the database file is locked

http://www.sqlite.org/faq.html

http://www.sqlite.org/c3ref/busy_handler.html

http://www.sqlite.org/c3ref/busy_timeout.html

Can multiple applications or multiple instances of the same application access a single database file at the same time?

Multiple processes can have the same database open at the same time. Multiple processes can be doing a SELECT at the same time. But only one process can be making changes to the database at any moment in time,however.

SQLite uses reader/writer locks to control access to the database. (Under Win95/98/ME which lacks support for reader/writer locks,a probabilistic simulation is used instead.) But use caution: this locking mechanism might not work correctly if the database file is kept on an NFS filesystem. This is because fcntl() file locking is broken on many NFS implementations. You should avoid putting SQLite database files on NFS if multiple processes might try to access the file at the same time. On Windows,Microsoft's documentation says that locking may not work under FAT filesystems if you are not running the Share.exe daemon. People who have a lot of experience with Windows tell me that file locking of network files is very buggy and is not dependable. If what they say is true,sharing an SQLite database between two or more Windows machines might cause unexpected problems.

We are aware of no other embedded SQL database engine that supports as much concurrency as SQLite. SQLite allows multiple processes to have the database file open at once,and for multiple processes to read the database at once. When any process wants to write,it must lock the entire database file for the duration of its update. But that normally only takes a few milliseconds. Other processes just wait on the writer to finish then continue about their business. Other embedded SQL database engines typically only allow a single process to connect to the database at once.

However,client/server database engines (such as PostgreSQL,MySQL,or Oracle) usually support a higher level of concurrency and allow multiple processes to be writing to the same database at the same time. This is possible in a client/server database because there is always a single well-controlled server process available to coordinate access. If your application has a need for a lot of concurrency,then you should consider using a client/server database. But experience suggests that most applications need much less concurrency than their designers imagine.

When SQLite tries to access a file that is locked by another process,the default behavior is to return SQLITE_BUSY. You can adjust this behavior from C code using the sqlite3_busy_handler() or sqlite3_busy_timeout() API functions.

Is SQLite threadsafe?

Threads are evil. Avoid them.

SQLite is threadsafe. We make this concession since many users choose to ignore the advice given in the previous paragraph. But in order to be thread-safe,SQLite must be compiled with the SQLITE_THREADSAFE preprocessor macro set to 1. Both the Windows and Linux precompiled binaries in the distribution are compiled this way. If you are unsure if the SQLite library you are linking against is compiled to be threadsafe you can call the sqlite3_threadsafe() interface to find out.

Prior to version 3.3.1,an sqlite3 structure could only be used in the same thread that called sqlite3_open() to create it. You could not open a database in one thread then pass the handle off to another thread for it to use. This was due to limitations (bugs?) in many common threading implementations such as on RedHat9. Specifically,an fcntl() lock created by one thread cannot be removed or modified by a different thread on the troublesome systems. And since SQLite uses fcntl() locks heavily for concurrency control,serious problems arose if you start moving database connections across threads.

The restriction on moving database connections across threads was relaxed somewhat in version 3.3.1. With that and subsequent versions,it is safe to move a connection handle across threads as long as the connection is not holding any fcntl() locks. You can safely assume that no locks are being held if no transaction is pending and all statements have been finalized.

Under Unix,you should not carry an open SQLite database across a fork() system call into the child process. Problems will result if you do.

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