Non-volatile storage library

[中文]

Introduction

Non-volatile storage (NVS) library is designed to store key-value pairs in flash. This section introduces some concepts used by NVS.

Underlying storage

Currently, NVS uses a portion of main flash memory through spi_flash_{read|write|erase} APIs. The library uses all the partitions with data type and nvs subtype. The application can choose to use the partition with the label nvs through the nvs_open API function or any other partition by specifying its name using the nvs_open_from_part API function.

Future versions of this library may have other storage backends to keep data in another flash chip (SPI or I2C), RTC, FRAM, etc.

Note

if an NVS partition is truncated (for example, when the partition table layout is changed), its contents should be erased. ESP-IDF build system provides a idf.py erase_flash target to erase all contents of the flash chip.

Note

NVS works best for storing many small values, rather than a few large values of the type ‘string’ and ‘blob’. If you need to store large blobs or strings, consider using the facilities provided by the FAT filesystem on top of the wear levelling library.

Keys and values

NVS operates on key-value pairs. Keys are ASCII strings; the maximum key length is currently 15 characters. Values can have one of the following types:

  • integer types: uint8_t, int8_t, uint16_t, int16_t, uint32_t, int32_t, uint64_t, int64_t
  • zero-terminated string
  • variable length binary data (blob)

Note

String values are currently limited to 4000 bytes. This includes the null terminator. Blob values are limited to 508000 bytes or 97.6% of the partition size - 4000 bytes, whichever is lower.

Additional types, such as float and double might be added later.

Keys are required to be unique. Assigning a new value to an existing key works as follows:

  • if the new value is of the same type as the old one, value is updated
  • if the new value has a different data type, an error is returned

Data type check is also performed when reading a value. An error is returned if the data type of the read operation does not match the data type of the value.

Namespaces

To mitigate potential conflicts in key names between different components, NVS assigns each key-value pair to one of namespaces. Namespace names follow the same rules as key names, i.e., the maximum length is 15 characters. Namespace name is specified in the nvs_open or nvs_open_from_part call. This call returns an opaque handle, which is used in subsequent calls to the nvs_get_*, nvs_set_*, and nvs_commit functions. This way, a handle is associated with a namespace, and key names will not collide with same names in other namespaces. Please note that the namespaces with the same name in different NVS partitions are considered as separate namespaces.

Security, tampering, and robustness

NVS is not directly compatible with the ESP32 flash encryption system. However, data can still be stored in encrypted form if NVS encryption is used together with ESP32 flash encryption. Please refer to NVS Encryption for more details.

If NVS encryption is not used, it is possible for anyone with physical access to the flash chip to alter, erase, or add key-value pairs. With NVS encryption enabled, it is not possible to alter or add a key-value pair and get recognized as a valid pair without knowing corresponding NVS encryption keys. However, there is no tamper-resistance against the erase operation.

The library does try to recover from conditions when flash memory is in an inconsistent state. In particular, one should be able to power off the device at any point and time and then power it back on. This should not result in loss of data, except for the new key-value pair if it was being written at the moment of powering off. The library should also be able to initialize properly with any random data present in flash memory.

Internals

Log of key-value pairs

NVS stores key-value pairs sequentially, with new key-value pairs being added at the end. When a value of any given key has to be updated, a new key-value pair is added at the end of the log and the old key-value pair is marked as erased.

Pages and entries

NVS library uses two main entities in its operation: pages and entries. Page is a logical structure which stores a portion of the overall log. Logical page corresponds to one physical sector of flash memory. Pages which are in use have a sequence number associated with them. Sequence numbers impose an ordering on pages. Higher sequence numbers correspond to pages which were created later. Each page can be in one of the following states:

Empty/uninitialized
Flash storage for the page is empty (all bytes are 0xff). Page is not used to store any data at this point and does not have a sequence number.
Active
Flash storage is initialized, page header has been written to flash, page has a valid sequence number. Page has some empty entries and data can be written there. No more than one page can be in this state at any given moment.
Full
Flash storage is in a consistent state and is filled with key-value pairs. Writing new key-value pairs into this page is not possible. It is still possible to mark some key-value pairs as erased.
Erasing
Non-erased key-value pairs are being moved into another page so that the current page can be erased. This is a transient state, i.e., page should never stay in this state at the time when any API call returns. In case of a sudden power off, the move-and-erase process will be completed upon the next power-on.
Corrupted
Page header contains invalid data, and further parsing of page data was canceled. Any items previously written into this page will not be accessible. The corresponding flash sector will not be erased immediately and will be kept along with sectors in uninitialized state for later use. This may be useful for debugging.

Mapping from flash sectors to logical pages does not have any particular order. The library will inspect sequence numbers of pages found in each flash sector and organize pages in a list based on these numbers.

+--------+     +--------+     +--------+     +--------+
| Page 1 |     | Page 2 |     | Page 3 |     | Page 4 |
| Full   +---> | Full   +---> | Active |     | Empty  |   <- states
| #11    |     | #12    |     | #14    |     |        |   <- sequence numbers
+---+----+     +----+---+     +----+---+     +---+----+
    |               |              |             |
    |               |              |             |
    |               |              |             |
+---v------+  +-----v----+  +------v---+  +------v---+
| Sector 3 |  | Sector 0 |  | Sector 2 |  | Sector 1 |    <- physical sectors
+----------+  +----------+  +----------+  +----------+

Structure of a page

For now, we assume that flash sector size is 4096 bytes and that ESP32 flash encryption hardware operates on 32-byte blocks. It is possible to introduce some settings configurable at compile-time (e.g., via menuconfig) to accommodate flash chips with different sector sizes (although it is not clear if other components in the system, e.g., SPI flash driver and SPI flash cache can support these other sizes).

