Documentation Source Text

then the two processes will be talking to different database files with 
the same name.  (Note that this is only possible on Posix and Posix-like
systems that permit a file to be unlinked while it is still open for
reading and writing.  Windows does not allow this to occur.)
Since rollback journals and WAL files are based on the name of the database
file, the two different database files will share the same rollback
journal or WAL file.  A rollback or recovery for one of the databases
might use content from the other database, resulting in corruption.

A similar problem occurs if a database file is renamed while it is
opened and a new file is created with the old name.</p>

<p>In other words, unlinking or renaming an open database file 
results in behavior that is undefined and probably undesirable.</p>

<p>Beginning with SQLite [version 3.7.17], the unix OS interface will
send SQLITE_WARNING messages to the [error log] if a database file is unlinked
while it is still in use.</p>

<tcl>hd_fragment alias {database filename aliasing}</tcl>
<h3>2.5 Multiple links to the same file</h3>

<p>If a single database file has multiple links (either hard or soft links)
then that is just another way of saying that the file has multiple names.
If two or more processes open the database using different names, then
they will use different rollback journals and WAL files.  That means that
if one process crashes, the other process will be unable to recover the
transaction in progress because it will be looking in the wrong place
for the appropriate journal.</p>

<p>In other words, opening and using a database file that has two or
more names results in behavior that is undefined and probably undesirable.</p>

<p>Beginning with SQLite [version 3.7.17], the unix OS interface will
send SQLITE_WARNING messages to the [error log] if a database file has 
multiple hard links.</p>


<p>Beginning with SQLite [version 3.10.0], the unix OS interface will
attempt to resolve symbolic links and open the database file by its
canonical name.  Prior to version 3.10.0, opening a database file 
through a symbolic link was similar to opening a database file
that had multiple hard links and resulted in undefined behavior.</p>

<h2>3.0 Failure to sync</h2>

<p>In order to guarantee that database files are always consistent, SQLite
will occasionally ask the operating system to flush all pending writes to
persistent storage then wait for that flush to complete.  This is 
accomplished using the <tt>fsync()</tt> system call under unix and

    <p>^When a new database is created, SQLite assigned a page size to
    the database based on platform and filesystem.  For many years,
    the default page size was almost always 1024 bytes, but beginning
    with SQLite [version 3.12.0] in 2016, the default page size increased
    to 4096.

    <p>^The page_size pragma will only cause an immediate change in the
    page size if it is issued while the database is still empty (prior 
    to the first CREATE statement) and if the database is not in
    [WAL mode].  ^(If the page_size pragma is
    used to specify a new page size just prior to
    running the [VACUUM] command and if the database is not in
    [WAL | WAL journal mode] then [VACUUM] will change the page
    size to the new value.)^</p>

    <p>^The [SQLITE_DEFAULT_PAGE_SIZE] compile-time option can be used
    to change the default page size assigned to new databases.