A core dump is a set of software state information that is automatically saved by the panic handler when a fatal error occurs. Core dumps are useful for conducting post-mortem analysis of the software's state at the moment of failure. ESP-IDF provides support for generating core dumps.
A core dump contains snapshots of all tasks in the system at the moment of failure, where each snapshot includes a task's control block (TCB) and stack. By analyzing the task snapshots, it is possible to find out what task, at what instruction (line of code), and what call stack of that task lead to the crash. It is also possible to dump the contents of variables on demand, provided those variables are assigned special core dump attributes.
Core dump data is saved to a core dump file according to a particular format, see :doc:`Core dump internals <core_dump_internals>` for more details. However, ESP-IDF's ``idf.py`` command provides special subcommands to decode and analyze the core dump file.
The :ref:`CONFIG_ESP_COREDUMP_TO_FLASH_OR_UART` option enables or disables core dump, and selects the core dump destination if enabled. When a crash occurs, the generated core dump file can either be saved to flash, or output to a connected host over UART.
The ELF format contains extended features and allows more information regarding erroneous tasks and crashed software to be saved. However, using the ELF format causes the core dump file to be larger. This format is recommended for new software designs and is flexible enough to be extended in future revisions to save more information.
Core dump files include a checksum, which can be used to verify the integrity of the core dump file, i.e., the file has not been corrupted. The :ref:`CONFIG_ESP_COREDUMP_CHECKSUM` option controls the type of checksum, namely CRC32 or SHA256 (only supported in the ELF format).
Core dump routines run from a separate stack due to core dump itself needing to parse and save all other task stacks. The :ref:`CONFIG_ESP_COREDUMP_STACK_SIZE` option controls the size of the core dump's stack in number of bytes.
Setting this option to 0 bytes will cause the core dump routines to run from the ISR stack, thus saving a bit of memory. Setting the option greater than zero will cause a separate stack to be instantiated.
If a separate stack is used, the recommended stack size should be larger than 800 bytes to ensure that the core dump routines themselves do not cause a stack overflow.
When the core dump file is saved to flash, the file is saved to a special core dump partition in flash. Specifying the core dump partition will reserve space on the flash chip to store the core dump file.
The core dump partition is automatically declared when using the default partition table provided by ESP-IDF. However, when using a custom partition table, you need to declare the core dump partition, as illustrated below:
There are no special requirements for the partition name. It can be chosen according to the application's needs, but the partition type should be ``data`` and the sub-type should be ``coredump``. Also, when choosing partition size, note that the core dump file introduces a constant overhead of 20 bytes and a per-task overhead of 12 bytes. This overhead does not include the size of TCB and stack for every task. So the partition size should be at least ``20 + max tasks number x (12 + TCB size + max task stack size)`` bytes.
When the core dump file is output to UART, the output file is Base64-encoded. The :ref:`CONFIG_ESP_COREDUMP_DECODE` option allows for selecting whether the output file is automatically decoded by the ESP-IDF monitor or kept encoded for manual decoding.
If :ref:`CONFIG_ESP_COREDUMP_DECODE` is set to automatically decode the UART core dump, ESP-IDF monitor will automatically decode the data, translate any function addresses to source code lines, and display it in the monitor. The output to ESP-IDF monitor would resemble the following output:
The :ref:`CONFIG_ESP_COREDUMP_UART_DELAY` allows for an optional delay to be added before the core dump file is output to UART.
================== CURRENT THREAD REGISTERS ===================
exccause 0x1d (StoreProhibitedCause)
excvaddr 0x0
epc1 0x40027657
epc2 0x0
...
==================== CURRENT THREAD STACK =====================
#0 0x400251cd in panic_abort (details=0x3ffc553b "abort() was called at PC 0x40087b84 on core 0") at /home/User/esp/esp-idf/components/esp_system/panic.c:452
#1 0x40028970 in esp_system_abort (details=0x3ffc553b "abort() was called at PC 0x40087b84 on core 0") at /home/User/esp/esp-idf/components/esp_system/port/esp_system_chip.c:93
...
======================== THREADS INFO =========================
Id Target Id Frame
* 1 process 1073501760 0x400251cd in panic_abort (details=0x3ffc553b "abort() was called at PC 0x40087b84 on core 0") at /home/User/esp/esp-idf/components/esp_system/panic.c:452
2 process 1073503644 vPortTaskWrapper (pxCode=0x0, pvParameters=0x0) at /home/User/esp/esp-idf/components/freertos/FreeRTOS-Kernel/portable/xtensa/port.c:161
#0 0x400251cd in panic_abort (details=0x3ffc553b "abort() was called at PC 0x40087b84 on core 0") at /home/User/esp/esp-idf/components/esp_system/panic.c:452
#1 0x40028970 in esp_system_abort (details=0x3ffc553b "abort() was called at PC 0x40087b84 on core 0") at /home/User/esp/esp-idf/components/esp_system/port/esp_system_chip.c:93
If you set :ref:`CONFIG_ESP_COREDUMP_DECODE` to no decoding, then the raw Base64-encoded body of core dump is output to UART between the following header and footer of the UART output:
<body of Base64-encoded core dump, save it to file on disk>
================= CORE DUMP END ===================
It is advised to manually save the core dump text body to a file. The ``CORE DUMP START`` and ``CORE DUMP END`` lines must not be included in a core dump text file. The saved text can the be decoded using the following command:
ESP-IDF provides special commands to help to retrieve and analyze core dumps:
*``idf.py coredump-info`` - prints crashed task's registers, call stack, list of available tasks in the system, memory regions, and contents of memory stored in core dump (TCBs and stacks).
*``idf.py coredump-debug`` - creates core dump ELF file and runs GDB debug session with this file. You can examine memory, variables, and task states manually. Note that since not all memory is saved in the core dump, only the values of variables allocated on the stack are meaningful.
It is a possible that at the moment of a crash, some tasks and/or the crashed task itself have one or more ROM functions in their call stacks. Since ROM is not part of the program ELF, it is impossible for GDB to parse such call stacks due to GDB analyzing functions' prologues to decode backtraces. Thus, call stack parsing will break with an error message upon the first ROM function that is encountered.
To overcome this issue, the `ROM ELF <https://github.com/espressif/esp-rom-elfs/releases>`_ provided by Espressif is loaded automatically by ESP-IDF monitor based on the target and its revision. More details about ROM ELFs can be found in `esp-rom-elfs <https://github.com/espressif/esp-rom-elfs/blob/master/README.md>`_.
Sometimes you want to read the last value of a variable to understand the root cause of a crash. Core dump supports retrieving variable data over GDB by applying special attributes to declared variables.
*``COREDUMP_DRAM_ATTR`` places the variable into the DRAM area, which is included in the dump.
:SOC_RTC_FAST_MEM_SUPPORTED or SOC_RTC_SLOW_MEM_SUPPORTED:* ``COREDUMP_RTC_ATTR`` places the variable into the RTC area, which is included in the dump.
:SOC_RTC_FAST_MEM_SUPPORTED:* ``COREDUMP_RTC_FAST_ATTR`` places the variable into the RTC_FAST area, which is included in the dump.
