Documentation Source Text

<h1 id=sqlite_database_files>SQLite Database Files</h1>
 
  <p>
    The bulk of this document, section <cite>database_file_format</cite>,
    contains the definition of a <i>well-formed SQLite database file</i>.
    SQLite is required to create database files that meet this definition.

  <p class=req id=FF0010>
    The system shall ensure that at the successful conclusion of a
    database transaction the contents of the database file constitute
    a <i>well-formed SQLite database file</i>.

  <p>
    Additionally, the database file should contain a serialized version
    of the logical database produced by the transaction. For all but the
    most trivial logical databases, there are many possible serial 
    representations.

  <p class=req id=FF0020>
    The system shall ensure that at the successful conclusion of a
    database transaction the contents of the database file are a valid
    serialization of the contents of the logical SQL database produced
    by the transaction.

  <p>
    Section <cite>database_file_manipulation</cite> contains requirements

	Some of the following requirements state that certain database header
        fields must contain defined constant values, even though the sqlite 
        database file format is designed to allow various values. This is
        done to artificially constrain the definition of a 
        <i>well-formed database</i> in order to make implementation and 
        testing more practical.

      <p class=req id=FF0030>
	The first 16 bytes of a well-formed database file contain the UTF-8
        encoding of the string "SQLite format 3" followed by a single 
        nul-terminator byte.

      <p>
        Following the 16 byte magic string in the file header is the
	<i>page size</i>, a 2-byte field. See section
        <cite>pages_and_page_types</cite> for details.

      <p class=req id=FF0040>
	The 19th byte (byte offset 18), the <i>file-format write version</i>, 
        of a well-formed database file contains the value 0x01.
      <p class=req id=FF0050>
	The 20th byte (byte offset 19), the <i>file-format read version</i>, 
        of a well-formed database file contains the value 0x01.

      <p class=req id=FF0060>
	The 21st byte (byte offset 20), the number of unused bytes on each
        page, of a well-formed database file shall contain the value 0x00.

      <p class=req id=FF0070>
	The 22nd byte (byte offset 21), the maximum fraction of an index 
        B-Tree page to use for embedded content, of a well-formed database 
        file shall contain the value 0x40.
      <p class=req id=FF0080>
	The 23rd byte (byte offset 22), the minimum fraction of an index 
        B-Tree page to use for embedded content when using overflow pages,
        of a well-formed database file contains the value 0x20.
      <p class=req id=FF0090>
	The 24th byte (byte offset 23), the minimum fraction of a table
        B-Tree page to use for embedded content when using overflow pages,
        of a well-formed database file contains the value 0x20.
      <p class=req id=FF0100>
        The 4 byte block starting at byte offset 24 of a well-formed 
        database file contains the <i>file change counter</i> formatted
        as a 4-byte big-endian integer.

      <p>
        Following the <i>file change counter</i> in the database header are
        two 4-byte fields related to the database file <i>free page list</i>.
        See section <cite>free_page_list</cite> for details.

      <p class=req id=FF0110>
        The 4 byte block starting at byte offset 40 of a well-formed 
        database file contains the <i>schema version</i> formatted
        as a 4-byte big-endian integer.

      <p class=req id=FF0120>
	The 4 byte block starting at byte offset 44 of a well-formed
        database file, the <i>schema layer file format</i>, contains a 
        big-endian integer value between 1 and 4, inclusive.

      <p class=req id=FF0130>
        The 4 byte block starting at byte offset 48 of a well-formed 
        database file contains the <i>default pager cache size</i> formatted
        as a 4-byte big-endian integer.

      <p class=req id=FF0140>
        The 4 byte block starting at byte offset 52 of a well-formed 
        database file contains the <i>auto-vacuum last root-page</i> 
        formatted as a 4-byte big-endian integer. If this value is non-zero,
        the database is said to be an <i>auto-vacuum database</i>.

      <p class=req id=FF0150>
	The 4 byte block starting at byte offset 56 of a well-formed
        database file, the <i>text encoding</i> contains a big-endian integer 
        value between 1 and 3, inclusive.

      <p class=req id=FF0160>
        The 4 byte block starting at byte offset 60 of a well-formed 
        database file contains the <i>user cookie</i> formatted
        as a 4-byte big-endian integer.

      <p class=req id=FF0170>
	The 4 byte block starting at byte offset 64 of a well-formed
        database file, the <i>incremental vaccum flag</i> contains a big-endian 
        integer value between 0 and 1, inclusive.

      <p class=req id=FF0180>
	In a well-formed non-autovacuum database (one with a zero stored
        in the 4-byte big-endian integer value beginning at byte offset
        52 of the database file header, the incremental vacuum flag is
        set to 0.

    <h3 id="pages_and_page_types">Pages and Page Types</h3>
      <p>

            permanently designated "pointer-map" pages. See section 
            <cite>pointer_map_pages</cite> for details.
        <li><b>The locking page</b>. The database page that starts at
            byte offset 2<sup>30</sup>, if it is large enough to contain
            such a page, is always left unused.
      </ul>

      <p class=req id=FF0190>
        The <i>database page size</i> of a well-formed database, stored as a
        2-byte big-endian unsigned integer at byte offset 16 of the file, 
        shall be an integer power of 2 between 512 and 32768, inclusive.
      <p class=req id=FF0200>
        The size of a <i>well formed database file</i> shall be an integer
        multiple of the <i>database page size</i>.
      <p class=req id=FF0210>
	Each page of a <i>well formed database file</i> is exactly one of a 
        <i>B-Tree page</i>, an <i>overflow page</i>, a <i>free page</i>, a 
        <i>pointer-map page</i> or the <i>locking page</i>.
      <p class=req id=FF0220>
        The database page that starts at byte offset 2<sup>30</sup>, the
        <i>locking page</i>, is never used for any purpose.
        

    <h3 id=schema_table>The Schema Table</h3>
      <p>
        Apart from being the page that contains the file-header, page 1 of

        <tr><td>index <td>i1 <td>abc <td>3 <td>CREATE INDEX i1 ON abc(b, c)
        <tr><td>table <td>def <td>def <td>4 <td>CREATE TABLE def(a PRIMARY KEY, b, c, UNIQUE(b, c))
        <tr><td>index <td>sqlite_autoindex_def_1 <td>def <td>5 <td>
        <tr><td>index <td>sqlite_autoindex_def_2 <td>def <td>6 <td>
        <tr><td>view <td>v1 <td>v1 <td>0 <td>CREATE VIEW v1 AS SELECT * FROM abc
      </table>

      <p class=req id=FF0230>
	In a <i>well-formed database file</i>, the portion of the first 
        database page not consumed by the database file-header (all but the 
        first 100 bytes) contains the root node of a table B-Tree, 
        the <i>schema table</i>.
      <p class=req id=FF0240>
	All records stored in the <i>schema table</i> contain exactly five
        fields.

      <p>The following requirements describe "table" records.

      <p class=req id=FF0250>
        For each SQL table in the database apart from itself 
        ("sqlite_master"), the <i>schema table</i> of a <i>well-formed 
        database file</i> contains an associated record.

      <p class=req id=FF0260>
        The first field of each <i>schema table</i> record associated with an
        SQL table shall be the text value "table".

      <p class=req id=FF0270>
        The second field of each <i>schema table</i> record associated with an
	SQL table shall be a text value set to the name of the SQL table.

      <p class=req id=FF0280>
        In a <i>well-formed database file</i>, the third field of all 
        <i>schema table</i> records associated with SQL tables shall contain 
        the same value as the second field.

