Documentation Source Text

serialize concurrent access attempts on a per-table basis. Tables support 
two types of locks, "read-locks" and "write-locks". Locks are granted to
connections - at any one time, each database connection has either a
read-lock, write-lock or no lock on each database table.
</p>

<p>At any one time, a single table may have any number of active read-locks
or a single active write lock. To read data from a table, a connection must 
first obtain a read-lock. To write to a table, a connection must obtain a 
write-lock on that table. If a required table lock cannot be obtained,
the query fails and SQLITE_LOCKED is returned to the caller.
</p> 

<p>Once a connection obtains a table lock, it is not released until the
current transaction (read or write) is concluded.

<h1>Shared Cache And In-Memory Databases</h1>

<p>
Beginning with SQLite [version 3.7.13] ([dateof:3.7.13]), 
shared cache can be used on
[in-memory databases], provided that the database is created using
a [URI filename].  For backwards compatibility, shared cache is always
disabled for in-memory
databases if the unadorned name ":memory:" is used to open the database.
Prior to version 3.7.13, shared cache was always
disabled for in-memory databases regardless of the database name used,
current system shared cache setting, or query parameters or flags.
</p>

<p>
Enabling shared-cache for an in-memory database allows two or more
database connections in the same process to have access to the same
in-memory database.  An in-memory database in shared cache is automatically
deleted and memory is reclaimed when the last connection to that database
closes.
</p>