Page consists of three parts: header, entry state bitmap, and entries themselves. To be compatible with ESP32 flash encryption, entry size is 32 bytes. For integer types, entry holds one key-value pair. For strings and blobs, an entry holds part of key-value pair (more on that in the entry structure description).

The following diagram illustrates the page structure. Numbers in parentheses indicate the size of each part in bytes.

+-----------+--------------+-------------+-------------------------+
| State (4) | Seq. no. (4) | version (1) | Unused (19) | CRC32 (4) |   Header (32)
+-----------+--------------+-------------+-------------------------+
|                Entry state bitmap (32)                           |
+------------------------------------------------------------------+
|                       Entry 0 (32)                               |
+------------------------------------------------------------------+
|                       Entry 1 (32)                               |
+------------------------------------------------------------------+
/                                                                  /
/                                                                  /
+------------------------------------------------------------------+
|                       Entry 125 (32)                             |
+------------------------------------------------------------------+

Page header and entry state bitmap are always written to flash unencrypted. Entries are encrypted if flash encryption feature of ESP32 is used.

Page state values are defined in such a way that changing state is possible by writing 0 into some of the bits. Therefore it is not necessary to erase the page to change its state unless that is a change to the erased state.

The version field in the header reflects the NVS format version used. For backward compatibility reasons, it is decremented for every version upgrade starting at 0xff (i.e., 0xff for version-1, 0xfe for version-2 and so on).

CRC32 value in the header is calculated over the part which does not include a state value (bytes 4 to 28). The unused part is currently filled with 0xff bytes.

The following sections describe the structure of entry state bitmap and entry itself.

Entry and entry state bitmap

Each entry can be in one of the following three states represented with two bits in the entry state bitmap. The final four bits in the bitmap (256 - 2 * 126) are not used.

Empty (2’b11)
Nothing is written into the specific entry yet. It is in an uninitialized state (all bytes are 0xff).
Written (2’b10)
A key-value pair (or part of key-value pair which spans multiple entries) has been written into the entry.
Erased (2’b00)
A key-value pair in this entry has been discarded. Contents of this entry will not be parsed anymore.

Structure of entry

For values of primitive types (currently integers from 1 to 8 bytes long), entry holds one key-value pair. For string and blob types, entry holds part of the whole key-value pair. For strings, in case when a key-value pair spans multiple entries, all entries are stored in the same page. Blobs are allowed to span over multiple pages by dividing them into smaller chunks. For tracking these chunks, an additional fixed length metadata entry is stored called “blob index”. Earlier formats of blobs are still supported (can be read and modified). However, once the blobs are modified, they are stored using the new format.

+--------+----------+----------+----------------+-----------+---------------+----------+
| NS (1) | Type (1) | Span (1) | ChunkIndex (1) | CRC32 (4) |    Key (16)   | Data (8) |
+--------+----------+----------+----------------+-----------+---------------+----------+

                                         Primitive  +--------------------------------+
                                        +-------->  |     Data (8)                   |
                                        | Types     +--------------------------------+
                   +-> Fixed length --
                   |                    |           +---------+--------------+---------------+-------+
                   |                    +-------->  | Size(4) | ChunkCount(1)| ChunkStart(1) | Rsv(2)|
    Data format ---+                    Blob Index  +---------+--------------+---------------+-------+
                   |
                   |                             +----------+---------+-----------+
                   +->   Variable length   -->   | Size (2) | Rsv (2) | CRC32 (4) |
                        (Strings, Blob Data)     +----------+---------+-----------+

Individual fields in entry structure have the following meanings:

NS
Namespace index for this entry. For more information on this value, see the section on namespaces implementation.
Type
One byte indicating the value data type. See the ItemType enumeration in nvs_types.h for possible values.
Span
Number of entries used by this key-value pair. For integer types, this is equal to 1. For strings and blobs, this depends on value length.
ChunkIndex
Used to store the index of a blob-data chunk for blob types. For other types, this should be 0xff.
CRC32
Checksum calculated over all the bytes in this entry, except for the CRC32 field itself.
Key
Zero-terminated ASCII string containing a key name. Maximum string length is 15 bytes, excluding a zero terminator.
Data

For integer types, this field contains the value itself. If the value itself is shorter than 8 bytes, it is padded to the right, with unused bytes filled with 0xff.

For “blob index” entry, these 8 bytes hold the following information about data-chunks:

  • Size
    (Only for blob index.) Size, in bytes, of complete blob data.
  • ChunkCount
    (Only for blob index.) Total number of blob-data chunks into which the blob was divided during storage.
  • ChunkStart
    (Only for blob index.) ChunkIndex of the first blob-data chunk of this blob. Subsequent chunks have chunkIndex incrementally allocated (step of 1).

For string and blob data chunks, these 8 bytes hold additional data about the value, which are described below:

  • Size
    (Only for strings and blobs.) Size, in bytes, of actual data. For strings, this includes zero terminators.
  • CRC32
    (Only for strings and blobs.) Checksum calculated over all bytes of data.

Variable length values (strings and blobs) are written into subsequent entries, 32 bytes per entry. The Span field of the first entry indicates how many entries are used.

Namespaces

As mentioned above, each key-value pair belongs to one of the namespaces. Namespace identifiers (strings) are stored as keys of key-value pairs in namespace with index 0. Values corresponding to these keys are indexes of these namespaces.