      <p class=req id=FF0290>
        In a <i>well-formed database file</i>, the fourth field of all 
	<i>schema table</i> records associated with SQL tables that are not
	virtual tables contains the page number (an integer value) of the root
        page of the associated <i>table B-Tree</i> structure within the 
        database file.

      <p class=req id=FF0300>
        If the associated database table is a virtual table, the fourth
        field of the <i>schema table</i> record shall contain an SQL NULL 
        value.

      <p class=req id=FF0310>
        In a well-formed database, the fifth field of all <i>schema table</i>
	records associated with SQL tables shall contain a "CREATE TABLE"
        or "CREATE VIRTUAL TABLE" statment (a text value).  The details
        of the statement shall be such that executing the statement
	would create a table of precisely the same name and schema as the
        existing database table.

      <p>The following requirements describe "implicit index" records.

      <p class=req id=FF0320>
        For each PRIMARY KEY or UNIQUE constraint present in the definition
        of each SQL table in the database, the schema table of a well-formed 
        database shall contain a record with the first field set to the text 
        value "index", and the second field set to a text value containing a
        string of the form "sqlite_autoindex_&lt;name&gt;_&lt;idx&gt;", where
        &lt;name&gt; is the name of the SQL table and &lt;idx&gt; is an 
        integer value.

      <p class=req id=FF0330>
        In a well-formed database, the third field of all schema table
        records associated with SQL PRIMARY KEY or UNIQUE constraints shall
	contain the name of the table to which the constraint applies (a 
        text value).
      <p class=req id=FF0340>
        In a well-formed database, the fourth field of all schema table
	records associated with SQL PRIMARY KEY or UNIQUE constraints shall
	contain the page number (an integer value) of the root page of the
        associated index B-Tree structure.
      <p class=req id=FF0350>
        In a well-formed database, the fifth field of all schema table
        records associated with SQL PRIMARY KEY or UNIQUE constraints shall
        contain an SQL NULL value.

      <p>The following requirements describe "explicit index" records.

      <p class=req id=FF0360>
	For each SQL index in the database, the schema table of a well-formed
	database shall contain a record with the first field set to the text
	value "index" and the second field set to a text value containing the
        name of the SQL index.
      <p class=req id=FF0370>
        In a well-formed database, the third field of all schema table
        records associated with SQL indexes shall contain the name of the
        SQL table that the index applies to.
      <p class=req id=FF0380>
        In a well-formed database, the fourth field of all schema table
        records associated with SQL indexes shall contain the page number 
        (an integer value) of the root page of the associated index B-Tree
        structure.
      <p class=req id=FF0390>
        In a well-formed database, the fifth field of all schema table
	records associated with SQL indexes shall contain an SQL "CREATE
	INDEX" statement (a text value). The details of the statement shall 
        be such that executing the statement would create an index of 
        precisely the same name and content as the existing database index.

      <p>The following requirements describe "view" records.

      <p class=req id=FF0400>
	For each SQL view in the database, the schema table of a well-formed
	database shall contain a record with the first field set to the text
	value "view" and the second field set to a text value containing the
        name of the SQL view.

      <p class=req id=FF0410>
        In a well-formed database, the third field of all schema table
        records associated with SQL views shall contain the same value as
        the second field.

      <p class=req id=FF0420>
        In a well-formed database, the third field of all schema table
        records associated with SQL views shall contain the integer value 0.

      <p class=req id=FF0430>
        In a well-formed database, the fifth field of all schema table
	records associated with SQL indexes shall contain an SQL "CREATE
	VIEW" statement (a text value). The details of the statement shall 
        be such that executing the statement would create a view of 
        precisely the same name and definition as the existing database view.

      <p>The following requirements describe "trigger" records.

      <p class=req id=FF0440>
	For each SQL trigger in the database, the schema table of a well-formed
	database shall contain a record with the first field set to the text
	value "trigger" and the second field set to a text value containing the
        name of the SQL trigger.

      <p class=req id=FF0450>
        In a well-formed database, the third field of all schema table
        records associated with SQL triggers shall contain the name of the
        database table or view to which the trigger applies.

      <p class=req id=FF0460>
        In a well-formed database, the third field of all schema table
        records associated with SQL triggers shall contain the integer value 0.

      <p class=req id=FF0470>
        In a well-formed database, the fifth field of all schema table
	records associated with SQL indexes shall contain an SQL "CREATE
	TRIGGER" statement (a text value). The details of the statement shall 
        be such that executing the statement would create a trigger of 
        precisely the same name and definition as the existing database trigger.

      <p>The following requirements describe the placement of B-Tree root 
         pages in auto-vacuum databases.

      <p class=req id=FF0480>
        In an auto-vacuum database, all pages that occur before the page
        number stored in the <i>auto-vacuum last root-page</i> field
        of the database file header (see FF0140) must be either B-Tree <i>root
        pages</i>, <i>pointer-map pages</i> or the <i>locking page</i>.

      <p class=req id=FF0490>
        In an auto-vacuum database, no B-Tree <i>root pages</i> may occur
        on or after the page number stored in the <i>auto-vacuum last root-page</i> field
        of the database file header (see FF0140) must be either B-Tree <i>root
        pages</i>, <i>pointer-map pages</i> or the <i>locking page</i>.


 
  <h2 id="btree_structures">B-Tree Structures</h2>
    <p>
      A large part of any SQLite database file is given over to one or more

          B-Tree structures are described in detail in section 
          <cite>table_btrees</cite>.
      <li>The <b>index B-Tree</b>, which uses database records as keys. Index
          B-Tree structures are described in detail in section 
          <cite>index_btrees</cite>.
    </ul>

    <p class=req id=FF0500>
      As well as the <i>schema table</i>, a <i>well-formed database file</i> 
      contains <i>N</i> table B-Tree structures, where <i>N</i> is the 
      number of non-virtual tables in the logical database, excluding the 
      sqlite_master table but including sqlite_sequence and other system 
      tables.
    <p class=req id=FF0510>
      A well-formed database file contains <i>N</i> table B-Tree structures,
      where <i>N</i> is the number of indexes in the logical database,
      including indexes created by UNIQUE or PRIMARY KEY clauses in the
      declaration of SQL tables.

    <h3 id="varint_format">Variable Length Integer Format</h3>
      <p>

	<tr><td>200815 <td>0x000000000003106F <td>0x8C 0xA0 0x6F
        <tr><td>-1     <td>0xFFFFFFFFFFFFFFFF 
            <td>0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF
        <tr><td>-78056 <td>0xFFFFFFFFFFFECD56
            <td>0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFD 0xCD 0x56
      </table>

    <p class=req id=FF0520>
      A 64-bit signed integer value stored in <i>variable length integer</i>
      format consumes from 1 to 9 bytes of space.

    <p class=req id=FF0530>
      The most significant bit of all bytes except the last in a serialized
      <i>variable length integer</i> is always set. Unless the serialized 
      form consumes the maximum 9 bytes available, then the most significant
      bit of the final byte of the representation is always cleared.

    <p class=req id=FF0540>
      The eight least significant bytes of the 64-bit twos-compliment
      representation of a value stored in a 9 byte <i>variable length
      integer</i> are stored in the final byte (byte offset 8) of the
      serialized <i>variable length integer</i>. The other 56 bits are
      stored in the 7 least significant bits of each of the first 8 bytes
      of the serialized <i>variable length integer</i>, in order from
      most significant to least significant.

