[][src]Struct sqlite3_header::SQLite3Header

pub struct SQLite3Header {
    pub magic_header_string: String,
    pub page_size: PageSize,
    pub file_format_write_version: FileFormat,
    pub file_format_read_version: FileFormat,
    pub reserved_bytes_per_page: u8,
    pub maximum_embedded_payload_fraction: u8,
    pub minimum_embedded_payload_fraction: u8,
    pub leaf_payload_fraction: u8,
    pub file_change_counter: u32,
    pub in_header_database_size: u32,
    pub freelist_page_index: u32,
    pub freelist_count: u32,
    pub schema_cookie: u32,
    pub schema_format: u32,
    pub default_page_cache_size: u32,
    pub largest_root_btree_page: u32,
    pub database_text_encoding: u32,
    pub user_version: u32,
    pub incremental_vacuum_mode: u32,
    pub application_id: u32,
    pub reserved: [u8; 20],
    pub version_valid_for_number: u32,
    pub sqlite_version_number: u32,
}

Fields

magic_header_string: String

Every valid SQLite database file begins with the following 16 bytes (in hex): 53 51 4c 69 74 65 20 66 6f 72 6d 61 74 20 33 00. This byte sequence corresponds to the UTF-8 string "SQLite format 3" including the nul terminator character at the end.

page_size: PageSize

The two-byte value beginning at offset 16 determines the page size of the database. For SQLite versions 3.7.0.1 (2010-08-04) and earlier, this value is interpreted as a big-endian integer and must be a power of two between 512 and 32768, inclusive. Beginning with SQLite version 3.7.1 (2010-08-23), a page size of 65536 bytes is supported. The value 65536 will not fit in a two-byte integer, so to specify a 65536-byte page size, the value at offset 16 is 0x00 0x01. This value can be interpreted as a big-endian 1 and thought of as a magic number to represent the 65536 page size. Or one can view the two-byte field as a little endian number and say that it represents the page size divided by 256. These two interpretations of the page-size field are equivalent.

file_format_write_version: FileFormat

The file format write version and file format read version at offsets 18 and 19 are intended to allow for enhancements of the file format in future versions of SQLite. In current versions of SQLite, both of these values are 1 for rollback journalling modes and 2 for WAL journalling mode. If a version of SQLite coded to the current file format specification encounters a database file where the read version is 1 or 2 but the write version is greater than 2, then the database file must be treated as read-only. If a database file with a read version greater than 2 is encountered, then that database cannot be read or written.

file_format_read_version: FileFormatreserved_bytes_per_page: u8

SQLite has the ability to set aside a small number of extra bytes at the end of every page for use by extensions. These extra bytes are used, for example, by the SQLite Encryption Extension to store a nonce and/or cryptographic checksum associated with each page. The "reserved space" size in the 1-byte integer at offset 20 is the number of bytes of space at the end of each page to reserve for extensions. This value is usually 0. The value can be odd. The "usable size" of a database page is the page size specified by the 2-byte integer at offset 16 in the header less the "reserved" space size recorded in the 1-byte integer at offset 20 in the header. The usable size of a page might be an odd number. However, the usable size is not allowed to be less than 480. In other words, if the page size is 512, then the reserved space size cannot exceed 32.

maximum_embedded_payload_fraction: u8

The maximum and minimum embedded payload fractions and the leaf payload fraction values must be 64, 32, and 32. These values were originally intended to be tunable parameters that could be used to modify the storage format of the b-tree algorithm. However, that functionality is not supported and there are no current plans to add support in the future. Hence, these three bytes are fixed at the values specified.

minimum_embedded_payload_fraction: u8leaf_payload_fraction: u8file_change_counter: u32

The file change counter is a 4-byte big-endian integer at offset 24 that is incremented whenever the database file is unlocked after having been modified. When two or more processes are reading the same database file, each process can detect database changes from other processes by monitoring the change counter. A process will normally want to flush its database page cache when another process modified the database, since the cache has become stale. The file change counter facilitates this. In WAL mode, changes to the database are detected using the wal-index and so the change counter is not needed. Hence, the change counter might not be incremented on each transaction in WAL mode.

in_header_database_size: u32

The 4-byte big-endian integer at offset 28 into the header stores the size of the database file in pages. If this in-header datasize size is not valid (see the next paragraph), then the database size is computed by looking at the actual size of the database file. Older versions of SQLite ignored the in-header database size and used the actual file size exclusively. Newer versions of SQLite use the in-header database size if it is available but fall back to the actual file size if the in-header database size is not valid. The in-header database size is only considered to be valid if it is non-zero and if the 4-byte change counter at offset 24 exactly matches the 4-byte version-valid-for number at offset 92. The in-header database size is always valid when the database is only modified using recent versions of SQLite, versions 3.7.0 (2010-07-21) and later. If a legacy version of SQLite writes to the database, it will not know to update the in-header database size and so the in-header database size could be incorrect. But legacy versions of SQLite will also leave the version-valid-for number at offset 92 unchanged so it will not match the change-counter. Hence, invalid in-header database sizes can be detected (and ignored) by observing when the change-counter does not match the version-valid-for number.