+-------------------------------------------+
| NS=0 Type=uint8_t Key="wifi" Value=1      |   Entry describing namespace "wifi"
+-------------------------------------------+
| NS=1 Type=uint32_t Key="channel" Value=6  |   Key "channel" in namespace "wifi"
+-------------------------------------------+
| NS=0 Type=uint8_t Key="pwm" Value=2       |   Entry describing namespace "pwm"
+-------------------------------------------+
| NS=2 Type=uint16_t Key="channel" Value=20 |   Key "channel" in namespace "pwm"
+-------------------------------------------+

Item hash list

To reduce the number of reads from flash memory, each member of the Page class maintains a list of pairs: item index; item hash. This list makes searches much quicker. Instead of iterating over all entries, reading them from flash one at a time, Page::findItem first performs a search for the item hash in the hash list. This gives the item index within the page if such an item exists. Due to a hash collision, it is possible that a different item will be found. This is handled by falling back to iteration over items in flash.

Each node in the hash list contains a 24-bit hash and 8-bit item index. Hash is calculated based on item namespace, key name, and ChunkIndex. CRC32 is used for calculation; the result is truncated to 24 bits. To reduce the overhead for storing 32-bit entries in a linked list, the list is implemented as a double-linked list of arrays. Each array holds 29 entries, for the total size of 128 bytes, together with linked list pointers and a 32-bit count field. The minimum amount of extra RAM usage per page is therefore 128 bytes; maximum is 640 bytes.

NVS Encryption

Data stored in NVS partitions can be encrypted using AES-XTS in the manner similar to the one mentioned in disk encryption standard IEEE P1619. For the purpose of encryption, each entry is treated as one sector and relative address of the entry (w.r.t. partition-start) is fed to the encryption algorithm as sector-number. The keys required for NVS encryption are stored in yet another partition, which is protected using Flash Encryption. Therefore, enabling Flash Encryption is a prerequisite for NVS encryption.

NVS key partition

An application requiring NVS encryption support needs to be compiled with a key-partition of the type data and subtype key. This partition should be marked as encrypted. Refer to Partition Tables for more details. The size of the partition should be 4096 bytes (minimum partition size). The structure of this partition is depicted below.

+-----------+--------------+-------------+----+
|              XTS encryption key(32)         |
+---------------------------------------------+
|              XTS tweak key (32)             |
+---------------------------------------------+
|                  CRC32(4)                   |
+---------------------------------------------+

This partition can be generated using nvs partition generator utility and flashed onto the device. Since the partition is marked encrypted and Flash Encryption is enabled, bootloader will encrypt this partition using flash encryption key on the first boot. Alternatively, the keys can be generated after startup using the nvs_flash_generate_keys API function provided by nvs_flash.h, which will then write those keys onto the key-partition in encrypted form.

It is possible for an application to use different keys for different NVS partitions and thereby have multiple key-partitions. However, it is a responsibility of the application to provide correct key-partition/keys for the purpose of encryption/decryption.

Encrypted Read/Write

The same NVS API functions nvs_get_* or nvs_set_* can be used for reading of, and writing to an encrypted nvs partition as well. However, the API functions for initialising NVS partitions are different: nvs_flash_secure_init and nvs_flash_secure_init_partition instead of nvs_flash_init and nvs_flash_init_partition respectively. The nvs_sec_cfg_t structure required for these API functions can be populated using nvs_flash_generate_keys or nvs_flash_read_security_cfg.

Applications are expected to follow the steps below in order to perform NVS read/write operations with encryption enabled.

  1. Find key partition and NVS data partition using esp_partition_find* API functions.
  2. Populate the nvs_sec_cfg_t struct using the nvs_flash_read_security_cfg or nvs_flash_generate_keys API functions.
  3. Initialise NVS flash partition using the nvs_flash_secure_init or nvs_flash_secure_init_partition API functions.
  4. Open a namespace using the nvs_open or nvs_open_from_part API functions.
  5. Perform NVS read/write operations using nvs_get_* or nvs_set_*.
  6. Deinitialise an NVS partition using nvs_flash_deinit.

NVS iterators

Iterators allow to list key-value pairs stored in NVS, based on specified partition name, namespace, and data type.

There are the following functions available:

  • nvs_entry_find returns an opaque handle, which is used in subsequent calls to the nvs_entry_next and nvs_entry_info functions.
  • nvs_entry_next returns iterator to the next key-value pair.
  • nvs_entry_info returns information about each key-value pair

If none or no other key-value pair was found for given criteria, nvs_entry_find and nvs_entry_next return NULL. In that case, the iterator does not have to be released. If the iterator is no longer needed, you can release it by using the function nvs_release_iterator.

NVS Partition Generator Utility

This utility helps generate NVS partition binary files which can be flashed separately on a dedicated partition via a flashing utility. Key-value pairs to be flashed onto the partition can be provided via a CSV file. For more details, please refer to NVS Partition Generator Utility.

Application Example

You can find two code examples in the storage directory of ESP-IDF examples:

storage/nvs_rw_value

Demonstrates how to read a single integer value from, and write it to NVS.

The value checked in this example holds the number of the ESP32 module restarts. The value’s function as a counter is only possible due to its storing in NVS.

The example also shows how to check if a read / write operation was successful, or if a certain value has not been initialized in NVS. The diagnostic procedure is provided in plain text to help you track the program flow and capture any issues on the way.

storage/nvs_rw_blob

Demonstrates how to read a single integer value and a blob (binary large object), and write them to NVS to preserve this value between ESP32 module restarts.

