On 02/09/2015 02:18 PM, Hick Gunter wrote:
In serialized mode, SQLite will acquire the mutex when it detects you are "starting to
use" the database handle (somewhere between entering sqlite3_prepare and the first
sqlite3_step) and then HANG ON TO IT, NOT LETTING GO until the calling thread is
"finished" (like when sqlite3_step returns SQLITE_DONE or the thread calls sqlite3_reset
or sqlite3_finalize).
This is quite inaccurate. Here is the implementation of sqlite3_prepare():
http://www.sqlite.org/src/artifact/173a5a4991384?ln=792
Notice that all it does is call sqlite3LockAndPrepare(). The
implemenation of which is here:
http://www.sqlite.org/src/artifact/173a5a4991384?ln=722-730
It grabs the database mutex, does its work, then releases the database
mutex.
sqlite3_step() does exactly the same. It grabs the db mutex, does its
work, releases the mutex.
SQLite does not hold the database mutex between non-nested API calls.
Dan.
In multithread mode, you are taking over this responsibility; if you take care,
you may nest several selects from different threads into a single transaction,
but need to be aware of the fact that they will all commit or rollback together.
-----Ursprüngliche Nachricht-----
Von: Neo Anderson [mailto:neo_in_mat...@msn.com]
Gesendet: Montag, 09. Februar 2015 06:34
An: General Discussion of SQLite Database
Betreff: Re: [sqlite] Multi-thread mode question
Does the application work if you configure SQLite to serialized mode?
Yes. But I am confused why serialized mode works while multi-thread mode always
cause crashes because I also wrap calls around statement handle.
even if you wrap the sqlite3_ calls... you'll need to wrap the entire
lifetime of the statment...
Do I need to do this in serialized mode (suppose I use a single connection
across multiple threads)?
----------------------------------------
Date: Sun, 8 Feb 2015 03:31:46 -0800
From: d3c...@gmail.com
To: sqlite-users@sqlite.org
Subject: Re: [sqlite] Multi-thread mode question
it's better to use a connection per thread... the connection resource
isn't very big...
even if you wrap the sqlite3_ calls... you'll need to wrap the entire
lifetime of the statment... if you do a execute and then start
stepping and getting values while another thread starts another
statement... that's 3 individual locks, but it doesn't lock the
context of the statement being used... it will lead to bizarre crashes
in the database; similar to double-releasing memory or delayed
reference of memory that has been released.
On Sun, Feb 8, 2015 at 3:00 AM, Dan Kennedy <danielk1...@gmail.com> wrote:
On 02/08/2015 04:30 PM, Neo Anderson wrote:
The doc says:
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.
I have a scenario that every sqlite3_calls around a single database
connection is protected by a recursive mutex, but I have very
strange runtime error in sqlite3.c and each time the error occurs at
a different place.
Does this mean the following statement is true:
In muti-thead mode, a single database connection cannot be shared
among threads even if any activity around the connection is protected by a
mutex.
Not true.
The only difference between multi-threaded and serialized mode is
that, internally, every sqlite3_xxx() API call grabs a recursive
mutex to prevent two threads from simultaneously accessing the database handle
structure.
i.e. the same thing your code is doing externally.
Note that calls on statement handles (i.e. sqlite3_step(),
sqlite3_column_text() etc.) count as calls on the database handle
that created them. So you need to protect them with the same mutex.
Does the application work if you configure SQLite to serialized mode?
Dan.
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Gunter Hick
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