Constant SQLITE_RESERVED_SPACE_OFFSET 

pub const SQLITE_RESERVED_SPACE_OFFSET: usize = 20;

Byte offset of the “reserved space per page” field in the SQLite header.

1-byte unsigned integer (file-format §1.3). Standard SQLite leaves this 0; a non-zero value is claimed by page-level extensions — encryption (SQLCipher/SEE) or a checksum VFS — and reduces the usable bytes per page.