  • value - tracks the number of the ESP32 module soft and hard restarts.
  • blob - contains a table with module run times. The table is read from NVS to dynamically allocated RAM. A new run time is added to the table on each manually triggered soft restart, and then the added run time is written to NVS. Triggering is done by pulling down GPIO0.

The example also shows how to implement the diagnostic procedure to check if the read / write operation was successful.

API Reference

Functions

esp_err_t nvs_flash_init(void)

Initialize the default NVS partition.

This API initialises the default NVS partition. The default NVS partition is the one that is labeled “nvs” in the partition table.

Return
  • ESP_OK if storage was successfully initialized.
  • ESP_ERR_NVS_NO_FREE_PAGES if the NVS storage contains no empty pages (which may happen if NVS partition was truncated)
  • ESP_ERR_NOT_FOUND if no partition with label “nvs” is found in the partition table
  • one of the error codes from the underlying flash storage driver

esp_err_t nvs_flash_init_partition(const char *partition_label)

Initialize NVS flash storage for the specified partition.

Return
  • ESP_OK if storage was successfully initialized.
  • ESP_ERR_NVS_NO_FREE_PAGES if the NVS storage contains no empty pages (which may happen if NVS partition was truncated)
  • ESP_ERR_NOT_FOUND if specified partition is not found in the partition table
  • one of the error codes from the underlying flash storage driver
Parameters
  • partition_label: Label of the partition. Must be no longer than 16 characters.

esp_err_t nvs_flash_deinit(void)

Deinitialize NVS storage for the default NVS partition.

Default NVS partition is the partition with “nvs” label in the partition table.

Return
  • ESP_OK on success (storage was deinitialized)
  • ESP_ERR_NVS_NOT_INITIALIZED if the storage was not initialized prior to this call

esp_err_t nvs_flash_deinit_partition(const char *partition_label)

Deinitialize NVS storage for the given NVS partition.

Return
  • ESP_OK on success
  • ESP_ERR_NVS_NOT_INITIALIZED if the storage for given partition was not initialized prior to this call
Parameters
  • partition_label: Label of the partition

esp_err_t nvs_flash_erase(void)

Erase the default NVS partition.

This function erases all contents of the default NVS partition (one with label “nvs”)

Return
  • ESP_OK on success
  • ESP_ERR_NOT_FOUND if there is no NVS partition labeled “nvs” in the partition table

esp_err_t nvs_flash_erase_partition(const char *part_name)

Erase specified NVS partition.

This function erases all contents of specified NVS partition

Return
  • ESP_OK on success
  • ESP_ERR_NOT_FOUND if there is no NVS partition with the specified name in the partition table
Parameters
  • part_name: Name (label) of the partition to be erased

esp_err_t nvs_flash_secure_init(nvs_sec_cfg_t *cfg)

Initialize the default NVS partition.

This API initialises the default NVS partition. The default NVS partition is the one that is labeled “nvs” in the partition table.

Return
  • ESP_OK if storage was successfully initialized.
  • ESP_ERR_NVS_NO_FREE_PAGES if the NVS storage contains no empty pages (which may happen if NVS partition was truncated)
  • ESP_ERR_NOT_FOUND if no partition with label “nvs” is found in the partition table
  • one of the error codes from the underlying flash storage driver
Parameters
  • cfg: Security configuration (keys) to be used for NVS encryption/decryption. If cfg is NULL, no encryption is used.

esp_err_t nvs_flash_secure_init_partition(const char *partition_label, nvs_sec_cfg_t *cfg)

Initialize NVS flash storage for the specified partition.

Return
  • ESP_OK if storage was successfully initialized.
  • ESP_ERR_NVS_NO_FREE_PAGES if the NVS storage contains no empty pages (which may happen if NVS partition was truncated)
  • ESP_ERR_NOT_FOUND if specified partition is not found in the partition table
  • one of the error codes from the underlying flash storage driver
Parameters
  • partition_label: Label of the partition. Note that internally a reference to passed value is kept and it should be accessible for future operations
  • cfg: Security configuration (keys) to be used for NVS encryption/decryption. If cfg is null, no encryption/decryption is used.

esp_err_t nvs_flash_generate_keys(const esp_partition_t *partition, nvs_sec_cfg_t *cfg)

Generate and store NVS keys in the provided esp partition.

Return
-ESP_OK, if cfg was read successfully; -or error codes from esp_partition_write/erase APIs.
Parameters
  • partition: Pointer to partition structure obtained using esp_partition_find_first or esp_partition_get. Must be non-NULL.
  • cfg: Pointer to nvs security configuration structure. Pointer must be non-NULL. Generated keys will be populated in this structure.

esp_err_t nvs_flash_read_security_cfg(const esp_partition_t *partition, nvs_sec_cfg_t *cfg)

Read NVS security configuration from a partition.

Note
Provided parition is assumed to be marked ‘encrypted’.
Return
-ESP_OK, if cfg was read successfully; -ESP_ERR_NVS_KEYS_NOT_INITIALIZED, if the partition is not yet written with keys. -ESP_ERR_NVS_CORRUPT_KEY_PART, if the partition containing keys is found to be corrupt -or error codes from esp_partition_read API.
Parameters
  • partition: Pointer to partition structure obtained using esp_partition_find_first or esp_partition_get. Must be non-NULL.
  • cfg: Pointer to nvs security configuration structure. Pointer must be non-NULL.