    <p class=req id=FF0550>
      A <i>variable length integer</i> that consumes less than 9 bytes of
      space contains a value represented as an <i>N</i>-bit unsigned 
      integer, where <i>N</i> is equal to the number of bytes consumed by 
      the serial representation (between 1 and 8) multiplied by 7. The
      <i>N</i> bits are stored in the 7 least significant bits of each
      byte of the serial representation, from most to least significant.
      

        the length of the data field is as described in the table above.
      <p>
        The data field associated with a string value contains the string
        encoded using the database encoding, as defined in the database
        file header (see section <cite>file_header</cite>). No 
        nul-terminator character is stored in the database.

      <p class=req id=FF0560>
	A <i>database record</i> consists of a <i>database record header</i>,
        followed by <i>database record data</i>. The first part of the
        <i>database record header</i> is a <i>variable length integer</i>
        containing the total size (including itself) of the header in bytes.

      <p class=req id=FF0570>
	Following the length field, the remainder of the <i>database record
	header</i> is populated with <i>N</i> <i>variable length integer</i>
        fields, where <i>N</i> is the number of database values stored in
        the record. 

      <p class=req id=FF0580>
	Following the <i>database record header</i>, the <i>database record
        data</i> is made up of <i>N</i> variable length blobs of data, where
        <i>N</i> is again the number of database values stored in the record.
        The <i>n</i> blob contains the data for the <i>n</i>th value in
        the database record. The size and format of each blob of data is
        encoded in the corresponding <i>variable length integer</i> field
        in the <i>database record header</i>.

      <p class=req id=FF0590>
        A value of 0 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL NULL. In this case
        the blob of data in the data area is 0 bytes in size.

      <p class=req id=FF0600>
        A value of 1 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 1-byte
        big-endian signed integer.
      <p class=req id=FF0610>
        A value of 2 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 2-byte
        big-endian signed integer.
      <p class=req id=FF0620>
        A value of 3 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 3-byte
        big-endian signed integer.
      <p class=req id=FF0630>
        A value of 4 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 4-byte
        big-endian signed integer.
      <p class=req id=FF0640>
        A value of 5 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 6-byte
        big-endian signed integer.
      <p class=req id=FF0650>
        A value of 6 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 8-byte
        big-endian signed integer.

      <p class=req id=FF0660>
        A value of 7 stored within the <i>database record header</i> indicates
	that the corresponding database value is an SQL real (floating
	point number). In this case the blob of data contains an 8-byte
        IEEE floating point number, stored in big-endian byte order.

      <p class=req id=FF0670>
        A value of 8 stored within the <i>database record header</i> indicates
	that the corresponding database value is an SQL integer, value 0.
        In this case the blob of data in the data area is 0 bytes in size.

      <p class=req id=FF0680>
        A value of 9 stored within the <i>database record header</i> indicates
	that the corresponding database value is an SQL integer, value 1.
        In this case the blob of data in the data area is 0 bytes in size.

      <p class=req id=FF0690>
	An even value greater than or equal to 12 stored within the 
        <i>database record header</i> indicates that the corresponding 
        database value is an SQL blob field. The blob of data contains the
        value data.  The blob of data is exactly (<i>n</i>-12)/2 bytes 
        in size, where <i>n</i> is the integer value stored in the 
        <i>database record header</i>.

      <p class=req id=FF0700>
	An odd value greater than or equal to 13 stored within the 
        <i>database record header</i> indicates that the corresponding 
        database value is an SQL text field. The blob of data contains the
        value text stored using the <i>database encoding</i>, with no 
        nul-terminator. The blob of data is exactly (<i>n</i>-12)/2 bytes 
        in size, where <i>n</i> is the integer value stored in the 
        <i>database record header</i>.

      <p>
        The following database file properties define restrictions on the 
        integer values that may be stored within a 
        <i>database record header</i>.

      <p class=req id=FF0710>
        In a well-formed database file, if the values 8 or 9 appear within
        any <i>database record header</i> within the database, then the
        <i>schema-layer file format</i> (stored at byte offset 44 of the
        database file header) must be set to 4.
      <p class=req id=FF0720>
        In a well-formed database file, the values 10 and 11, and all
        negative values may not appear within any <i>database record header</i> 
        in the database.

    <h3 id=index_btrees>Index B-Trees</h3>
      <p>
        As specified in section <cite>fileformat_overview</cite>, index 

        Figure <cite>figure_indextree</cite> depicts one possible record
        distribution for an index B-Tree containing records R1 to R26, assuming
        that for all values of N, <i>R(N+1)&gt;R(N)</i>. In total the B-Tree
        structure uses 11 database file pages. Internal tree nodes contain
        database records and references to child node pages. Leaf nodes contain
        database records only.

      <p class=req id=FF0730>
        The pages in an index B-Tree structures are arranged into a tree
        structure such that all leaf pages are at the same depth.

      <p class=req id=FF0740>
        Each leaf node page in an index B-Tree contains one or more
        B-Tree cells, where each cell contains a database record.

      <p class=req id=FF0750>
        Each internal node page in an index B-Tree contains one or more
        B-Tree cells, where each cell contains a child page number, <i>C</i>,
        and a database record <i>R</i>. All database records stored within
        the sub-tree headed by page <i>C</i> are smaller than record <i>R</i>,
        according to the index sort order (see below). Additionally, unless 
	<i>R</i> is the smallest database record stored on the internal node
	page, all integer keys within the sub-tree headed by <i>C</i> are
	greater than <i>R<sub>-1</sub></i>, where <i>R<sub>-1</sub></i> is the
        largest database record on the internal node page that is smaller 
        than <i>R</i>.

      <p class=req id=FF0760>
        As well as child page numbers associated with B-Tree cells, each
        internal node page in an index B-Tree contains the page number
        of an extra child page, the <i>right-child page</i>. All database
        records stored in all B-Tree cells within the sub-tree headed by the 
        <i>right-child page</i> are greater than all database records 
        stored within B-Tree cells on the internal node page.

          <p>
            An index B-Tree contains an entry for each row in its associated
            database table. The fields of the record used as the index B-Tree
            key are copies of each of the indexed columns of the associated 
            database row, in order, followed by the rowid value of the same 
            row. See figure <cite>figure_examplepop</cite> for an example.

        <p class=req id=FF0770>
          In a well-formed database, each index B-Tree contains a single entry
          for each row in the indexed logical database table.

        <p class=req id=FF0780>
	  Each <i>database record</i> (key) stored by an index B-Tree in a
	  well-formed database contains the same number of values, the number
          of indexed columns plus one.

        <p class=req id=FF0790>
	  The final value in each <i>database record</i> (key) stored by an
	  index B-Tree in a well-formed database contains the rowid (an integer
          value) of the corresponding logical database row.

        <p class=req id=FF0800>
          The first <i>N</i> values in each <i>database record</i> (key) 
	  stored in an index B-Tree where <i>N</i> is the number of indexed
	  columns, contain the values of the indexed columns from the
	  corresponding logical database row, in the order specified for the
          index.
 
      <h4 id="index_btree_compare_func">Record Sort Order</h4>

            the page) of the next block in the list stored as a big-endian
            unsigned integer. The first two bytes of the final block in the 
            list are set to zero. The third and fourth bytes of each free
            block contain the total size of the free block in bytes, stored
            as a 2 byte big-endian unsigned integer.
      </ul>

      <p class=req id=FF0810>
	The <i>b-tree page flags</i> field (the first byte) of each database
	page used as an internal node of an index B-Tree structure is set to
        0x02.  
      <p class=req id=FF0820>
	The <i>b-tree page flags</i> field (the first byte) of each database
        page used as a leaf node of an index B-Tree structure is set to 0x0A.

      <p>
        The following requirements describe the <i>B-Tree page header</i>
        present at the start of both index and table B-Tree pages.

