sqlite: check reader/writer lock

Question

I have 2 processes which both access an sqlite3 database. While reading is not a problem in sqlite, only one process can write to the database. According to the faq: http://www.sqlite.org/faq.html#q5 sqlite uses reader/writer locks.

How do i check if the database is locked for writing by another process, both from python and c++?

[edit] I mean executing a query is an option, but it takes performance depending on the query. So the question is also what type of query do i use to minimize this effect. I also would like to lock/unlock the database myself.

Answer 1

When SQLite tries to access a locked database, it's default behaviour is to return SQLITE_BUSY. The documentation describes ways to add custom handling to this event: http://www.sqlite.org/faq.html#q5.

I understand from the documentation that any function you call that tries to write to this locked database will simply return SQLITE_BUSY, and that will be your notification that the database is locked.

If you are worried about performing a write that won't be completed, you could implement a busy handler callback (sqlite3_busy_handler) in conjuction with a busy timeout (sqlite3_busy_timeout), which would retry the write after x milliseconds.

[edit] In http://www.sqlite.org/c3ref/io_methods.html, it mentions a struct of function pointers, one of which is xCheckReservedLock(), which returns true if the file is locked. In this struct are all the other lock related functions. I am unsure about accessing these from outside the sqlite library, but it could be worth investigating. I think you do it through this interface: http://www.sqlite.org/c3ref/file_control.html