Esp8266 GDBSTUB for Sming


This is a rewrite of gdbstub based on the esp8266 Arduino project.

To use the GNU Debugger (GDB) with Sming requires your application to include some code (gdbstub) which communicates via the serial port. On the ESP8266 only UART0 may be used for this as UART1 is transmit-only.

The gdbstub code will only be built if you specify ENABLE_GDB =1 when compiling your application. At startup, before your init() function is called, it will claim UART0 so your application will be unable to use it directly. Therefore, the default port for Serial is changed to UART2.

UART2 is a ‘virtual’ serial port to enable serial communications to work correctly when GDB-enabled. Read/write calls and serial callbacks are handled via gdbstub. Baud rate changes affect UART0 directly.

Note that target refers to the application being debugged, and host the development system running the GDB program.

Refer to the official GDB documentation for further details.


This is the application which runs on your development system and talks to gdbstub.

  • Linux: A version of this should be available in $ESP_HOME/xtensa-lx106-elf/bin/xtensa-lx106-elf-gdb

  • Windows: At time of writing, UDK doesn’t provide a GDB application - Download and run the executable installer at SysProgs

    • Copy the C:\SysGCC\esp8266\opt\xtensa-lx106-elf\bin\xtensa-lx106-elf-gdb.exe to a suitable location.

  • Mac: ?


  • Configure gdbstub by editing gdbstub-cfg.h as required. You can also configure the options by setting :USER_CFLAGS in your project’s file. e.g USER_CFLAGS=-DGDBSTUB_BREAK_ON_INIT=0.

  • Optional: Add gdb_do_break() statements to your application.

  • Run make clean, then make ENABLE_GDB=1 flash to build and flash the application with debugging enabled

  • Run gdb, depending on your configuration immediately after resetting the board or after it has run into an exception. The easiest way to do it is to use the provided script: make gdb.

To run manually, see the following variables which you can inspect using make list-config.


Command line used to run GDB.


Location of the GDB stub component, and the gdbcmds file.


Defaults to COM_PORT, but if necessary you can change it to a different value.


Same as COM_SPEED_SERIAL, which is the value compiled into the gdbstub code.


If you need to use alternate serial pins, enable this option GDB_UART_SWAP=1


Path to the GDB executable being used.

Useful GDB commands

c Continue execution

q Quit and detach

where Display current stopped location

bt Show stack backtrace

disass Disassemble, disass/m to mix with source code

print expr Display a variable or other value

print func() Call a function, display result, or call func() to discard result

tui enable Provides a windowed interface within the console (only seems to work in Linux)

x/32xw $sp Display contents of stack

info reg Display register values

info break Display details of currently set breakpoints

delete Delete all breakpoints

br Set a breakpoint at the given address or function name

hbr Set a hardware breakpoint

watch Set a hardware watchpoint to detect when the value of a variable changes

These commands require GDBSTUB_ENABLE_HOSTIO to be enabled:

remote get targetfile hostfile Read a file from SPIFFS (on the target)

remote put hostfile targetfile Write a file to SPIFFS

remote delete targetfile Delete a file from SPIFFS



  • Ensure Use external console for inferior is checked.

  • In connection settings, specify COM port like with leading /, e.g. /COM4

Problems connecting?

  • Switch to the debug perspective before connecting

  • Ensure serial baud rate matches your application

  • Remove or disable all breakpoints before attaching. Eclipse will attempt to set these on connection, and if any are invalid it will hang and timeout.

  • Check connectivity using command-line GDB

GDB System Calls

Applications may interact with GDB directly using system calls, for example reading input from the GDB command prompt. See the Live Debug sample for a demonstration.

Note that system calls are disabled in the default configuration, so set GDBSTUB_ENABLE_SYSCALL =1 to use this feature with your application.

Known Issues and Limitations

  • Unable to set requested break/watch points
    • Cause: Due to hardware limitations, only one hardware breakpount and one hardware watchpoint are available

    • Solution: None (hardware limitation)

  • System crashes if debugger is paused for too long
    • Cause: The WiFi hardware is designed to be serviced by software periodically. It has some buffers so it will behave OK when some data comes in while the processor is busy, but these buffers are not infinite. If the WiFi hardware receives lots of data while the debugger has stopped the CPU, it is bound to crash. This will happen mostly when working with UDP and/or ICMP; TCP-connections in general will not send much more data when the other side doesn’t send any ACKs.

    • Solution: In such situations avoid pausing the debugger for extended periods

  • Software breakpoints/watchpoints (‘break’ and ‘watch’) don’t work on flash code
    • Cause: GDB handles these by replacing code with a debugging instruction, therefore the code must be in RAM.