      <p class=req id=FF0830>
        The first byte of each database page used as a B-Tree page contains
	the <i>b-tree page flags</i> field. On page 1, the <i>b-tree page
        flags</i> field is stored directly after the 100 byte file header
        at byte offset 100.

      <p class=req id=FF0840>
        The number of B-Tree cells stored on a B-Tree page is stored as a
        2-byte big-endian integer starting at byte offset 3 of the B-Tree 
        page. On page 1, this field is stored at byte offset 103.

      <p class=req id=FF0850>
        The 2-byte big-endian integer starting at byte offset 5 of each
        B-Tree page contains the byte-offset from the start of the page
        to the start of the <i>cell content area</i>, which consumes all space
        from this offset to the end of the usable region of the page.
	On page 1, this field is stored at byte offset 105.  All B-Tree 
        cells on the page are stored within the cell-content area.

      <p class=req id=FF0860>
        On each page used as an internal node a of B-Tree structures, the
        page number of the rightmost child node in the B-Tree structure is
        stored as a 4-byte big-endian unsigned integer beginning at byte
        offset 8 of the database page, or byte offset 108 on page 1.

      <p>
        This requirement describes the cell content offset array. It applies
        to both B-Tree variants.

      <p class=req id=FF0870>
        Immediately following the <i>page header</i> on each B-Tree page is the
        <i>cell offset array</i>, consisting of <i>N</i> 2-byte big-endian
        unsigned integers, where <i>N</i> is the number of cells stored
        on the B-Tree page (FF0840). On an internal node B-Tree page,
        the cell offset array begins at byte offset 12, or on a leaf
        page, byte offset 8. For the B-Tree node on page 1, these
        offsets are 112 and 108, respectively.

      <p class=req id=FF0880>
        The <i>cell offset array</i> and the <i>cell content area</i> (FF0850)
        may not overlap.

      <p class=req id=FF0890>
        Each value stored in the <i>cell offset array</i> must be greater
        than or equal to the offset to the <i>cell content area</i> (FF0850),
        and less than the database <i>page size</i>.

      <p class=req id=FF0900>
	The <i>N</i> values stored within the <i>cell offset array</i> are the
        byte offsets from the start of the B-Tree page to the beginning of 
        each of the <i>N</i> cells stored on the page.

      <p class=req id=FF0910>
	No two B-Tree cells may overlap.

      <p>
	The following requirements govern management of free-space within the
        page content area (both table and index B-Tree pages).

      <p class=req id=FF0920>
	Within the <i>cell content area</i>, all blocks of contiguous
	free-space (space not used by B-Tree cells) greater than 3 bytes in
        size are linked together into a linked list, the <i>free block list</i>.
        Such blocks of free space are known as <i>free blocks</i>.

      <p class=req id=FF0930>
        The first two bytes of each <i>free block</i> contain the offset
        of the next <i>free block</i> in the <i>free block list</i> formatted
	as a 2-byte big-endian integer, relative to the start of the database
        page. If there is no next <i>free block</i>, then the first two 
        bytes are set to 0x00.

      <p class=req id=FF0940>
        The second two bytes (byte offsets 2 and 3) of each <i>free block</i> 
        contain the total size of the <i>free block</i>, formatted as a 2-byte
        big-endian integer.

      <p class=req id=FF0950>
        On all B-Tree pages, the offset of the first <i>free block</i> in the 
        <i>free block list</i>, relative to the start of the database page,
        is stored as a 2-byte big-endian integer starting at byte offset 
        1 of the database page. If there is no first <i>free block</i> 
        (because the <i>free block list</i> is empty), then the two bytes
        at offsets 1 and 2 of the database page are set to 0x00. On page 1,
        this field is stored at byte offset 101 of the page.


      <p class=req id=FF0960>
        Within the cell-content area, all blocks of contiguous free-space 
        (space not used by B-Tree cells) less than or equal to 3 bytes in 
        size are known as <i>fragments</i>. The total size of all 
        <i>fragments</i> on a B-Tree page is stored as a 1-byte unsigned 
        integer at byte offset 7 of the database page. On page 1, this
        field is stored at byte offset 107.

          the values read from byte offsets 21 and 22 of the file header,
          respectively.
        <center><img src="images/fileformat/indexlongrecord.gif">
        <p><i>Figure <span class=fig id=figure_indexlongrecord></span> - 
          Large Record Index B-Tree Cell.</i>
        </center>

      <p class=req id=FF0970>
        Each B-Tree cell belonging to an internal node page of an index 
        B-Tree consists of a 4-byte big-endian unsigned integer, the 
        <i>child page number</i>, followed by a <i>variable length integer</i>
        field, followed by a <i>database record</i>. The 
        <i>variable length integer</i> field contains the length of the
        database record in bytes.

      <p class=req id=FF0980>
	Each B-Tree cell belonging to an leaf page of an index B-Tree 
        consists of a <i>variable length integer</i> field, followed by 
        a <i>database record</i>. The <i>variable length integer</i> field 
        contains the length of the database record in bytes.

      <p class=req id=FF0990>
	If the database record stored in an index B-Tree page is 
        sufficiently small, then the entire cell is stored within the 
        index B-Tree page.  Sufficiently small is defined as equal to or 
        less than <i>max-local</i>, where:
        <code>
            <i>max-local</i> := (<i>usable-size</i> - 12) * 64 / 255 - 23</code>
        
      <p class=req id=FF1000>
        If the database record stored as part of an index B-Tree cell is too
        large to be stored entirely within the B-Tree page (as defined by
        FF0520), then only a prefix of the <i>database record</i> is stored
        within the B-Tree page and the remainder stored in an <i>overflow
        chain</i>. In this case, the database record prefix is immediately
	followed by the page number of the first page of the 
        <i>overflow chain</i>, formatted as a 4-byte big-endian unsigned 
        integer.

      <p class=req id=FF1010>
        When a <i>database record</i> belonging to a table B-Tree cell is
        stored partially within an <i>overflow page chain</i>, the size
        of the prefix stored within the index B-Tree page is <i>N</i> bytes,
        where <i>N</i> is calculated using the following algorithm:
        <code>
            <i>min-local</i> := (<i>usable-size</i> - 12) * 32 / 255 - 23
            <i>max-local</i> := (<i>usable-size</i> - 12) * 64 / 255 - 23
            <i>N</i> := <i>min-local</i> + ((<i>record-size</i> - <i>min-local</i>) % (<i>usable-size</i> - 4))
            if( <i>N</i> &gt; <i>max-local</i> ) <i>N</i> := <i>min-local</i></code>

      <p>
        Requirements FF1010 and FF0990 are similar to the algorithms 
        presented in the text above. However instead of 
        <i>min-embedded-fraction</i> and <i>max-embedded-fraction</i> the
        requirements use the constant values 32 and 64, as well-formed 
        database files are required by FF0080 and FF0070 to store these 
        values in the relevant database file header fields.

    <h3 id=table_btrees>Table B-Trees</h3>
      <p>
        As noted in section <cite>fileformat_overview</cite>, table B-Trees
        store a set of unique 64-bit signed integer keys. Associated with
        each key is a database record. As with index B-Trees, the database

        Figure <cite>figure_tabletree</cite> depicts a table B-Tree containing
	a contiguous set of 14 integer keys starting with 1. Each key <i>n</i>
	has an associated database record R<i>n</i>. All the keys and their
	associated records are stored in the leaf pages. The internal node
	pages contain no database data, their only purpose is to provide
	a way to navigate the tree structure.

      <p class=req id=FF1020>
        The pages in a table B-Tree structures are arranged into a tree
        structure such that all leaf pages are at the same depth.