Structures

struct nvs_sec_cfg_t

Key for encryption and decryption.

Public Members

uint8_t eky[NVS_KEY_SIZE]

XTS encryption and decryption key

uint8_t tky[NVS_KEY_SIZE]

XTS tweak key

Macros

NVS_KEY_SIZE

Functions

esp_err_t nvs_set_i8(nvs_handle_t handle, const char *key, int8_t value)

set value for given key

This family of functions set value for the key, given its name. Note that actual storage will not be updated until nvs_commit function is called.

Return
  • ESP_OK if value was set successfully
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • ESP_ERR_NVS_READ_ONLY if storage handle was opened as read only
  • ESP_ERR_NVS_INVALID_NAME if key name doesn’t satisfy constraints
  • ESP_ERR_NVS_NOT_ENOUGH_SPACE if there is not enough space in the underlying storage to save the value
  • ESP_ERR_NVS_REMOVE_FAILED if the value wasn’t updated because flash write operation has failed. The value was written however, and update will be finished after re-initialization of nvs, provided that flash operation doesn’t fail again.
  • ESP_ERR_NVS_VALUE_TOO_LONG if the string value is too long
Parameters
  • handle: Handle obtained from nvs_open function. Handles that were opened read only cannot be used.
  • key: Key name. Maximal length is determined by the underlying implementation, but is guaranteed to be at least 15 characters. Shouldn’t be empty.
  • value: The value to set. For strings, the maximum length (including null character) is 4000 bytes.

esp_err_t nvs_set_u8(nvs_handle_t handle, const char *key, uint8_t value)
esp_err_t nvs_set_i16(nvs_handle_t handle, const char *key, int16_t value)
esp_err_t nvs_set_u16(nvs_handle_t handle, const char *key, uint16_t value)
esp_err_t nvs_set_i32(nvs_handle_t handle, const char *key, int32_t value)
esp_err_t nvs_set_u32(nvs_handle_t handle, const char *key, uint32_t value)
esp_err_t nvs_set_i64(nvs_handle_t handle, const char *key, int64_t value)
esp_err_t nvs_set_u64(nvs_handle_t handle, const char *key, uint64_t value)
esp_err_t nvs_set_str(nvs_handle_t handle, const char *key, const char *value)
esp_err_t nvs_get_i8(nvs_handle_t handle, const char *key, int8_t *out_value)

get value for given key

These functions retrieve value for the key, given its name. If key does not exist, or the requested variable type doesn’t match the type which was used when setting a value, an error is returned.

In case of any error, out_value is not modified.

All functions expect out_value to be a pointer to an already allocated variable of the given type.

// Example of using nvs_get_i32:
int32_t max_buffer_size = 4096; // default value
esp_err_t err = nvs_get_i32(my_handle, "max_buffer_size", &max_buffer_size);
assert(err == ESP_OK || err == ESP_ERR_NVS_NOT_FOUND);
// if ESP_ERR_NVS_NOT_FOUND was returned, max_buffer_size will still
// have its default value.

Return
  • ESP_OK if the value was retrieved successfully
  • ESP_ERR_NVS_NOT_FOUND if the requested key doesn’t exist
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • ESP_ERR_NVS_INVALID_NAME if key name doesn’t satisfy constraints
  • ESP_ERR_NVS_INVALID_LENGTH if length is not sufficient to store data
Parameters
  • handle: Handle obtained from nvs_open function.
  • key: Key name. Maximal length is determined by the underlying implementation, but is guaranteed to be at least 15 characters. Shouldn’t be empty.
  • out_value: Pointer to the output value. May be NULL for nvs_get_str and nvs_get_blob, in this case required length will be returned in length argument.

esp_err_t nvs_get_u8(nvs_handle_t handle, const char *key, uint8_t *out_value)
esp_err_t nvs_get_i16(nvs_handle_t handle, const char *key, int16_t *out_value)
esp_err_t nvs_get_u16(nvs_handle_t handle, const char *key, uint16_t *out_value)
esp_err_t nvs_get_i32(nvs_handle_t handle, const char *key, int32_t *out_value)
esp_err_t nvs_get_u32(nvs_handle_t handle, const char *key, uint32_t *out_value)
esp_err_t nvs_get_i64(nvs_handle_t handle, const char *key, int64_t *out_value)
esp_err_t nvs_get_u64(nvs_handle_t handle, const char *key, uint64_t *out_value)
esp_err_t nvs_get_str(nvs_handle_t handle, const char *key, char *out_value, size_t *length)

get value for given key

These functions retrieve the data of an entry, given its key. If key does not exist, or the requested variable type doesn’t match the type which was used when setting a value, an error is returned.

In case of any error, out_value is not modified.

All functions expect out_value to be a pointer to an already allocated variable of the given type.

nvs_get_str and nvs_get_blob functions support WinAPI-style length queries. To get the size necessary to store the value, call nvs_get_str or nvs_get_blob with zero out_value and non-zero pointer to length. Variable pointed to by length argument will be set to the required length. For nvs_get_str, this length includes the zero terminator. When calling nvs_get_str and nvs_get_blob with non-zero out_value, length has to be non-zero and has to point to the length available in out_value. It is suggested that nvs_get/set_str is used for zero-terminated C strings, and nvs_get/set_blob used for arbitrary data structures.

