:`CONFIG_GDBSTUB` option.
The serial backend for GDB stub can be enabled with the :kconfig:option:`CONFIG_GDBSTUB_SERIAL_BACKEND` option.
Since serial backend utilizes UART devices to send and receive GDB commands,
- If there are spare UART devices on the board, set
zephyr,gdbstub-uartproperty of the chosen node to the spare UART device so that :c:func:`printk` and log messages are not being printed to the same UART device used for GDB. - For boards with only one UART device, :c:func:`printk` and logging must be disabled if they are also using the same UART device for output. GDB related messages may interleave with log messages which may have unintended consequences. Usually this can be done by disabling :kconfig:option:`CONFIG_PRINTK` and :kconfig:option:`CONFIG_LOG`.
Build with GDB stub and serial backend enabled.
Flash built image onto board and reset the board.
- Execution should now be paused at :c:func:`gdb_init`.
Execute GDB on development machine and connect to the GDB stub.
target remote <serial device>
For example,
target remote /dev/ttyUSB1
GDB commands can be used to start debugging.
There is a test application :zephyr_file:`tests/subsys/debug/gdbstub` with one of its
test cases debug.gdbstub.breakpoints demonstrating how the Zephyr GDB stub can be used.
The test also has a case to connect to the QEMU's GDB stub implementation (at a custom
port tcp:1235) as a reference to validate the test script itself.
Run the test with the following command from your :envvar:`ZEPHYR_BASE` directory:
./scripts/twister -p qemu_x86 -T tests/subsys/debug/gdbstub
The test should run successfully, and now let's do something similar step-by-step to demonstrate how the Zephyr GDB stub works from the GDB user's perspective.
In the snippets below use and expect your appropriate directories instead of
<SDK install directory>, <build_directory>, <ZEPHYR_BASE>.
Open two terminal windows.
On the first terminal, build and run the test application:
.. zephyr-app-commands:: :zephyr-app: tests/subsys/debug/gdbstub :host-os: unix :board: qemu_x86 :gen-args: '-DCONFIG_QEMU_EXTRA_FLAGS="-serial tcp:localhost:5678,server"' :goals: build run
Note how we set :kconfig:option:`CONFIG_QEMU_EXTRA_FLAGS` to direct QEMU serial console port to the
localhostTCP port5678to wait for a connection from the GDBremotecommand we are going to do on the next steps.On the second terminal, start GDB:
<SDK install directory>/x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gdb
Tell GDB where to look for the built ELF file:
(gdb) symbol-file <build directory>/zephyr/zephyr.elf
Response from GDB:
Reading symbols from <build directory>/zephyr/zephyr.elf...
Tell GDB to connect to the Zephyr gdbstub serial backend which is exposed earlier as a server through the TCP port
-serialredirection at QEMU.(gdb) target remote localhost:5678
Response from GDB:
Remote debugging using localhost:5678 arch_gdb_init () at <ZEPHYR_BASE>/arch/x86/core/ia32/gdbstub.c:252 252 }
GDB also shows where the code execution is stopped. In this case, it is at :zephyr_file:`arch/x86/core/ia32/gdbstub.c`, line 252.