      <p class=req id=FF1030>
        Each leaf page in a table B-Tree structure contains one or more
        B-Tree cells, where each cell contains a 64-bit signed integer key
        value and a database record.

      <p class=req id=FF1040>
        Each internal node page in a table B-Tree structure contains one or 
        more B-Tree cells, where each cell contains a 64-bit signed integer 
	key value, <i>K</i>, and a child page number, <i>C</i>. All integer key
        values in all B-Tree cells within the sub-tree headed by page <i>C</i>
        are less than or equal to <i>K</i>. Additionally, unless <i>K</i>
        is the smallest integer key value stored on the internal node page,
        all integer keys within the sub-tree headed by <i>C</i> are greater
        than <i>K<sub>-1</sub></i>, where <i>K<sub>-1</sub></i> is the largest
        integer key on the internal node page that is smaller than <i>K</i>.

      <p class=req id=FF1050>
        As well as child page numbers associated with B-Tree cells, each
        internal node page in a table B-Tree contains the page number
        of an extra child page, the <i>right-child page</i>. All key values
        in all B-Tree cells within the sub-tree headed by the <i>right-child
        page</i> are greater than all key values stored within B-Tree cells
        on the internal node page.

          are SQL NULL. If the schema layer file-format is greater than
          2, then the values associated with the "missing" fields are 
	  determined by the default value of the associated database table 
          columns.
	  <span class=todo>Reference to CREATE TABLE syntax. How are default
          values determined?</span>

        <p class=req id=FF1060>
          In a well-formed database, each table B-Tree contains a single entry
          for each row in the corresponding logical database table.

        <p class=req id=FF1070>
	  The key value (a 64-bit signed integer) for each B-Tree entry is
	  the same as the value of the rowid field of the corresponding
          logical database row.

        <p class=req id=FF1080>
	  The SQL values serialized to make up each <i>database record</i>
	  stored as ancillary data in a table B-Tree shall be the equal to the
	  values taken by the <i>N</i> leftmost columns of the corresponding
	  logical database row, where <i>N</i> is the number of values in the
          database record.

        <p class=req id=FF1090>
	    If a logical database table column is declared as an "INTEGER
            PRIMARY KEY", then instead of its integer value, an SQL NULL
	    shall be stored in its place in any database records used as
            ancillary data in a table B-Tree.

        <p>The following database properties discuss table B-Tree records 
           with implicit (default) values.

          <p class=req id=FF1100>
	    If the database <i>schema layer file-format</i> (the value stored 
            as a 4-byte integer at byte offset 44 of the file header) is 1, 
            then all database records stored as ancillary data in a table 
            B-Tree structure have the same number of fields as there are 
            columns in the corresponding logical database table.

          <p class=req id=FF1110>
	    If the database <i>schema layer file-format</i> value is two or
            greater and the rightmost <i>M</i> columns of a row contain SQL NULL
            values, then the corresponding record stored as ancillary data in
	    the table B-Tree has between <i>N</i>-<i>M</i> and <i>N</i> fields,
	    where <i>N</i> is the number of columns in the logical database
            table.

          <p class=req id=FF1120>
	    If the database <i>schema layer file-format</i> value is three or
	    greater and the rightmost <i>M</i> columns of a row contain their
            default values according to the logical table declaration, then the
	    corresponding record stored as ancillary data in the table B-Tree
	    may have as few as <i>N</i>-<i>M</i> fields, where <i>N</i> is the
            number of columns in the logical database table.

              section <cite>table_btree_cell_format</cite> for details.
          <li>The format of page 1 is the same as any other table B-Tree,
              except that 100 bytes less than usual is available for content.
              The first 100 bytes of page 1 is consumed by the database
              file header.
        </ul>

      <p class=req id=FF1130>
	In a <i>well-formed database file</i>, the first byte of each page used
        as an internal node of a table B-Tree structure is set to 0x05.
      <p class=req id=FF1140>
	In a <i>well-formed database file</i>, the first byte of each page used
        as a leaf node of a table B-Tree structure is set to 0x0D.
        
      <p>
        Most of the requirements specified in section 
        <cite>index_btree_page_format</cite> also apply to table B-Tree 
        pages. The wording of the requirements make it clear when this is

          header).

        <p>
          The following requirements describe the format of table B-Tree 
          cells, and the distribution thereof between B-Tree and overflow
          pages.

        <p class=req id=FF1150>
          B-Tree cells belonging to table B-Tree internal node pages consist
	  of exactly two fields, a 4-byte big-endian unsigned integer
          immediately followed by a <i>variable length integer</i>. These
          fields contain the child page number and key value respectively
          (see FF1030).

        <p class=req id=FF1160>
          B-Tree cells belonging to table B-Tree leaf node pages consist
	  of three fields, two <i>variable length integer</i> values
          followed by a database record. The size of the database record
          in bytes is stored in the first of the two 
          <i>variable length integer</i> fields. The second of the two
          <i>variable length integer</i> fields contains the 64-bit signed 
          integer key (see FF1030).

        <p class=req id=FF1170>
          If the size of the record stored in a table B-Tree leaf page cell 
          is less than or equal to (<i>usable page size</i>-35) bytes, then 
          the entire cell is stored on the B-Tree leaf page. In a well-formed
	  database, <i>usable page size</i> is the same as the database 
          <i>page size</i>.

        <p class=req id=FF1180>
          If a table B-Tree cell is too large to be stored entirely on
          a leaf page (as defined by FF1170), then a prefix of the cell
          is stored on the leaf page, and the remainder stored in an
          <i>overflow page chain</i>. In this case the cell prefix
          stored on the B-Tree leaf page is immediately followed by a 
          4-byte big-endian unsigned integer containing the page number
          of the first overflow page in the chain.

        <p class=req id=FF1190>
          When a table B-Tree cell is stored partially in an 
          <i>overflow page chain</i>, the prefix stored on the B-Tree
          leaf page consists of the two <i>variable length integer</i> fields,
          followed by the first <i>N</i> bytes of the database record, where
          <i>N</i> is determined by the following algorithm:
      <code>
        <i>min-local</i> := (<i>usable-size</i> - 12) * 255 / 32 - 23
        <i>max-local</i> := (<i>usable-size</i> - 35)
        <i>N</i> := <i>min-local</i> + (<i>record-size</i> - <i>min-local</i>) % (<i>usable-size</i> - 4)
        if( <i>N</i> &gt; <i>max-local</i> ) N := <i>min-local</i>
      </code>
        
        <p>
          Requirement FF1190 is very similar to the algorithm presented in
          the text above. Instead of <i>min-embedded-fraction</i>, it uses
          the constant value 32, as well-formed database files are required
          by FF0090 to store this value in the relevant database file 
          header field.

    <h3 id="overflow_page_chains">Overflow Page Chains</h3>
      <p>
        Sometimes, a database record stored in either an index or table 
        B-Trees is too large to fit entirely within a B-Tree cell. In this
        case part of the record is stored within the B-Tree cell and the

      <p>
        Each overflow page except for the last one in the linked list 
        contains <i>available-space</i> bytes of record data. The last
        page in the list contains the remaining data, starting at byte
        offset 4. The value of the "next page" field on the last page
        in an overflow chain is undefined.

      <p class=req id=FF1200>
	A single <i>overflow page</i> may store up to <i>available-space</i>
        bytes of database record data, where <i>available-space</i> is equal
        to (<i>usable-size</i> - 4).

      <p class=req id=FF1210>
        When a database record is too large to store within a B-Tree page
        (see FF1170 and FF1000), a prefix of the record is stored within
        the B-Tree page and the remainder stored across <i>N</i> overflow
        pages. In this case <i>N</i> is the minimum number of pages required 
        to store the portion of the record not stored on the B-Tree page,
        given the maximum payload per overflow page defined by FF1200.

