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    <p>I've never used SQLite, but I've spent a decent amount of time reading its docs because I plan on using it in the future (and the docs are interesting). I'd say that thread safety is independent of whether multiple processes can access the same database file at once. SQLite, regardless of what threading mode it is in, will <a href="http://www.sqlite.org/lockingv3.html" rel="noreferrer">lock the database file</a>, so that multiple processes can read from the database at once but only one can write.</p> <p><a href="http://www.sqlite.org/threadsafe.html" rel="noreferrer">Thread safety</a> only affects how your process can use SQLite. Without any thread safety, you can only call SQLite functions from one thread. But it should still, say, take an EXCLUSIVE lock before writing, so that other processes can't corrupt the database file. Thread safety just protects data in your process's memory from getting corrupted if you use multiple threads. So I don't think you ever need to worry about what another process (in this case iOS) is doing with an SQLite database.</p> <p><strong>Edit:</strong> To clarify, any time you write to the database, including a plain <code>INSERT</code>/<code>UPDATE</code>/<code>DELETE</code>, it will automatically take an EXCLUSIVE lock, write to the database, then release the lock. (And it actually takes a SHARED lock, then a RESERVED lock, then a PENDING lock, then an EXCLUSIVE lock before writing.) By default, if the database is already locked (say from another process), then SQLite will return SQLITE_BUSY without waiting. You can call <a href="http://www.sqlite.org/c3ref/busy_timeout.html" rel="noreferrer"><code>sqlite3_busy_timeout()</code></a> to tell it to wait longer.</p>
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    1. COWhen you say "*it should still, say, take an EXCLUSIVE lock before writing*", do you mean that SQLite will automatically do this for me? Or that I (or the iOS SMS framework) am responsible for this? For example, do I (or iOS) need to issue a `BEGIN EXCLUSIVE TRANSACTION;` statement before writing?
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    2. COI added an edit. `BEGIN EXCLUSIVE TRANSACTION` is for taking an EXCLUSIVE lock at the beginning of a transaction. By default, SQLite waits as long as it can. From the docs: *Note that the BEGIN command does not acquire any locks on the database. After a BEGIN command, a SHARED lock will be acquired when the first SELECT statement is executed. A RESERVED lock will be acquired when the first INSERT, UPDATE, or DELETE statement is executed.* ...
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    3. CO... *No EXCLUSIVE lock is acquired until either the memory cache fills up and must be spilled to disk or until the transaction commits. In this way, the system delays blocking read access to the file file until the last possible moment.* Also, even if you don't use `BEGIN TRANSACTION`, all statements are still in an implicit transaction.
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