SQLite supports three different threading modes:
Single-thread. In this mode, all mutexes are disabled and SQLite is unsafe to use in more than a single thread at once.
Multi-thread. In this mode, SQLite can be safely used by multiple threads provided that no single database connection is used simultaneously in two or more threads.
Serialized. In serialized mode, SQLite can be safely used by multiple threads with no restriction.
The threading mode can be selected at compile-time (when the SQLite library is being compiled from source code) or at start-time (when the application that intends to use SQLite is initializing) or at run-time (when a new SQLite database connection is being created). Generally speaking, run-time overrides start-time and start-time overrides compile-time. Except, single-thread mode cannot be overridden once selected.
The default mode is serialized.
Use the SQLITE_THREADSAFE compile-time parameter to select the threading mode. If no SQLITE_THREADSAFE compile-time parameter is present, then serialized mode is used. This can be made explicit with -DSQLITE_THREADSAFE=1. With -DSQLITE_THREADSAFE=0 the threading mode is single-thread. With -DSQLITE_THREADSAFE=2 the threading mode is multi-thread.
The return value of the sqlite3_threadsafe() interface is the value of SQLITE_THREADSAFE set at compile-time. It does not reflect changes to the threading mode made at runtime via the sqlite3_config() interface or by flags given as the third argument to sqlite3_open_v2().
If single-thread mode is selected at compile-time, then critical mutexing logic is omitted from the build and it is impossible to enable either multi-thread or serialized modes at start-time or run-time.
Assuming that the compile-time threading mode is not single-thread, then the threading mode can be changed during initialization using the sqlite3_config() interface. The SQLITE_CONFIG_SINGLETHREAD verb puts SQLite into single-thread mode, the SQLITE_CONFIG_MULTITHREAD verb sets multi-thread mode, and the SQLITE_CONFIG_SERIALIZED verb sets serialized mode.
If single-thread mode has not been selected at compile-time or start-time, then individual database connections can be created as either multi-thread or serialized. It is not possible to downgrade an individual database connection to single-thread mode. Nor is it possible to escalate an individual database connection if the compile-time or start-time mode is single-thread.
The threading mode for an individual database connection is determined by flags given as the third argument to sqlite3_open_v2(). The SQLITE_OPEN_NOMUTEX flag causes the database connection to be in the multi-thread mode and the SQLITE_OPEN_FULLMUTEX flag causes the connection to be in serialized mode. If neither flag is specified or if sqlite3_open() or sqlite3_open16() are used instead of sqlite3_open_v2(), then the default mode determined by the compile-time and start-time settings is used.