Even, Thank you for your input on that. I figured that was a risky idea and it is good to know the definitive answer.
Best regards, Martin -----Original Message----- From: Even Rouault [mailto:even.roua...@mines-paris.org] Sent: Thursday, August 02, 2012 2:13 AM To: Martin Chapman Cc: 'kedardeshpande87'; gdal-dev@lists.osgeo.org Subject: Re: [gdal-dev] Corrupted .shp error when reading shapefile from multiple threads Selon Martin Chapman <chapm...@pixia.com>: > Kedar, > > Think about it...how does GetNextFeature() know what feature is next? > It's because somewhere inside the datasource handle there is probably > a variable that is keeping track of what record is next. If multiple > threads are all using the same datasource handle then this variable is > probably getting updated / read by different threads at the same time > and then all hell will break loose. > > The answer to your problem is to open a separate datasource handle for > every thread you have so there is no sharing across threads. Right now > your getZipCode() function is sharing a global datasource handle and > the code is not synchronized. If you synchronize this code then it > will be no better than a single threaded application so if I was you I would either: > > 1. Open a datasource handle on each thread and pass it to the zipcode > function from each thread ... or > > 2. Create a class that encapsulates the getZipCode() function and has > a datasource member variable that opens the datasource on construction > and then create a separate instance of this class for each thread on startup > so each thread has its own copy of the class. Or .... > > 3. See if there is a function on the handle that will get the next > feature given an index (like GetFeature(int index)) that you pass into > the function so there is no internal record keeping needed by the handle. > This is a risky way to do it though because even though it will work > there may be other global variables the handle is keeping so unless > Frank can vouch for that method I would go with curtain number 2. Martin, I concur with your points. About 3), (using GetFeature(index)), this won't work either. Because GetFeature() needs to seek in the .shp and .dbf files. So if 2 threads use GetFeature() on the same layer object, which has a single file pointer, the seeks may conflict with each other and you might end up reading the wrong content. The only safe solution is to use a datasource object for each thread. _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
