[gdal-dev] Thread safety of GDAL projection machinery?

[Even Rouault]

Charles,

At first sight, you're likely doing things the right way and probably 
discovered a bug related to multi-threading in GDAL. The use of 
GDALCreateGenImgProjTransformer() should be thread-safe (provided that you 
don't use the same dataset in the different threads)

By reviewing the code of 2 files that are used in the retrieving and parsing 
of the csv files, there are a few variables that are supposed to be 
thread-local (CPL_THREADLOCAL), but this is currently a no-op.

But as it's just a guess and maybe not the real cause for your problems, 
ideally, you could file a bug in the GDAL Trac with a small program that 
reproduces your issue, so that it can be investigated further.

Best regards,

Even

Le Wednesday 27 May 2009 20:41:57 Charles Karney, vous avez écrit :
> I have a multi-threaded application with reads the geodata out of many
> files using GDAL.
>
> For each image, it calls
>
>   GDALCreateGenImgProjTransformer(// Source
> 				  dataset,GDALGetProjectionRef(dataset),
> 				  // Dest (NULL means dest is geo)
> 				  NULL, WGS84.c_str(),
> 				  // Don't use GCPs
> 				  0, 0, 0 );
>
> and I then compute the lat/long of the 4 image corners.
>
> I call
>
>     GDALAllRegister();
>
> before entering the multi-threaded portion of the code.
>
> With files that require that pcs.csv be read, I get the following error
> messages
>
>     ERROR 3: Read of file c:/Program Files (x86)/FWTools2.2.8/data/pcs.csv
> failed. ERROR 3: Read of file c:/Program Files
> (x86)/FWTools2.2.8/data/pcs.csv failed. ERROR 2: CPLRealloc(): Out of
> memory allocating 1956 bytes.
>     ERROR 3: Read of file c:/Program Files (x86)/FWTools2.2.8/data/pcs.csv
> failed. ERROR 3: Read of file c:/Program Files
> (x86)/FWTools2.2.8/data/pcs.csv failed. ERROR 3: Read of file c:/Program
> Files (x86)/FWTools2.2.8/data/pcs.csv failed. ERROR 3: Read of file
> c:/Program Files (x86)/FWTools2.2.8/data/pcs.csv failed. ERROR 3: Read of
> file c:/Program Files (x86)/FWTools2.2.8/data/datum.csv failed. ERROR 3:
> Read of file c:/Program Files (x86)/FWTools2.2.8/data/ellipsoid.csv failed.
> ERROR 2: CPLRealloc(): Out of memory allocating 409 bytes.
>     ERROR 1: No PROJ.4 translation for source SRS, coordinate
>     transformation initialization has failed.
>
> This is with 6 threads.  The error messages change from invocation to
> invocation as is typical with multi-threading bugs.
>
> If I change the code so that one a single thread is handling the GDAL
> stuff, everything works OK.
>
> * Is this "supposed" to work?
>
> * Do I need to do something outside the threaded portion of the code to
>   initialize the projection machinery?
>
> * If it's known not to work, what GDAL calls do I need to protect with
>   locks?
>
> Thanks for the help!