App Image Format

An application image consists of the following structures:

  1. The esp_image_header_t structure describes the mode of SPI flash and the count of memory segments.

  2. The 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:

The count of each segment is defined in the segment_count field that is stored in esp_image_header_t. The count cannot be more than ESP_IMAGE_MAX_SEGMENTS.

To get the list of your image segments, please run the following command: --chip esp32 image_info build/app.bin 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
Checksum: e8 (valid)Validation Hash: 407089ca0eae2bbf83b4120979d3354b1c938a49cb7a0c997f240474ef2ec76b (valid)

You can also see the information on segments in the IDF logs while your application is booting:

I (443) esp_image: segment 0: paddr=0x00020020 vaddr=0x3f400020 size=0x13ce0 ( 81120) map
I (489) esp_image: segment 1: paddr=0x00033d08 vaddr=0x3ff80000 size=0x00000 ( 0) load
I (530) esp_image: segment 2: paddr=0x00033d10 vaddr=0x3ff80000 size=0x00000 ( 0) load
I (571) esp_image: segment 3: paddr=0x00033d18 vaddr=0x3ffb0000 size=0x028e0 ( 10464) load
I (612) esp_image: segment 4: paddr=0x00036600 vaddr=0x3ffb28e0 size=0x00000 ( 0) load
I (654) esp_image: segment 5: paddr=0x00036608 vaddr=0x40080000 size=0x00400 ( 1024) load
I (695) esp_image: segment 6: paddr=0x00036a10 vaddr=0x40080400 size=0x09600 ( 38400) load
I (737) esp_image: segment 7: paddr=0x00040018 vaddr=0x400d0018 size=0x62e4c (405068) map
I (847) esp_image: segment 8: paddr=0x000a2e6c vaddr=0x40089a00 size=0x06cec ( 27884) load
I (888) esp_image: segment 9: paddr=0x000a9b60 vaddr=0x400c0000 size=0x00000 ( 0) load
I (929) esp_image: segment 10: paddr=0x000a9b68 vaddr=0x50000000 size=0x00004 ( 4) load
I (971) esp_image: segment 11: paddr=0x000a9b74 vaddr=0x50000004 size=0x00000 ( 0) load
I (1012) esp_image: segment 12: paddr=0x000a9b7c vaddr=0x50000004 size=0x00000 ( 0) load

For more details on the type of memory segments and their address ranges, see ESP32 Technical Reference Manual > System and Memory > Embedded Memory [PDF].

  1. 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.

  2. If the hash_appended field from 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.

  3. If the options CONFIG_SECURE_SIGNED_APPS_SCHEME is set to ECDSA then the application image will have additional 68 bytes for an ECDSA signature, which includes:

  • version word (4 bytes),

  • signature data (64 bytes).

Application Description

The DROM segment starts with the esp_app_desc_t structure which carries specific fields describing the application:

  • secure_version - see Anti-rollback.

  • version - see App version. *

  • 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(esp_image_header_t) + sizeof(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:

const __attribute__((section(".rodata_custom_desc"))) esp_custom_app_desc_t custom_app_desc = { ... }

Offset for custom structure is sizeof(esp_image_header_t) + sizeof(esp_image_segment_header_t) + sizeof(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

  • For Cmake: add target_link_libraries(${COMPONENT_TARGET} "-u custom_app_desc") into CMakeLists.txt

API Reference


struct esp_image_header_t

Main header of binary image.

Public Members

uint8_t magic


uint8_t segment_count

Count of memory segments

uint8_t spi_mode

flash read mode (esp_image_spi_mode_t as uint8_t)

uint8_t spi_speed : 4

flash frequency (esp_image_spi_freq_t as uint8_t)

uint8_t spi_size : 4

flash chip size (esp_image_flash_size_t as uint8_t)

uint32_t entry_addr

Entry address

uint8_t wp_pin

WP pin when SPI pins set via efuse (read by ROM bootloader, the IDF bootloader uses software to configure the WP pin and sets this field to 0xEE=disabled)

uint8_t spi_pin_drv[3]

Drive settings for the SPI flash pins (read by ROM bootloader)

esp_chip_id_t chip_id

Chip identification number

uint8_t min_chip_rev

Minimum chip revision supported by image

uint8_t reserved[8]

Reserved bytes in additional header space, currently unused

uint8_t hash_appended

If 1, a SHA256 digest “simple hash” (of the entire image) is appended after the checksum. Included in image length. This digest is separate to secure boot and only used for detecting corruption. For secure boot signed images, the signature is appended after this (and the simple hash is included in the signed data).

struct esp_image_segment_header_t

Header of binary image segment.

Public Members

uint32_t load_addr

Address of segment

uint32_t data_len

Length of data

struct esp_app_desc_t

Description about application.

Public Members

uint32_t magic_word


uint32_t secure_version

Secure version

uint32_t reserv1[2]


char version[32]

Application version

char project_name[32]

Project name

char time[16]

Compile time

char date[16]

Compile date

char idf_ver[32]

Version IDF

uint8_t app_elf_sha256[32]

sha256 of elf file

uint32_t reserv2[20]




The magic word for the esp_image_header_t structure.


Max count of segments in the image.


The magic word for the esp_app_desc structure that is in DROM.


enum esp_chip_id_t

ESP chip ID.


ESP_CHIP_ID_ESP32 = 0x0000

chip ID: ESP32

ESP_CHIP_ID_ESP32S2 = 0x0002

chip ID: ESP32-S2

ESP_CHIP_ID_ESP32S3 = 0x0004

chip ID: ESP32-S3

ESP_CHIP_ID_ESP32C3 = 0x0005

chip ID: ESP32-C3


Invalid chip ID (we defined it to make sure the esp_chip_id_t is 2 bytes size)

enum esp_image_spi_mode_t

SPI flash mode, used in esp_image_header_t.



SPI mode QIO




SPI mode DIO







enum esp_image_spi_freq_t

SPI flash clock frequency.



SPI clock frequency 40 MHz


SPI clock frequency 26 MHz


SPI clock frequency 20 MHz


SPI clock frequency 80 MHz

enum esp_image_flash_size_t

Supported SPI flash sizes.



SPI flash size 1 MB


SPI flash size 2 MB


SPI flash size 4 MB


SPI flash size 8 MB


SPI flash size 16 MB


SPI flash size MAX