      <p class=req id=FF1220>
        The list of overflow pages used to store a single database record
        are linked together in a singly linked list known as an 
	<i>overflow chain</i>. The first four bytes of each page except the
        last in an <i>overflow chain</i> are used to store the page number
	of the next page in the linked list, formatted as an unsigned
        big-endian integer. The first four bytes of the last page in an
        <i>overflow chain</i> are set to 0x00.

      <p class=req id=FF1230>
        Each overflow page except the last in an <i>overflow chain</i> 
	contains <i>N</i> bytes of record data starting at byte offset 4 of
	the page, where <i>N</i> is the maximum payload per overflow page, 
        as defined by FF1200. The final page in an <i>overflow chain</i> 
        contains the remaining data, also starting at byte offset 4.

  <h2 id=free_page_list>The Free Page List</h2>
    <p>
      Sometimes, after deleting data from the database, SQLite removes pages
      from B-Tree structures. If these pages are not immediately required
      for some other purpose, they are placed on the free page list. The

    <p>
      All trunk pages in the free-list except for the first contain the 
      maximum possible number of references to leaf pages. <span class=todo>Is this actually true in an auto-vacuum capable database?</span> The page number
      of the first page in the linked list of free-list trunk pages is 
      stored as a 4-byte big-endian unsigned integer at offset 32 of the
      file header (section <cite>file_header</cite>).

    <p class=req id=FF1240>
      All <i>free pages</i> in a <i>well-formed database file</i> are part of
      the database <i>free page list</i>.

    <p class=req id=FF1250>
      Each free page is either a <i>free list trunk</i> page or a
      <i>free list leaf</i> page.

    <p class=req id=FF1260>
      All <i>free list trunk</i> pages are linked together into a singly
      linked list. The first 4 bytes of each page in the linked list 
      contains the page number of the next page in the list, formatted
      as an unsigned big-endian integer. The first 4 bytes of the last
      page in the linked list are set to 0x00.

    <p class=req id=FF1270>
      The second 4 bytes of each <i>free list trunk</i> page contains
      the number of </i>free list leaf</i> page numbers stored on the free list
      trunk page, formatted as an unsigned big-endian integer.

    <p class=req id=FF1280>
      Beginning at byte offset 8 of each <i>free list trunk</i> page are
      <i>N</i> page numbers, each formatted as a 4-byte unsigned big-endian 
      integers, where <i>N</i> is the value described in requirement FF1270.

    <p class=req id=FF1290>
      All page numbers stored on all <i>free list trunk</i> pages refer to
      database pages that are <i>free list leaves</i>.

    <p class=req id=FF1300>
      The page number of each <i>free list leaf</i> page in a well-formed
      database file appears exactly once within the set of pages numbers
      stored on <i>free list trunk</i> pages.

    <p>The following statements govern the two 4-byte big-endian integers
       associated with the <i>free page list</i> structure in the database
       file header.

    <p class=req id=FF1310>
      The total number of pages in the free list, including all <i>free list
      trunk</i> and <i>free list leaf</i> pages, is stored as a 4-byte unsigned
      big-endian integer at offset 36 of the database file header.

    <p class=req id=FF1320>
      The page number of the first page in the linked list of <i>free list
      trunk</i> pages is stored as a 4-byte big-endian unsigned integer at
      offset 32 of the database file header. If there are no <i>free list
      trunk</i> pages in the database file, then the value stored at
      offset 32 of the database file header is 0.
  

      table entries for the <i>num-entries</i> pages that follow it in the
      database file:
    <pre>
        <i>pointer-map-page-number</i> := 2 + <i>n</i> * <i>num-entries</i>
</pre>


    <p class=req id=FF1330>
      Non auto-vacuum databases do not contain pointer map pages.

    <p class=req id=FF1340>
      In an auto-vacuum database file, every <i>(num-entries + 1)</i>th 
      page beginning with page 2 is designated a pointer-map page, where 
      <i>num-entries</i> is calculated as:
      <code>
        <i>num-entries</i> := <i>database-usable-page-size</i> / 5
      </code>

    <p class=req id=FF1350>
      In an auto-vacuum database file, each pointer-map page contains
      a pointer map entry for each of the <i>num-entries</i> (defined by 
      FF1340) pages that follow it, if they exist.

    <p class=req id=FF1360>
      Each pointer-map page entry consists of a 1-byte page type and a 
      4-byte page parent number, 5 bytes in total.

    <p class=req id=FF1370>
	Pointer-map entries are packed into the pointer-map page in order,
        starting at offset 0. The entry associated with the database 
        page that immediately follows the pointer-map page is located at
        offset 0. The entry for the following page at offset 5 etc.

    <p>
      The following requirements govern the content of pointer-map entries.

    <p class=req id=FF1380>
	For each page except page 1 in an auto-vacuum database file that is 
	the root page of a B-Tree structure, the page type of the 
      corresponding pointer-map entry is set to the value 0x01 and the
      parent page number is zero.
    <p class=req id=FF1390>
	For each page that is a part of an auto-vacuum database file free-list, 
	the page type of the corresponding pointer-map entry is set to the
      value 0x02 and the parent page number is zero.
    <p class=req id=FF1400>
	For each page in a well-formed auto-vacuum database that is the first
	page in an overflow chain, the page type of the corresponding
      pointer-map entry is set to 0x03 and the parent page number field
      is set to the page number of the B-Tree page that contains the start
      of the B-Tree cell stored in the overflow-chain.
    <p class=req id=FF1410>
	For each page that is the second or a subsequent page in an overflow
	chain, the page type of the corresponding pointer-map entry is set to
	0x04 and the parent page number field is set to the page number of the
      preceding page in the overflow chain.
    <p class=req id=FF1420>
	For each page that is not a root page but is a part of a B-Tree tree
	structure (not part of an overflow chain), the page type of the
	corresponding pointer-map entry is set to the value 0x05 and the parent
	page number field is set to the page number of the parent node in the
        B-Tree structure.

      is to determine the <i>page-size</i> used by the database file. 
      Because it is not possible to be certain as to the <i>page-size</i> 
      without holding at least a <i>shared lock</i> on the database file
      (because some other <i>database connection</i> might have changed it
      since the <i>database file header</i> was read), the value read from the
      <i>database file header</i> is known as the <i>expected page size</i>. 

    <p class=req id=FS0060>
      When a new <i>database connection</i> is required, SQLite shall attempt
      to open a file-handle on the database file. If the attempt fails, then
      no new <i>database connection</i> is created and an error returned.

    <p class=req id=FS0070>
      When a new <i>database connection</i> is required, after opening the
      new file-handle, SQLite shall attempt to read the first 100 bytes
      of the database file. If the attempt fails for any other reason than
      that the opened file is less than 100 bytes in size, then 
      the file-handle is closed, no new <i>database connection</i> is created
      and an error returned instead.

    <p class=req id=FS0080>
      If the <i>database file header</i> is successfully read from a newly
      opened database file, the connections <i>expected page-size</i> shall 
      be set to the value stored in the <i>page-size field</i> of the 
      database header.

    <p class=req id=FS0090>
      If the <i>database file header</i> cannot be read from a newly opened 
      database file (because the file is less than 100 bytes in size), the 
      connections <i>expected page-size</i> shall be set to the compile time
      value of the SQLITE_DEFAULT_PAGESIZE option.