freelist_page_index: u32

Unused pages in the database file are stored on a freelist. The 4-byte big-endian integer at offset 32 stores the page number of the first page of the freelist, or zero if the freelist is empty. The 4-byte big-endian integer at offset 36 stores stores the total number of pages on the freelist.

freelist_count: u32schema_cookie: u32

The schema cookie is a 4-byte big-endian integer at offset 40 that is incremented whenever the database schema changes. A prepared statement is compiled against a specific version of the database schema. When the database schema changes, the statement must be reprepared. When a prepared statement runs, it first checks the schema cookie to ensure the value is the same as when the statement was prepared and if the schema cookie has changed, the statement either automatically reprepares and reruns or it aborts with an SQLITE_SCHEMA error.

schema_format: u32

The schema format number is a 4-byte big-endian integer at offset 44. The schema format number is similar to the file format read and write version numbers at offsets 18 and 19 except that the schema format number refers to the high-level SQL formatting rather than the low-level b-tree formatting. Four schema format numbers are currently defined:

  1. Format 1 is understood by all versions of SQLite back to version 3.0.0 (2004-06-18).
  2. Format 2 adds the ability of rows within the same table to have a varying number of columns, in order to support the ALTER TABLE ... ADD COLUMN functionality. Support for reading and writing format 2 was added in SQLite version 3.1.3 on 2005-02-20.
  3. Format 3 adds the ability of extra columns added by ALTER TABLE ... ADD COLUMN to have non-NULL default values. This capability was added in SQLite version 3.1.4 on 2005-03-11.
  4. Format 4 causes SQLite to respect the DESC keyword on index declarations. (The DESC keyword is ignored in indexes for formats 1, 2, and 3.) Format 4 also adds two new boolean record type values (serial types 8 and 9). Support for format 4 was added in SQLite 3.3.0 on 2006-01-10. New database files created by SQLite use format 4 by default. The legacy_file_format pragma can be used to cause SQLite to create new database files using format 1. The format version number can be made to default to 1 instead of 4 by setting SQLITE_DEFAULT_FILE_FORMAT=1 at compile-time.
default_page_cache_size: u32

The 4-byte big-endian signed integer at offset 48 is the suggested cache size in pages for the database file. The value is a suggestion only and SQLite is under no obligation to honor it. The absolute value of the integer is used as the suggested size. The suggested cache size can be set using the default_cache_size pragma.

largest_root_btree_page: u32database_text_encoding: u32

The 4-byte big-endian integer at offset 56 determines the encoding used for all text strings stored in the database. A value of 1 means UTF-8. A value of 2 means UTF-16le. A value of 3 means UTF-16be. No other values are allowed. The sqlite3.h header file defines C-preprocessor macros SQLITE_UTF8 as 1, SQLITE_UTF16LE as 2, and SQLITE_UTF16BE as 3, to use in place of the numeric codes for the text encoding.

user_version: u32

The 4-byte big-endian integer at offset 60 is the user version which is set and queried by the user_version pragma. The user version is not used by SQLite.

incremental_vacuum_mode: u32

The two 4-byte big-endian integers at offsets 52 and 64 are used to manage the auto_vacuum and incremental_vacuum modes. If the integer at offset 52 is zero then pointer-map (ptrmap) pages are omitted from the database file and neither auto_vacuum nor incremental_vacuum are supported. If the integer at offset 52 is non-zero then it is the page number of the largest root page in the database file, the database file will contain ptrmap pages, and the mode must be either auto_vacuum or incremental_vacuum. In this latter case, the integer at offset 64 is true for incremental_vacuum and false for auto_vacuum. If the integer at offset 52 is zero then the integer at offset 64 must also be zero.

application_id: u32

The 4-byte big-endian integer at offset 68 is an "Application ID" that can be set by the PRAGMA application_id command in order to identify the database as belonging to or associated with a particular application. The application ID is intended for database files used as an application file-format. The application ID can be used by utilities such as file(1) to determine the specific file type rather than just reporting "SQLite3 Database". A list of assigned application IDs can be seen by consulting the magic.txt file in the SQLite source repository.

reserved: [u8; 20]

All other bytes of the database file header are reserved for future expansion and must be set to zero.

version_valid_for_number: u32sqlite_version_number: u32

The 4-byte big-endian integer at offset 96 stores the SQLITE_VERSION_NUMBER value for the SQLite library that most recently modified the database file. The 4-byte big-endian integer at offset 92 is the value of the change counter when the version number was stored. The integer at offset 92 indicates which transaction the version number is valid for and is sometimes called the "version-valid-for number".

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