GDB stub


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 the client while Zephyr acts as the server. When this feature is enabled, Zephyr stops its execution after 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

Enabling GDB Stub

GDB stub can be enabled with the CONFIG_GDBSTUB option.

Using Serial Backend

The serial backend for GDB stub can be enabled with the 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 CONFIG_GDBSTUB_SERIAL_BACKEND_NAME to the spare UART device so that printk() and log messages are not being printed to the same UART device used for GDB.

  • For boards with only one UART device, 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 CONFIG_PRINTK and CONFIG_LOG.


Using Serial Backend

  1. Build with GDB stub and serial backend enabled.

  2. Flash built image onto board and reset the board.

    • Execution should now be paused at gdb_init().

  3. Execute GDB on development machine and connect to the GDB stub.

    target remote <serial device>

    For example,

    target remote /dev/ttyUSB1
  4. GDB commands can be used to start debugging.


This is an example using samples/subsys/debug/gdbstub to demonstrate how GDB stub works.

  1. Open two terminal windows.

  2. On the first terminal, build and run the sample:

    # From the root of the zephyr repository
    west build -b qemu_x86 samples/subsys/debug/gdbstub
    west build -t run
  3. On the second terminal, start GDB:

    <SDK install directory>/x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gdb
    1. Tell GDB where to look for the built ELF file:

      (gdb) file <build directory>/zephyr/zephyr.elf

      Response from GDB:

      Reading symbols from <build directory>/zephyr/zephyr.elf...
    2. Tell GDB to connect to the server:

      (gdb) target remote localhost:5678

      Note that QEMU is setup to redirect the serial used for GDB stub in the Zephyr image to a networking port. Hence the connection to localhost, port 5678.

      Response from GDB:

      Remote debugging using :5678
      arch_gdb_init () at <ZEPHYR_BASE>/arch/x86/core/ia32/gdbstub.c:232
      232     }

      GDB also shows where the code execution is stopped. In this case, it is at arch/x86/core/ia32/gdbstub.c, line 232.

    3. Use command bt or backtrace to show the backtrace of stack frames.

      (gdb) bt
      #0  arch_gdb_init () at <ZEPHYR_BASE>/arch/x86/core/ia32/gdbstub.c:232
      #1  0x00105068 in gdb_init (arg=0x0) at <ZEPHYR_BASE>/subsys/debug/gdbstub.c:833
      #2  0x00109d6f in z_sys_init_run_level (level=0x1) at <ZEPHYR_BASE>/kernel/device.c:72
      #3  0x0010a40b in z_cstart () at <ZEPHYR_BASE>/kernel/init.c:423
      #4  0x00105383 in z_x86_prep_c (arg=0x9500) at <ZEPHYR_BASE>/arch/x86/core/prep_c.c:58
      #5  0x001000a9 in __csSet () at <ZEPHYR_BASE>/arch/x86/core/ia32/crt0.S:273
    4. Use command list to show the source code and surroundings where code execution is stopped.

      (gdb) list
      227     }
      229     void arch_gdb_init(void)
      230     {
      231             __asm__ volatile ("int3");
      232     }
      234     /* Hook current IDT. */
      235     _EXCEPTION_CONNECT_NOCODE(z_gdb_debug_isr, IV_DEBUG, 3);
      236     _EXCEPTION_CONNECT_NOCODE(z_gdb_break_isr, IV_BREAKPOINT, 3);
    5. Use command s or step to step through program until it reaches a different source line. Now that it finished executing arch_gdb_init() and is continuing in gdb_init().

      (gdb) s
      gdb_init (arg=0x0) at /home/dleung5/zephyr/rtos/zephyr/subsys/debug/gdbstub.c:834
      834     return 0;
      (gdb) list
      829                     LOG_ERR("Could not initialize gdbstub backend.");
      830                     return -1;
      831             }
      833             arch_gdb_init();
      834             return 0;
      835     }
      837     #ifdef CONFIG_XTENSA
      838     /*
    6. Use command br or break to setup a breakpoint. This example sets up a breakpoint at main(), and let code execution continue without any intervention using command c (or continue).

      (gdb) break main
      Breakpoint 1 at 0x1005a9: file <ZEPHYR_BASE>/samples/subsys/debug/gdbstub/src/main.c, line 32.
      (gdb) continue

      Once code execution reaches main(), execution will be stopped and GDB prompt returns.

      Breakpoint 1, main () at <ZEPHYR_BASE>/samples/subsys/debug/gdbstub/src/main.c:32
      32           ret = test();

      Now GDB is waiting at the beginning of main():

      (gdb) list
      28     void main(void)
      29     {
      30             int ret;
      32             ret = test();
      33             printk("%d\n", ret);
      34     }
      36     K_THREAD_DEFINE(thread, STACKSIZE, thread_entry, NULL, NULL, NULL,
    7. To examine the value of ret, the command p or print can be used.

      (gdb) p ret
      $1 = 0x11318c

      Since ret has not been assigned a value yet, what it contains is simply a random value.

    8. If step (s or step) is used here, it will continue execution until printk() is reached, thus skipping the interior of test(). To examine code execution inside test(), a breakpoint can be set for test(), or simply using si (or stepi) to execute one machine instruction, where it has the side effect of going into the function.

      (gdb) si
      test () at <ZEPHYR_BASE>/samples/subsys/debug/gdbstub/src/main.c:13
      13     {
      (gdb) list
      8      #include <zephyr/sys/printk.h>
      10     #define STACKSIZE 512
      12     static int test(void)
      13     {
      14             int a;
      15             int b;
      17             a = 10;
    9. Here, step can be used to go through all code inside test() until it returns. Or the command finish can be used to continue execution without intervention until the function returns.

      (gdb) finish
      Run till exit from #0  test () at <ZEPHYR_BASE>/samples/subsys/debug/gdbstub/src/main.c:13
      0x001005ae in main () at <ZEPHYR_BASE>/samples/subsys/debug/gdbstub/src/main.c:32
      32             ret = test();
      Value returned is $2 = 0x1e

      And now, execution is back to main().

    10. Examine ret again which should have the return value from test(). Sometimes, the assignment is not done until another step is 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 = 0x11318c
      (gdb) s
      33              printk("%d\n", ret);
      (gdb) p ret
      $4 = 0x1e
    11. If continue is 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 GDB stub does not support this functionality (yet).