  <h2>Closing a Connection</h2>

    pages are read one at a time. SQLite never reads partial pages and
    always uses a single call to xRead() for each required page. After
    reading the data for a database page, SQLite adds it to the connections
    <i>page cache</i> so that it does not have to be read if required
    again. Refer to section <cite>page_cache_algorithms</cite> for a
    description of how this affects the IO performed by SQLite.

  <p class=req id=FS0010>
    Except for the read operation required by FS0070 and those reads made
    as part of opening a read-only transaction, SQLite shall only read
    data from a <i>database connection</i> while the 
    <i>database connection</i> has an open read-only or read/write transaction.

  <p>
    In the above requirement, reading data from a database connection
    includes retrieving data from the connections <i>page cache</i>.

  <p class=req id=FS0020>
    Aside from those read operations described by FS0070 and FSXXXX, SQLite
    shall read data from the database in aligned blocks of <i>page-size</i>
    bytes, where <i>page-size</i> is the database page size used by the
    database file.

  <h2 id=open_read_only_trans>Opening a Read-Only Transaction</h2>
    <p>
      Before data may be read from a <i>database connection</i>, a 

      enumerated above. If this happens, then the <i>shared-lock</i> is 
      released (if it was obtained) and an error returned to the user. 
      Step 2 of the procedure above is described in more detail in section
      <cite>hot_journal_detection</cite>. Section <cite>cache_validation</cite>
      describes the process identified by step 3 above. Further detail
      on step 4 may be found in section <cite>read_page_one</cite>.

    <p class=req id=FS0100>
      When required to open a <i>read-only transaction</i> using a 
      <i>database connection</i>, SQLite shall first attempt to obtain 
      a <i>shared-lock</i> on the file-handle open on the database file.

    <p class=req id=FS0110>
      If, while opening a <i>read-only transaction</i>, SQLite fails to obtain
      the <i>shared-lock</i> on the database file, then the process is
      abandoned, no transaction is opened and an error returned to the user.

    <p>
      The most common reason an attempt to obtain a <i>shared-lock</i> may
      fail is that some other connection is holding an <i>exclusive</i> or
      <i>pending lock</i>. However it may also fail because some other
      error (e.g. IO, comms related) occurs within the call to the xLock()
      method.

    <p class=req id=FS0030>
      While opening a <i>read-only transaction</i>, after successfully
      obtaining a <i>shared lock</i> on the database file, SQLite shall 
      attempt to detect and roll back a <i>hot journal file</i> associated 
      with the same database file.

    <p class=req id=FS0120>
      If, while opening a <i>read-only transaction</i>, SQLite encounters
      an error while attempting to detect or roll back a <i>hot journal
      file</i>, then the <i>shared-lock</i> on the database file is released,
      no transaction is opened and an error returned to the user.

    <p>
      Section <cite>hot_journal_detection</cite> contains a description of
      and requirements governing the detection of a hot-journal file refered
      to in the above requirements.

    <p class=req id=FS0040>
      Assuming no errors have occured, then after attempting to detect and
      roll back a <i>hot journal file</i>, if the connections 
      <i>page cache</i> is not empty, then SQLite shall validate the contents 
      of the <i>page cache</i> by testing the <i>file change counter</i>. 
      This procedure is known as <i>cache validiation</i>.

    <p class=req id=FS0050>
      If the contents of the <i>page cache</i> are found to be invalid by
      the check prescribed by F20040, SQLite shall discard the cache contents
      before continuing.
      

  <h3 id=hot_journal_detection>Hot Journal Detection</h3>
    <p>

            <cite>rollback</cite>).
      </ol>

    <p>
      The following requirements describe step 1 of the above procedure in
      more detail.

    <p class=req id=FS0140>
      When required to attempt to detect a <i>hot-journal file</i>, SQLite
      shall first use the xAccess() method of the VFS layer to check if a
      journal file exists in the file-system.

    <p class=req id=FS0150>
      When required to attempt to detect a <i>hot-journal file</i>, if the
      call to xAccess() required by FS0140 indicates that a journal file does
      not exist, then the attempt to detect a <i>hot-journal file</i> is
      finished. A <i>hot-journal file</i> was not detected.

    <p>
      The following requirements describe step 2 of the above procedure in
      more detail.

    <p class=req id=FS0160>
      When required to attempt to detect a <i>hot-journal file</i>, if the
      call to xAccess() required by FS0140 indicates that a journal file
      is present, then the xCheckReservedLock() method of the database file
      file-handle is invoked to determine whether or not some other 
      process is holding a <i>reserved</i> or greater lock on the database 
      file.

    <p class=req id=FS0170>
      If the call to xCheckReservedLock() required by FS0160 indicates that
      some other <i>database connection</i> is holding a <i>reserved</i>
      or greater lock on the database file, 

    <p class=todo>
      Finish this section.

  <h3 id=cache_validation>Cache Validation</h3>

      opening a new <i>read-only transaction</i>, SQLite checks the value
      of the <i>file change counter</i> stored in the database file. If the
      value has not changed since the database file was unlocked, then the
      contents of the <i>page cache</i> can be trusted. If the value has
      changed, then the <i>page cache</i> cannot be trusted and all data
      is discarded.
   
    <p class=req id=FS0180>
      When a file-handle open on a database file is unlocked, if the
      <i>page cache</i> belonging to the associated <i>database connection</i>
      is not empty, SQLite shall store the value of the <i>file change
      counter</i> internally.

    <p class=req id=FS0190>
      When required to perform <i>cache validation</i> as part of opening
      a <i>read transaction</i>, SQLite shall read a 16 byte block 
      starting at byte offset 24 of the <i>database file</i> using the xRead()
      method of the <i>database connections</i> file handle.

    <p class=todo>
      Why a 16 byte block? Why not 4? (something to do with encrypted
      databases).

    <p class=req id=FS0200>
      While performing <i>cache validation</i>, after loading the 16 byte
      block as required by FS0190, SQLite shall compare the 32-bit big-endian
      integer stored in the first 4 bytes of the block to the most
      recently stored value of the <i>file change counter</i> (see FS0180).
      If the values are not the same, then SQLite shall conclude that
      the contents of the cache are invalid.

    <p>
      Requirement FS0050 (section <cite>open_read_only_trans</cite>) 
      specifies the action SQLite is required to take upon determining that 
      the cache contents are invalid.


  <h3 id=read_page_one>Page 1 and the Expected Page Size</h3>
    <p>
      As the last step in opening a <i>read transaction</i> on a database

      the connections <i>expected page size</i>. The <i>expected page size</i>
      is the value of the <i>page-size</i> field read from the 
      <i>database file header</i> while opening the database connection 
      (see section <cite>open_new_connection</cite>), or the <i>page-size</i>
      stored of the database file when the most <i>read transaction</i> was
      concluded.

    <p class=req id=FS0210>
      During the conclusing of a <i>read transaction</i>, before unlocking
      the database file, SQLite shall set the connections 
      <i>expected page size</i> to the current database <i>page-size</i>.

    <p class=req id=FS0220>
      As part of opening a new <i>read transaction</i>, immediately after 
      performing <i>cache validation</i>, if there is no data for database
      page 1 in the <i>page cache</i>, SQLite shall read <i>N</i> bytes from
      the start of the database file using the xRead() method of the 
      connections file handle, where <i>N</i> is the connections current 
      <i>expected page size</i> value.

    <p class=req id=FS0230>
      If page 1 data is read as required by FS0230, then the value of the
      <i>page-size</i> field that appears in the database file header that
      consumes the first 100 bytes of the read block is not the same as the
      connections current <i>expected page size</i>, then the 
      <i>expected page size</i> is set to this value, the database file is
      unlocked and the entire procedure to open a <i>read transaction</i>
      is repeated.

