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:
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 :c:macro:`ESP_IMAGE_MAX_SEGMENTS`.
For more details on the type of memory segments and their address ranges, see **{IDF_TARGET_NAME} Technical Reference Manual** > **System and Memory** > **Embedded Memory** [`PDF <{IDF_TARGET_TRM_EN_URL}#sysmem>`__].
For more details on the type of memory segments and their address ranges, see **{IDF_TARGET_NAME} Technical Reference Manual** > **System and Memory** > **Internal Memory** [`PDF <{IDF_TARGET_TRM_EN_URL}#sysmem>`__].
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 the SHA256 hash is calculated on the range from the first byte and up to this field. The length of this field is 32 bytes.
5. If the option :ref:`CONFIG_SECURE_SIGNED_APPS_SCHEME` is set to ECDSA then the application image will have an additional 68 bytes for an ECDSA signature, which includes:
6. If the option :ref:`CONFIG_SECURE_SIGNED_APPS_SCHEME` is set to RSA or ECDSA (V2) then the application image will have an additional signature sector of 4 KB in size. For more details on the format of this signature sector, please refer to :ref:`signature-block-format`.
The ``DROM`` segment of the application binary starts with the :cpp:type:`esp_app_desc_t` structure which carries specific fields describing the application:
..[#f1] The maximum length is 32 characters, including null-termination character. For example, if the length of ``PROJECT_NAME`` exceeds 31 characters, the excess characters will be disregarded.
This structure is useful for identification of images uploaded via Over-the-Air (OTA) updates 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 with or not.
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 ``target_link_libraries(${COMPONENT_TARGET} "-u custom_app_desc")`` into ``CMakeLists.txt``.
