2. The :cpp:type:`esp_image_segment_header_t` structure describes each segment, its length, and its location in {IDF_TARGET_NAME}'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:
For more details on the type of memory segments and their address ranges, see the {IDF_TARGET_NAME} 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``