Use command
btorbacktraceto show the backtrace of stack frames.(gdb) bt #0 arch_gdb_init () at <ZEPHYR_BASE>/arch/x86/core/ia32/gdbstub.c:252 #1 0x00104140 in gdb_init () at <ZEPHYR_BASE>/zephyr/subsys/debug/gdbstub.c:852 #2 0x00109c13 in z_sys_init_run_level (level=INIT_LEVEL_PRE_KERNEL_2) at <ZEPHYR_BASE>/kernel/init.c:360 #3 0x00109e73 in z_cstart () at <ZEPHYR_BASE>/kernel/init.c:630 #4 0x00104422 in z_prep_c (arg=0x1245bc <x86_cpu_boot_arg>) at <ZEPHYR_BASE>/arch/x86/core/prep_c.c:80 #5 0x001000c9 in __csSet () at <ZEPHYR_BASE>/arch/x86/core/ia32/crt0.S:290 #6 0x001245bc in uart_dev () #7 0x00134988 in z_interrupt_stacks () #8 0x00000000 in ?? ()
Use command
listto show the source code and surroundings where code execution is stopped.(gdb) list 247 __asm__ volatile ("int3"); 248 249 #ifdef CONFIG_GDBSTUB_TRACE 250 printk("gdbstub:%s GDB is connected\n", __func__); 251 #endif 252 } 253 254 /* Hook current IDT. */ 255 _EXCEPTION_CONNECT_NOCODE(z_gdb_debug_isr, IV_DEBUG, 3); 256 _EXCEPTION_CONNECT_NOCODE(z_gdb_break_isr, IV_BREAKPOINT, 3);Use command
sorstepto step through program until it reaches a different source line. Now that it finished executing :c:func:`arch_gdb_init` and is continuing in :c:func:`gdb_init`.(gdb) s gdb_init () at <ZEPHYR_BASE>/subsys/debug/gdbstub.c:857 857 return 0;
(gdb) list 852 arch_gdb_init(); 853 854 #ifdef CONFIG_GDBSTUB_TRACE 855 printk("gdbstub:%s exit\n", __func__); 856 #endif 857 return 0; 858 } 859 860 #ifdef CONFIG_XTENSA 861 /*Use command
brorbreakto setup a breakpoint. For this example set up a breakpoint at :c:func:`main`, and let code execution continue without any intervention using commandc(orcontinue).(gdb) break main Breakpoint 1 at 0x10064d: file <ZEPHYR_BASE>/tests/subsys/debug/gdbstub/src/main.c, line 27.
(gdb) continue Continuing.
Once code execution reaches :c:func:`main`, execution will be stopped and GDB prompt returns.
Breakpoint 1, main () at <ZEPHYR_BASE>/tests/subsys/debug/gdbstub/src/main.c:27 27 printk("%s():enter\n", __func__);Now GDB is waiting at the beginning of :c:func:`main`:
(gdb) list 22 23 int main(void) 24 { 25 int ret; 26 27 printk("%s():enter\n", __func__); 28 ret = test(); 29 printk("ret=%d\n", ret); 30 return 0; 31 }To examine the value of
ret, the commandporprintcan be used.(gdb) p ret $1 = 1273788
Since
rethas not been initialized, it contains some random value.If step (
sorstep) is used here, it will continue execution skipping the interior of :c:func:`test`. To examine code execution inside :c:func:`test`, a breakpoint can be set for :c:func:`test`, or simply usingsi(orstepi) to execute one machine instruction, where it has the side effect of going into the function. The GDB commandfinishcan be used to continue execution without intervention until the function returns.(gdb) finish Run till exit from #0 test () at <ZEPHYR_BASE>/tests/subsys/debug/gdbstub/src/main.c:17 0x00100667 in main () at <ZEPHYR_BASE>/tests/subsys/debug/gdbstub/src/main.c:28 28 ret = test(); Value returned is $2 = 30
Examine
retagain which should have the return value from :c:func:`test`. Sometimes, the assignment is not done until anotherstepis issued, as in this case. This is due to the assignment code is done after returning from function. The assignment code is generated by the toolchain as machine instructions which are not visible when viewing the corresponding C source file.(gdb) p ret $3 = 1273788 (gdb) step 29 printk("ret=%d\n", ret); (gdb) p ret $4 = 30If
continueis issued here, code execution will continue indefinitely as there are no breakpoints to further stop execution. Breaking execution in GDB via Ctrl-C does not currently work as the Zephyr gdbstub does not support this functionality yet. Switch to the first console with QEMU running the Zephyr image and stop it manually with Ctrl+a x. When the same test is executed by Twister, it automatically takes care of stopping the QEMU instance.