-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
On 11/05/2014 02:05 PM, nicolas riesch wrote:
> This means that EACH SUCCESSIVE function in the sequence above can
> be processed on a DIFFERENT OS THREAD.
That works just fine with SQLite, with one caveat. You should also
make sure the wrapper itself is threadsafe. For example what does the
wrapper do if you call close/finalize in one thread and step in
another concurrently?
The caveat is an implementation decision in SQLite and its error apis.
Usually the OS error model is to have the errors be per thread.
SQLite instead has them be attached to the connection. This means the
error apis return the most recent error for a connection no matter
which thread it happened on. (In your case that is possibly
desireable.) An example of how this can give the wrong information is
if thread 1 does a step followed by looking at the error info, but
between those two calls thread 2 does a sqlite operation on the same
database connection. The error info thread 1 looks at could be from
its earlier call or from the the thread 2 call.
The second error issue is the api that returns a pointer to the error
string (sqlite3_errmsg). By the time the pointer is used it could be
pointing to garbage or even now unmapped memory because a SQLite call
elsewhere on the connection caused that pointer to be freed. Unmapped
memory will cause a crash, and who knows what the garbage will result in.
You can tell if a wrapper got multithreading right if the code looks
like this around every SQLite API call:
// acquire db mutex
sqlite3_db_mutex(db)
// make sqlite call
sqlite3_step()
// copy error details if previous gave an error
if (error) {
// make a copy of the error message
saved=strdup(sqlite3_errmsg(db));
}
// release mutex
Note this has to be done for every sqlite call that can set the error
message which is approximately all of them. Here for example is the
macro I use in my Python wrapper to do this:
https://github.com/rogerbinns/apsw/blob/master/src/util.c#L36
Roger
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1
iEYEARECAAYFAlRbsk0ACgkQmOOfHg372QQMNQCfTfLUpodmuqnqUhe3tlXRAUBf
N7EAoJeUlu4Ir2h5WCHY9k1Ey9U7icm/
=vmiH
-----END PGP SIGNATURE-----
_______________________________________________
sqlite-users mailing list
sqlite-users@sqlite.org
http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
