Documentation Source Text

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]
([dateof:3.7.1]), 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 is 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.</p>

<tcl>hd_fragment vnums {file format version numbers}</tcl>
<h3>File format version numbers</h3>

................................................................................
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.</p>

<tcl>hd_fragment usable_size {usable size}</tcl>
<p>The "usable size" of a database page is the page size specify 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.)^</p>

<h3>Payload fractions</h3>
................................................................................

<p>A database file might contain one or more pages that are not in
active use.  Unused pages can come about, for example, when information
is deleted from the database.  Unused pages are stored on the freelist
and are reused when additional pages are required.</p>

<p>The freelist is organized as a linked list of freelist trunk pages
with each trunk pages containing page numbers for zero or more freelist
leaf pages.</p>

<p>A freelist trunk page consists of an array of 4-byte big-endian integers.
The size of the array is as many integers as will fit in the usable space
of a page.  The minimum usable space is 480 bytes so the array will always
be at least 120 entries in length.  ^The first integer on a freelist trunk
page is the page number of the next freelist trunk page in the list or zero 
................................................................................
values. 
A varint is between 1 and 9 bytes in length.  The varint consists of either
zero or more bytes which have the high-order bit set followed by a single byte
with the high-order bit clear, or nine bytes, whichever is shorter.
The lower seven bits of each of the first eight bytes and all 8 bits of
the ninth byte are used to reconstruct the 64-bit twos-complement integer.
Varints are big-endian: bits taken from the earlier byte of the varint
are the more significant than bits taken from the later bytes. </p>

<p>The format of a cell depends on which kind of b-tree page the cell
appears on.  The following table shows the elements of a cell, in
order of appearance, for the various b-tree page types.

<dl>
<dt><p>Table B-Tree Leaf Cell (header 0x0d):</p></dt>
................................................................................
WAL".  ^At the start of the first new
write transaction, the WAL header salt-1 value is incremented
and the salt-2 value is randomized.  These changes to the salts invalidate
old frames in the WAL that have already been checkpointed but not yet
overwritten, and prevent them from being checkpointed again.</p>

<p>The WAL file can optionally be truncated on a reset, but it need not be.
Performance is usually a little better if the WAL is not truncate, since
filesystems generally will overwrite an existing file faster than they
will grow a file.

<tcl>hd_fragment walread {WAL read algorithm}</tcl>
<h2>Reader Algorithm</h2>

<p>^(To read a page from the database (call it page number P), a reader

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]
([dateof:3.7.1]), 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.</p>

<tcl>hd_fragment vnums {file format version numbers}</tcl>
<h3>File format version numbers</h3>

................................................................................
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.</p>

<tcl>hd_fragment usable_size {usable size}</tcl>
<p>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.)^</p>

<h3>Payload fractions</h3>
................................................................................

<p>A database file might contain one or more pages that are not in
active use.  Unused pages can come about, for example, when information
is deleted from the database.  Unused pages are stored on the freelist
and are reused when additional pages are required.</p>

<p>The freelist is organized as a linked list of freelist trunk pages
with each trunk page containing page numbers for zero or more freelist
leaf pages.</p>

<p>A freelist trunk page consists of an array of 4-byte big-endian integers.
The size of the array is as many integers as will fit in the usable space
of a page.  The minimum usable space is 480 bytes so the array will always
be at least 120 entries in length.  ^The first integer on a freelist trunk
page is the page number of the next freelist trunk page in the list or zero 
................................................................................
values. 
A varint is between 1 and 9 bytes in length.  The varint consists of either
zero or more bytes which have the high-order bit set followed by a single byte
with the high-order bit clear, or nine bytes, whichever is shorter.
The lower seven bits of each of the first eight bytes and all 8 bits of
the ninth byte are used to reconstruct the 64-bit twos-complement integer.
Varints are big-endian: bits taken from the earlier byte of the varint
are more significant than bits taken from the later bytes. </p>

<p>The format of a cell depends on which kind of b-tree page the cell
appears on.  The following table shows the elements of a cell, in
order of appearance, for the various b-tree page types.

<dl>
<dt><p>Table B-Tree Leaf Cell (header 0x0d):</p></dt>
................................................................................
WAL".  ^At the start of the first new
write transaction, the WAL header salt-1 value is incremented
and the salt-2 value is randomized.  These changes to the salts invalidate
old frames in the WAL that have already been checkpointed but not yet
overwritten, and prevent them from being checkpointed again.</p>

<p>The WAL file can optionally be truncated on a reset, but it need not be.
Performance is usually a little better if the WAL is not truncated, since
filesystems generally will overwrite an existing file faster than they
will grow a file.

<tcl>hd_fragment walread {WAL read algorithm}</tcl>
<h2>Reader Algorithm</h2>

<p>^(To read a page from the database (call it page number P), a reader