// Example (without error checking) of using nvs_get_str to get a string into dynamic array:
size_t required_size;
nvs_get_str(my_handle, "server_name", NULL, &required_size);
char* server_name = malloc(required_size);
nvs_get_str(my_handle, "server_name", server_name, &required_size);

// Example (without error checking) of using nvs_get_blob to get a binary data
into a static array:
uint8_t mac_addr[6];
size_t size = sizeof(mac_addr);
nvs_get_blob(my_handle, "dst_mac_addr", mac_addr, &size);

Return
  • ESP_OK if the value was retrieved successfully
  • ESP_ERR_NVS_NOT_FOUND if the requested key doesn’t exist
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • ESP_ERR_NVS_INVALID_NAME if key name doesn’t satisfy constraints
  • ESP_ERR_NVS_INVALID_LENGTH if length is not sufficient to store data
Parameters
  • handle: Handle obtained from nvs_open function.
  • key: Key name. Maximal length is determined by the underlying implementation, but is guaranteed to be at least 15 characters. Shouldn’t be empty.
  • out_value: Pointer to the output value. May be NULL for nvs_get_str and nvs_get_blob, in this case required length will be returned in length argument.
  • length: A non-zero pointer to the variable holding the length of out_value. In case out_value a zero, will be set to the length required to hold the value. In case out_value is not zero, will be set to the actual length of the value written. For nvs_get_str this includes zero terminator.

esp_err_t nvs_get_blob(nvs_handle_t handle, const char *key, void *out_value, size_t *length)
esp_err_t nvs_open(const char *name, nvs_open_mode_t open_mode, nvs_handle_t *out_handle)

Open non-volatile storage with a given namespace from the default NVS partition.

Multiple internal ESP-IDF and third party application modules can store their key-value pairs in the NVS module. In order to reduce possible conflicts on key names, each module can use its own namespace. The default NVS partition is the one that is labelled “nvs” in the partition table.

Return
  • ESP_OK if storage handle was opened successfully
  • ESP_ERR_NVS_NOT_INITIALIZED if the storage driver is not initialized
  • ESP_ERR_NVS_PART_NOT_FOUND if the partition with label “nvs” is not found
  • ESP_ERR_NVS_NOT_FOUND id namespace doesn’t exist yet and mode is NVS_READONLY
  • ESP_ERR_NVS_INVALID_NAME if namespace name doesn’t satisfy constraints
  • other error codes from the underlying storage driver
Parameters
  • name: Namespace name. Maximal length is determined by the underlying implementation, but is guaranteed to be at least 15 characters. Shouldn’t be empty.
  • open_mode: NVS_READWRITE or NVS_READONLY. If NVS_READONLY, will open a handle for reading only. All write requests will be rejected for this handle.
  • out_handle: If successful (return code is zero), handle will be returned in this argument.

esp_err_t nvs_open_from_partition(const char *part_name, const char *name, nvs_open_mode_t open_mode, nvs_handle_t *out_handle)

Open non-volatile storage with a given namespace from specified partition.

The behaviour is same as nvs_open() API. However this API can operate on a specified NVS partition instead of default NVS partition. Note that the specified partition must be registered with NVS using nvs_flash_init_partition() API.

Return
  • ESP_OK if storage handle was opened successfully
  • ESP_ERR_NVS_NOT_INITIALIZED if the storage driver is not initialized
  • ESP_ERR_NVS_PART_NOT_FOUND if the partition with specified name is not found
  • ESP_ERR_NVS_NOT_FOUND id namespace doesn’t exist yet and mode is NVS_READONLY
  • ESP_ERR_NVS_INVALID_NAME if namespace name doesn’t satisfy constraints
  • other error codes from the underlying storage driver
Parameters
  • part_name: Label (name) of the partition of interest for object read/write/erase
  • name: Namespace name. Maximal length is determined by the underlying implementation, but is guaranteed to be at least 15 characters. Shouldn’t be empty.
  • open_mode: NVS_READWRITE or NVS_READONLY. If NVS_READONLY, will open a handle for reading only. All write requests will be rejected for this handle.
  • out_handle: If successful (return code is zero), handle will be returned in this argument.

esp_err_t nvs_set_blob(nvs_handle_t handle, const char *key, const void *value, size_t length)

set variable length binary value for given key

This family of functions set value for the key, given its name. Note that actual storage will not be updated until nvs_commit function is called.

Return
  • ESP_OK if value was set successfully
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • ESP_ERR_NVS_READ_ONLY if storage handle was opened as read only
  • ESP_ERR_NVS_INVALID_NAME if key name doesn’t satisfy constraints
  • ESP_ERR_NVS_NOT_ENOUGH_SPACE if there is not enough space in the underlying storage to save the value
  • ESP_ERR_NVS_REMOVE_FAILED if the value wasn’t updated because flash write operation has failed. The value was written however, and update will be finished after re-initialization of nvs, provided that flash operation doesn’t fail again.
  • ESP_ERR_NVS_VALUE_TOO_LONG if the value is too long
Parameters
  • handle: Handle obtained from nvs_open function. Handles that were opened read only cannot be used.
  • key: Key name. Maximal length is 15 characters. Shouldn’t be empty.
  • value: The value to set.
  • length: length of binary value to set, in bytes; Maximum length is 508000 bytes or (97.6% of the partition size - 4000) bytes whichever is lower.

esp_err_t nvs_erase_key(nvs_handle_t handle, const char *key)

Erase key-value pair with given key name.

Note that actual storage may not be updated until nvs_commit function is called.

