SQLITE和多线程

sqlite支持三种不同的线程模式：

1)单线程single-thread。这种模式中，所有的互斥锁都失效，并且sqlite超过一个线程就不能安全的使用。

2)多线程multi-thread。这种模式中，sqlite可以安全的被多个线程使用，假如此时在两个或者更多线程中同时使用不止一个database的连接。只要此时没有两个或者更多的线程同时使用一个database的连接。

3)串行化serialized。在串行化模式中，sqlite可以不受限制的使用安全的被多个线程使用。

线程模式可以在编译阶段(当sqlite库从源码中进行编译时)，或者在启动阶段(当准备使用sqlite的应用程序正在初始化时)，或者运行阶段(当新的sqlite数据库连接正在被创建时)。一般上来讲，运行阶段可以覆盖启动阶段，而启动阶段可以覆盖编译阶段。除了，单线程模式一旦选定就不能被覆盖更改。

默认模式是串行化模式。

编译阶段线程模式的选择

使用SQLITE_THREADSAFE的编译阶段参数来选择线程模式。如果不存在SQLITE_THREADSAFE的编译阶段参数，那么使用serialized mode。可以通过使用-DSQLITE_THREADSAFE=1显式的指定。使用-DSQLITE_THREADSAFE=1则线程模式是single-thread.使用-DSQLITE_THREADSAFE=2 则线程模式是multi-thread.

sqlite3_threadsafe()接口的返回值是由编译阶段线程模式的选择来决定的。如果在编译阶段选择了single-thread模式，那么sqlite3_threadsafe()返回false。如果选择的是multi-thread或者serialized，那么sqlite3_threadsafe()返回true。sqlite3_threadsafe()接口早于multi-thread模式和启动阶段以及运行阶段的模式选择，所以不能区分multi-thread和serialized，也不能报告通知启动阶段和运行阶段模式的变更。

如果single-thread模式在编译阶段被选择了，那么关键的互斥逻辑在构建时就被省略掉了，也不可能在启动阶段或者运行阶段让multi-thread和serialized使能。

启动阶段线程模式的选择

假设编译阶段线程模型不是single-thread，那么线程模型就可以使用sqlite3_config() 接口在初始化过程中更改。SQLITE_CONFIG_SINGLETHREAD动词将Sqlite置于单线程模式，而SQLITE_CONFIG_MULTITHREAD则职位多线程模式，SQLITE_CONFIG_SERIALIZED是serialized模式。

运行阶段线程模式的选择

如果在编译阶段或者启动阶段线程模型选择的不是single-thread，那么单个的数据库连接可以以multi-thread和serialized创建。但是将一个独立的数据库连接降级成single-thread模式是不可能的。也不能再编译阶段或者启动阶段模式是single-thread时将一个单独的数据库连接的模式增强。

一个单独的数据库的线程模式是由sqlite3_open_v2()的第三个参数中的flags给出决定的。SQLITE_OPEN_NOMUTEX的标志将使得数据库连接处于multi-thread，而SQLITE_OPEN_FULLMUTEX标志则将连接处于serialized模式。如果没有flag被指定或者没有使用sqlite3_open_v2()而是sqlite3_open() or sqlite3_open16()，那么默认使用的模式将由编译阶段和启动阶段的设置来决定。

SQLite And Multiple Threads

SQLite support three different threading modes:

1. Single-thread. In this mode,all mutexes are disabled and SQLite is unsafe to use in more than a single thread at once.

2. 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.

3. 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.

Compile-time selection of threading mode

Use the SQLITE_THREADSAFE compile-time parameter to selected the threading mode. If noSQLITE_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 determined by the compile-time threading mode selection. If single-thread mode is selected at compile-time,thensqlite3_threadsafe() returns false. If either the multi-thread or serialized modes are selected,thensqlite3_threadsafe() returns true. Thesqlite3_threadsafe() interface predates the multi-thread mode and start-time and run-time mode selection and so is unable to distinguish between multi-thread and serialized mode nor is it able to report start-time or run-time mode changes.

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.

Start-time selection of threading mode

Assuming that the compile-time threading mode is not single-thread,then the threading mode can be changed during initialization using thesqlite3_config() interface. TheSQLITE_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.

Run-time selection of threading 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 tosqlite3_open_v2(). TheSQLITE_OPEN_NOMUTEX flag causes the database connection to be in the multi-thread mode and theSQLITE_OPEN_FULLMUTEX flag causes the connection to be in serialized mode. If neither flag is specified or ifsqlite3_open() orsqlite3_open16() are used instead ofsqlite3_open_v2(),then the default mode determined by the compile-time and start-time settings is used.

http://sqlite.org/threadsafe.html

（编辑：李大同）

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