An application image consists of the following structures:
1. The :cpp:type:`esp_image_header_t` structure describes the mode of SPI flash and the count of memory segments.
2. The :cpp:type:`esp_image_segment_header_t` structure describes each segment, its length, and its location in ESP32's memory, followed by the data with a length of ``data_len``. The data offset for each segment in the image is calculated in the following way:
* offset for 0 Segment = sizeof(:cpp:type:`esp_image_header_t`) + sizeof(:cpp:type:`esp_image_segment_header_t`).
* offset for 1 Segment = offset for 0 Segment + length of 0 Segment + sizeof(:cpp:type:`esp_image_segment_header_t`).
* offset for 2 Segment = offset for 1 Segment + length of 1 Segment + sizeof(:cpp:type:`esp_image_segment_header_t`).
* ...
The count of each segment is defined in the ``segment_count`` field that is stored in :cpp:type:`esp_image_header_t`. The count cannot be more than :cpp:type:`ESP_IMAGE_MAX_SEGMENTS`.
To get the list of your image segments, please run the following command:
::
esptool.py --chip esp32 image_info build/app.bin
::
esptool.py v2.3.1
Image version: 1
Entry point: 40080ea4
13 segments
Segment 1: len 0x13ce0 load 0x3f400020 file_offs 0x00000018 SOC_DROM
Segment 2: len 0x00000 load 0x3ff80000 file_offs 0x00013d00 SOC_RTC_DRAM
Segment 3: len 0x00000 load 0x3ff80000 file_offs 0x00013d08 SOC_RTC_DRAM
Segment 4: len 0x028e0 load 0x3ffb0000 file_offs 0x00013d10 DRAM
Segment 5: len 0x00000 load 0x3ffb28e0 file_offs 0x000165f8 DRAM
Segment 6: len 0x00400 load 0x40080000 file_offs 0x00016600 SOC_IRAM
Segment 7: len 0x09600 load 0x40080400 file_offs 0x00016a08 SOC_IRAM
Segment 8: len 0x62e4c load 0x400d0018 file_offs 0x00020010 SOC_IROM
Segment 9: len 0x06cec load 0x40089a00 file_offs 0x00082e64 SOC_IROM
Segment 10: len 0x00000 load 0x400c0000 file_offs 0x00089b58 SOC_RTC_IRAM
Segment 11: len 0x00004 load 0x50000000 file_offs 0x00089b60 SOC_RTC_DATA
Segment 12: len 0x00000 load 0x50000004 file_offs 0x00089b6c SOC_RTC_DATA
Segment 13: len 0x00000 load 0x50000004 file_offs 0x00089b74 SOC_RTC_DATA
For more details on the type of memory segments and their address ranges, see the ESP32 Technical Reference Manual, Section 1.3.2 *Embedded Memory*.
3. The image has a single checksum byte after the last segment. This byte is written on a sixteen byte padded boundary, so the application image might need padding.
4. If the ``hash_appended`` field from :cpp:type:`esp_image_header_t` is set then a SHA256 checksum will be appended. The value of SHA256 is calculated on the range from first byte and up to this field. The length of this field is 32 bytes.
5. If the options :ref:`CONFIG_SECURE_SIGNED_APPS_SCHEME` is set to ECDSA then the application image will have additional 68 bytes for an ECDSA signature, which includes:
The ``DROM`` segment starts with the :cpp:type:`esp_app_desc_t` structure which carries specific fields describing the application:
*``secure_version`` - see :doc:`Anti-rollback</api-reference/system/ota>`.
*``version`` - see :doc:`App version</api-reference/system/system>`. ``*``
*``project_name`` is filled from ``PROJECT_NAME``. ``*``
*``time`` and ``date`` - compile time and date.
*``idf_ver`` - version of ESP-IDF. ``*``
*``app_elf_sha256`` - contains sha256 for the elf application file.
``*`` - The maximum length is 32 characters, including null-termination character. For example, if the length of ``PROJECT_NAME`` exceeds 32 characters, the excess characters will be disregarded.
This structure is useful for identification of images uploaded OTA because it has a fixed offset = sizeof(:cpp:type:`esp_image_header_t`) + sizeof(:cpp:type:`esp_image_segment_header_t`). As soon as a device receives the first fragment containing this structure, it has all the information to determine whether the update should be continued or not.
Adding a Custom Structure to an Application
-------------------------------------------
Customer also has the opportunity to have similar structure with a fixed offset relative to the beginning of the image.
The following pattern can be used to add a custom structure to your image:
Offset for custom structure is sizeof(:cpp:type:`esp_image_header_t`) + sizeof(:cpp:type:`esp_image_segment_header_t`) + sizeof(:cpp:type:`esp_app_desc_t`).
To guarantee that the custom structure is located in the image even if it is not used, you need to add:
* For Make: add ``COMPONENT_ADD_LDFLAGS += -u custom_app_desc`` into ``component.mk``
* For Cmake: add ``target_link_libraries(${COMPONENT_TARGET} "-u custom_app_desc")`` into ``CMakeLists.txt``