Return
  • ESP_OK if erase operation was successful
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • ESP_ERR_NVS_READ_ONLY if handle was opened as read only
  • ESP_ERR_NVS_NOT_FOUND if the requested key doesn’t exist
  • other error codes from the underlying storage driver
Parameters
  • handle: Storage handle obtained with nvs_open. Handles that were opened read only cannot be used.
  • key: Key name. Maximal length is determined by the underlying implementation, but is guaranteed to be at least 15 characters. Shouldn’t be empty.

esp_err_t nvs_erase_all(nvs_handle_t handle)

Erase all key-value pairs in a namespace.

Note that actual storage may not be updated until nvs_commit function is called.

Return
  • ESP_OK if erase operation was successful
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • ESP_ERR_NVS_READ_ONLY if handle was opened as read only
  • other error codes from the underlying storage driver
Parameters
  • handle: Storage handle obtained with nvs_open. Handles that were opened read only cannot be used.

esp_err_t nvs_commit(nvs_handle_t handle)

Write any pending changes to non-volatile storage.

After setting any values, nvs_commit() must be called to ensure changes are written to non-volatile storage. Individual implementations may write to storage at other times, but this is not guaranteed.

Return
  • ESP_OK if the changes have been written successfully
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL
  • other error codes from the underlying storage driver
Parameters
  • handle: Storage handle obtained with nvs_open. Handles that were opened read only cannot be used.

void nvs_close(nvs_handle_t handle)

Close the storage handle and free any allocated resources.

This function should be called for each handle opened with nvs_open once the handle is not in use any more. Closing the handle may not automatically write the changes to nonvolatile storage. This has to be done explicitly using nvs_commit function. Once this function is called on a handle, the handle should no longer be used.

Parameters
  • handle: Storage handle to close

esp_err_t nvs_get_stats(const char *part_name, nvs_stats_t *nvs_stats)

Fill structure nvs_stats_t. It provides info about used memory the partition.

This function calculates to runtime the number of used entries, free entries, total entries, and amount namespace in partition.

// Example of nvs_get_stats() to get the number of used entries and free entries:
nvs_stats_t nvs_stats;
nvs_get_stats(NULL, &nvs_stats);
printf("Count: UsedEntries = (%d), FreeEntries = (%d), AllEntries = (%d)\n",
       nvs_stats.used_entries, nvs_stats.free_entries, nvs_stats.total_entries);

Return
  • ESP_OK if the changes have been written successfully. Return param nvs_stats will be filled.
  • ESP_ERR_NVS_PART_NOT_FOUND if the partition with label “name” is not found. Return param nvs_stats will be filled 0.
  • ESP_ERR_NVS_NOT_INITIALIZED if the storage driver is not initialized. Return param nvs_stats will be filled 0.
  • ESP_ERR_INVALID_ARG if nvs_stats equal to NULL.
  • ESP_ERR_INVALID_STATE if there is page with the status of INVALID. Return param nvs_stats will be filled not with correct values because not all pages will be counted. Counting will be interrupted at the first INVALID page.
Parameters
  • part_name: Partition name NVS in the partition table. If pass a NULL than will use NVS_DEFAULT_PART_NAME (“nvs”).
  • nvs_stats: Returns filled structure nvs_states_t. It provides info about used memory the partition.

esp_err_t nvs_get_used_entry_count(nvs_handle_t handle, size_t *used_entries)

Calculate all entries in a namespace.

Note that to find out the total number of records occupied by the namespace, add one to the returned value used_entries (if err is equal to ESP_OK). Because the name space entry takes one entry.

// Example of nvs_get_used_entry_count() to get amount of all key-value pairs in one namespace:
nvs_handle_t handle;
nvs_open("namespace1", NVS_READWRITE, &handle);
...
size_t used_entries;
size_t total_entries_namespace;
if(nvs_get_used_entry_count(handle, &used_entries) == ESP_OK){
    // the total number of records occupied by the namespace
    total_entries_namespace = used_entries + 1;
}

Return
  • ESP_OK if the changes have been written successfully. Return param used_entries will be filled valid value.
  • ESP_ERR_NVS_NOT_INITIALIZED if the storage driver is not initialized. Return param used_entries will be filled 0.
  • ESP_ERR_NVS_INVALID_HANDLE if handle has been closed or is NULL. Return param used_entries will be filled 0.
  • ESP_ERR_INVALID_ARG if used_entries equal to NULL.
  • Other error codes from the underlying storage driver. Return param used_entries will be filled 0.
Parameters
  • handle: Handle obtained from nvs_open function.
  • used_entries: Returns amount of used entries from a namespace.

nvs_iterator_t nvs_entry_find(const char *part_name, const char *namespace_name, nvs_type_t type)

Create an iterator to enumerate NVS entries based on one or more parameters. 

// Example of listing all the key-value pairs of any type under specified partition and namespace
nvs_iterator_t it = nvs_entry_find(partition, namespace, NVS_TYPE_ANY);
while (it != NULL) {
        nvs_entry_info_t info;
        nvs_entry_info(it, &info);
        it = nvs_entry_next(it);
        printf("key '%s', type '%d' \n", info.key, info.type);
};
// Note: no need to release iterator obtained from nvs_entry_find function when
//       nvs_entry_find or nvs_entry_next function return NULL, indicating no other
//       element for specified criteria was found.
}

Return
Iterator used to enumerate all the entries found, or NULL if no entry satisfying criteria was found. Iterator obtained through this function has to be released using nvs_release_iterator when not used any more.
Parameters
  • part_name: Partition name
  • namespace_name: Set this value if looking for entries with a specific namespace. Pass NULL otherwise.
  • type: One of nvs_type_t values.

nvs_iterator_t nvs_entry_next(nvs_iterator_t iterator)

Returns next item matching the iterator criteria, NULL if no such item exists.