    • Solution: Use hardware breakpoint (‘hbreak’) or use GDB_IRAM_ATTR for code which requires testing

  • If hardware breakpoint is set, single-stepping won’t work unless code is in RAM.
    • Cause: GDB reverts to software breakpoints if no hardware breakpoints are available

    • Solution: Delete hardware breakpoint before single-stepping

  • Crash occurs when setting breakpoint in HardwareTimer callback routine
    • Cause: By default, HardwareTimer uses Non-maskable Interrupts (NMI) which keep running when the debugger is paused

    • Solution: Use the timer in non-maskable mode, or enable GDBSTUB_PAUSE_HARDWARE_TIMER option

  • If gdbstub isn’t initialised then UART2 won’t work, though initialisation will succeed
    • Cause: By design, uart callbacks can be registered for UART2 at any time, before or after initialisation

    • Solution: Not really an issue, just something to be aware of

  • Error reported, “packet reply is too long”
    • Cause: Mismatch between GDB version and stub code

    • Solution: Set GDBSTUB_GDB_PATCHED =1 or use an unpatched version of GDB

  • Whilst GDB is attached, input cannot be passed to application
    • Cause: GDB buffers keystrokes and replays them only when the target is interrupted (e.g. via ctrl+C), rather than passing them via serial connection.

    • Solution: Application may use gdb_syscall interface to communicate with debugger. See $(SMING_HOME)/system/gdb_syscall.h, and Live Debug sample.

  • No apparent way to have second ‘console’ (windows terminology) separate from GDB interface
    • Cause: Unknown

    • Solution: Is this possible with remote targets?

  • GDB (in Windows) doesn’t respond at all to Ctrl+C
    • Cause: Unknown

    • Solution: Press Ctrl+Break to ‘hard kill’ GDB. You’ll probably need to do the next step as well to get it back

  • When GDB is running under windows, appears to hang when target reset or restarted
    • Cause: Unknown, may not happen on all devboards but presents with NodeMCU

    • Solution
      • quit GDB quit

      • Start terminal make terminal

      • reset board

      • quit terminal

      • run GDB again make gdb

  • Debug messages don’t appear in Eclipse
    • Cause: Unknown

    • Solution: Use command-line GDB, or a better visual debugger




When enabled, an exception or crash dumps a stack trace to debug output Default is ON for debug builds, OFF for release builds

Note: Not dependent upon ENABLE_GDB


When enabled, an unexpected reset (i.e. system crash) dumps a stack trace to debug output Default is ON for debug builds, OFF for release builds

Note: Not dependent upon ENABLE_GDB


When defined, GDB communications are echoed to UART1 for testing GDB stub operation.

0: No debug output 1: Show decoded commands and responses 2: Show packet content 3: Show debug output for internal routines


Espressif provide a patched version of GDB which emits only those registered present in the lx106. Set to 0 if an unpatched version of GDB is used.


Enable this to make the exception and debugging handlers switch to a private stack. This will use up 1K of RAM, but may be useful if you’re debugging stack or stack pointer corruption problems. It’s normally disabled because not many situations need it. If for some reason the GDB communication stops when you run into an error in your code, try enabling this.


Enable this to cause the program to pause and wait for gdb to be connected when an exception is encountered.


Enable this to cause the program to pause and wait for gdb to be connected when an unexpected system restart occurs.


If this is defined, gdbstub will break the program when you press Ctrl-C in gdb. It does this by monitoring for the ‘x03’ character in the serial receive routine. Any preceding characters are passed through to the application via UART2. If your application uses the serial port for terminal (text) communications you should be OK, but binary transfers are likely to cause problems and this option should probably be disabled. Instead, use GDBSTUB_BREAK_ON_INIT, or call gdb_do_break() in your application.

Specify: 0 to disable Ctrl+C break checking completely 1 to allow Ctrl+C break only when debugger is attached 2 to allow Ctrl+C break at any time. Ensure you have set remote interrupt-on-connect on in GDB command file, so it will send a Ctrl+C sequence when attempting to connect


The GDB stub has exclusive access to UART0, so applications cannot use it directly and attempts to open it will fail.

If this option is enabled, the default serial port will be changed to UART2 to allow debug output (from m_printf, debug_*, os_printf, etc.) to show up in your GDB session.

Outside of GDB terminal applications should work as normal, with the following caveats:

    If GDBSTUB_BREAK_ON_INIT is defined, then at startup your application will display `$T05#b9` and stop.
    A similar thing will happen if GDBSTUB_CTRLC_BREAK=2 and you type Ctrl+C.
    Continue by typing `$D#44` (without the quotes), or exit the terminal and start GDB.
See GDB remote serial protocol for further details.

Disabling this option releases some IRAM. You may be instead able to use UART1 for debug output, adding Serial.setPort(UART_ID_1); in your application’s init() function.


Enable gdb_syscall_* functions for use by application. If undefined, calls will do nothing and return -1.


Enable Host I/O capability, where files may be accessed via GDB command prompt using remote get, remote put and remote delete commands.


Enable this if you want the GDB stub to wait for you to attach GDB before running. It does this by breaking in the init routine; use the gdb ‘c’ command (continue) to start the program.


Some commands are not required by GDB, so if necessary can be disabled to save memory.


Specify a timeout (in milliseconds) when stub is reading from serial port. Set to 0 to wait indefinitely.


Wherever possible gdbstub code is placed in flash memory. This is fine for most cases, but if debugging whilst flash is disabled or busy (eg during SPI operations or flash write/erase) then you will need to enable this option to move stub code into IRAM.


Used by

Environment Variables

SoC support

  • esp8266