    <p class=req id=FS0240>
      If page 1 data is read as required by FS0230, then the value of the
      <i>page-size</i> field that appears in the database file header that
      consumes the first 100 bytes of the read block is the same as the
      connections current <i>expected page size</i>, then the block of data
      read is added to the connections <i>page cache</i> as page 1.

  <h2>Ending a Read-only Transaction</h2>
    <p>
      To end a <i>read-only transaction</i>, SQLite simply relinquishes the
      <i>shared lock</i> on the file-handle open on the database file. No
      other action is required.

    <p class=req id=FS0130>
      When required to end a <i>read-only transaction</i>, SQLite shall
      relinquish the <i>shared lock</i> held on the database file by
      calling the xUnlock() method of the file-handle.

    <p>
      See also requirements FS0180 and FS0210 above.

<h1 id=writing_data>Writing Data</h1>
  <p>
    Safely writing data to a database file is also a complex procedure. The
    database file must be updated in such a way that if a power failure,
    operating system crash or application fault occurs while SQLite is midway
    through writing to the database file the database contents are still

    started may be required. This may occur if the user explicitly requests
    transaction rollback (i.e. by issuing a "ROLLBACK" command), or 
    automatically, as a result of encountering an SQL constraint (see
    <cite>sql_sqlitert_requirements</cite>). For this reason, the original
    page content is stored in the <i>journal file</i> before the page is 
    even modified within the <i>page cache</i>.

  <p class=req id=FS0250>
    Before modifying or adding any in-memory <i>page cache</i> pages in 
    preparation for writing to the <i>database file</i>, the 
    <i>database connection</i> shall open a <i>write transaction</i> on 
    the database file.

  <p class=req id=FS0260>
    Before modifying the <i>page cache</i> image of a database page that
    existed and was not a <i>free-list leaf</i> page when the current <i>write
    transaction</i> began, SQLite shall ensure that the original page content
    has been written to the journal file (<i>journalled</i>).

  <p class=todo>
    If the sector size is larger than the page-size, coresident pages must

          the VFS xOpen method), and a <i>journal file header</i> written 
          to the start of it using a single call to the file handles xWrite 
          method.
    </ol>

    <p>
      Requirements describing step 1 of the above procedure in detail:
    <p class=req id=FS0350>
      When required to open a <i>write transaction</i> on the database, 
      SQLite shall first open a <i>read transaction</i>, if the <i>database
      connection</i> in question has not already opened one.

    <p class=req id=FS0360>
      When required to open a <i>write transaction</i> on the database, after
      ensuring a <i>read transaction</i> has already been opened, SQLite 
      shall obtain a <i>reserved lock</i> on the database file by calling
      the xLock method of the file-handle open on the database file.

    <p>
      Requirements describing step 2 of the above procedure in detail:

    <p class=req id=FS0370>
      When required to open a <i>write transaction</i> on the database, after
      obtaining a <i>reserved lock</i> on the database file, SQLite shall
      open a read/write file-handle on the corresponding <i>journal file</i>.

    <p class=req id=FS0380>
      When required to open a <i>write transaction</i> on the database, after
      opening a file-handle on the <i>journal file</i>, SQLite shall write
      a <i>journal header</i> into the first <i>sector-size</i> bytes of the
      journal file, using single call to the xWrite method of the recently
      opened file-handle.

    <p>
      Requirements describing the <i>journal header</i> written to 
      the <i>journal file</i>:

    <p class=req id=FS0390>
      The first 8 bytes of the <i>journal header</i> required to be written
      by FS0380 shall be:
    

    <p class=todo>
      Reqirements describing the details of opening a <i>write transaction</i>.

    <p class=todo>
      Reqirement for error handling?

      back.

    <p>
      A page is journalled by adding a <i>journal record</i> to the <i>
      journal file</i>. The format of a <i>journal record</i> is described
      in section <cite>journal_record_format</cite>.

    <p class=req id=FS0270>
      When required to <i>journal a database page</i>, SQLite shall first
      append the <i>page number</i> of the page being journalled to the
      <i>journal file</i>, formatted as a 4-byte big-endian unsigned integer,
      using a single call to the xWrite method of the file-handle opened
      on the journal file.

    <p class=req id=FS0280>
      When required to <i>journal a database page</i>, if the attempt to 
      append the <i>page number</i> to the journal file is successful, 
      then the current page data (<i>page-size</i> bytes) shall be appended 
      to the journal file, using a single call to the xWrite method of the 
      file-handle opened on the journal file.

    <p class=req id=FS0290>
      When required to <i>journal a database page</i>, if the attempt to 
      append the current page data to the journal file is successful, 
      then SQLite shall append a 4-byte big-endian integer checksum value 
      to the to the journal file, using a single call to the xWrite method 
      of the file-handle opened on the journal file.

    <p>
      The checksum value written to the <i>journal file</i> immediately after
      the page data (requirement FS0290), is a function of both the page
      data and the <i>checksum initializer</i> field stored in the 
      <i>journal header</i> (see section <cite>journal_header_format</cite>).
      Specifically, it is the sum of the <i>checksum initializer</i> and
      the value of every 200th byte of page data interpreted as an 8-bit
      unsigned integer, starting with the (<i>page-size</i> % 200)'th 
      byte of page data. For example, if the <i>page-size</i> is 1024 bytes,
      then a checksum is calculated by adding the values of the bytes at
      offsets 23, 223, 423, 623, 823 and 1023 (the last byte of the page)
      together with the value of the <i>checksum initializer</i>.

    <p class=req id=FS0300>
      The checksum value written to the <i>journal file</i> by the write
      required by FS0290 shall be equal to the sum of the <i>checksum
      initializer</i> field stored in the <i>journal header</i> (FS0XXX) and
      every 200th byte of the page data, beginning with the 
      (<i>page-size</i> % 200)th byte.

    <p>
      The '%' character is used in the two paragraphs to represent the modulo
      operator, just as it is in programming languages such as C, Java and
      Javascript.

      prevent any other <i>database connection</i> from reading the database
      after a subset of the modifications that have been or will be made by
      a <i>write transaction</i> have been written into the database file.

    <p class=todo>
      Journal header operations?

    <p class=req id=FS0310>
      Unless a <i>pending</i> or <i>exclusive</i> lock has already been
      obtained, when SQLite is required to <i>write out a page cache</i>, it
      shall first upgrade the lock on the database file to a 
      <i>pending lock</i> using a call to the xLock method of the file-handle
      open on the database file.

    <p class=req id=FS0320>
      Unless one has already been obtained, when SQLite is required to 
      <i>write out a page cache</i>, after successfully obtaining a 
      <i>pending lock</i> it shall upgrade the lock on the database file 
      to an <i>exclusive lock</i> using a call to the xLock method of the
      file-handle open on the database file.

    <p class=todo>
      If obtaining the lock fails?

    <p class=req id=FS0330>
      When SQLite is required to <i>write out a page cache</i>, if the 
      required <i>exclusive lock</i> is already held or successfully 
      obtained, SQLite shall copy the contents of all pages that have been
      modified within the <i>page cache</i> to the database file, using a
      single write of <i>page-size</i> bytes for each.

    <p class=req id=FS0340>
      When the modified contents of a <i>page cache</i> is copied into the
      database file, as required by FS0330, the write operations shall 
      occur in <i>page number</i> order, from lowest to highest.

    <p>
      The above requirement to write data to the database file in the order
      in which it occurs in the file is added to improve performance. On
      many systems, sorting the regions of the file to be written before 
      writing to them allows the storage hardware to operate more efficiently.