Note that any copies of the iterator will be invalid after this call.

Return
NULL if no entry was found, valid nvs_iterator_t otherwise.
Parameters
  • iterator: Iterator obtained from nvs_entry_find function. Must be non-NULL.

void nvs_entry_info(nvs_iterator_t iterator, nvs_entry_info_t *out_info)

Fills nvs_entry_info_t structure with information about entry pointed to by the iterator.

Parameters
  • iterator: Iterator obtained from nvs_entry_find or nvs_entry_next function. Must be non-NULL.
  • out_info: Structure to which entry information is copied.

void nvs_release_iterator(nvs_iterator_t iterator)

Release iterator.

Parameters
  • iterator: Release iterator obtained from nvs_entry_find function. NULL argument is allowed.

Structures

struct nvs_entry_info_t

information about entry obtained from nvs_entry_info function

Public Members

char namespace_name[16]

Namespace to which key-value belong

char key[16]

Key of stored key-value pair

nvs_type_t type

Type of stored key-value pair

struct nvs_stats_t

Note
Info about storage space NVS.

Public Members

size_t used_entries

Amount of used entries.

size_t free_entries

Amount of free entries.

size_t total_entries

Amount all available entries.

size_t namespace_count

Amount name space.

Macros

ESP_ERR_NVS_BASE

Starting number of error codes

ESP_ERR_NVS_NOT_INITIALIZED

The storage driver is not initialized

ESP_ERR_NVS_NOT_FOUND

Id namespace doesn’t exist yet and mode is NVS_READONLY

ESP_ERR_NVS_TYPE_MISMATCH

The type of set or get operation doesn’t match the type of value stored in NVS

ESP_ERR_NVS_READ_ONLY

Storage handle was opened as read only

ESP_ERR_NVS_NOT_ENOUGH_SPACE

There is not enough space in the underlying storage to save the value

ESP_ERR_NVS_INVALID_NAME

Namespace name doesn’t satisfy constraints

ESP_ERR_NVS_INVALID_HANDLE

Handle has been closed or is NULL

ESP_ERR_NVS_REMOVE_FAILED

The value wasn’t updated because flash write operation has failed. The value was written however, and update will be finished after re-initialization of nvs, provided that flash operation doesn’t fail again.

ESP_ERR_NVS_KEY_TOO_LONG

Key name is too long

ESP_ERR_NVS_PAGE_FULL

Internal error; never returned by nvs API functions

ESP_ERR_NVS_INVALID_STATE

NVS is in an inconsistent state due to a previous error. Call nvs_flash_init and nvs_open again, then retry.

ESP_ERR_NVS_INVALID_LENGTH

String or blob length is not sufficient to store data

ESP_ERR_NVS_NO_FREE_PAGES

NVS partition doesn’t contain any empty pages. This may happen if NVS partition was truncated. Erase the whole partition and call nvs_flash_init again.

ESP_ERR_NVS_VALUE_TOO_LONG

String or blob length is longer than supported by the implementation

ESP_ERR_NVS_PART_NOT_FOUND

Partition with specified name is not found in the partition table

ESP_ERR_NVS_NEW_VERSION_FOUND

NVS partition contains data in new format and cannot be recognized by this version of code

ESP_ERR_NVS_XTS_ENCR_FAILED

XTS encryption failed while writing NVS entry

ESP_ERR_NVS_XTS_DECR_FAILED

XTS decryption failed while reading NVS entry

ESP_ERR_NVS_XTS_CFG_FAILED

XTS configuration setting failed

ESP_ERR_NVS_XTS_CFG_NOT_FOUND

XTS configuration not found

ESP_ERR_NVS_ENCR_NOT_SUPPORTED

NVS encryption is not supported in this version

ESP_ERR_NVS_KEYS_NOT_INITIALIZED

NVS key partition is uninitialized

ESP_ERR_NVS_CORRUPT_KEY_PART

NVS key partition is corrupt

ESP_ERR_NVS_CONTENT_DIFFERS

Internal error; never returned by nvs API functions. NVS key is different in comparison

NVS_DEFAULT_PART_NAME

Default partition name of the NVS partition in the partition table

NVS_PART_NAME_MAX_SIZE

maximum length of partition name (excluding null terminator)

Type Definitions

typedef uint32_t nvs_handle_t

Opaque pointer type representing non-volatile storage handle

typedef nvs_handle_t nvs_handle
typedef nvs_open_mode_t nvs_open_mode
typedef struct nvs_opaque_iterator_t *nvs_iterator_t

Opaque pointer type representing iterator to nvs entries

Enumerations

enum nvs_open_mode_t

Mode of opening the non-volatile storage.

Values:

NVS_READONLY

Read only

NVS_READWRITE

Read and write

enum nvs_type_t

Types of variables.

Values:

NVS_TYPE_U8 = 0x01

Type uint8_t

NVS_TYPE_I8 = 0x11

Type int8_t

NVS_TYPE_U16 = 0x02

Type uint16_t

NVS_TYPE_I16 = 0x12

Type int16_t

NVS_TYPE_U32 = 0x04

Type uint32_t

NVS_TYPE_I32 = 0x14

Type int32_t

NVS_TYPE_U64 = 0x08

Type uint64_t

NVS_TYPE_I64 = 0x18

Type int64_t

NVS_TYPE_STR = 0x21

Type string

NVS_TYPE_BLOB = 0x42

Type blob

NVS_TYPE_ANY = 0xff

Must be last