[gdal-dev] Properly reinitializing PROJ_LIB search paths after supplementary file download

[David Klaus]

Hello GDAL community,

The product that I work on uses the CPP GDAL library for a number of
routines. Occasionally, this requires supplementary files -- Currently, we
supply proj-data 1.20
<https://github.com/OSGeo/PROJ-data/releases/tag/1.20.0> -- which we
download when we detect that the dataset is needed. Here's the problem,
often GDALRegisterAll() is called before the proj-data 1.20 files are
downloaded. From my testing, it appears that if the proj-data 1.20 files
are not available when GDALRegisterAll() is called, GDAL will not use them
regardless of whether or not these files are available later.

Now, what I think might be a good solution is appropriately updating GDAL's
state after proj-data is downloaded s.t. it is able to detect the proj-data
files. But, I'm not sure how to do this. Calling the following seems to
produce correct results:

GDALDestroy()
GDALAllRegister()

However, the documentation says the following:

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////FROM
DOCUMENTATION///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Finalize GDAL/OGR library.

This function calls GDALDestroyDriverManager() and OGRCleanupAll() and
finalize Thread Local Storage variables.

Prior to GDAL 2.4.0, this function should normally be explicitly called by
application code if GDAL is dynamically linked (but that does not hurt),
since it was automatically called through the unregistration mechanisms of
dynamic library loading.

Since GDAL 2.4.0, this function may be called by application code, since it
is no longer called automatically, on non-MSVC builds, due to ordering
problems with respect to automatic destruction of global C++ objects.

*Note: no GDAL/OGR code should be called after this call!*
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////FROM
DOCUMENTATION///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

So, this doesn't seem like the correct way to address this problem. How
should I go about updating GDAL's state s.t. it will detect and read the
downloaded supplementary proj-data files? Or, is there something more I
should be doing before GDALAllRegister() is called?

GDAL Version: GDAL 3.10.3, released 2025/04/01
Proj Version: 9.6.0

P.S. To get out in front of what I expect to be the first question, yes I
am setting the PROJ_LIB environmental variable at runtime to the correct
folder. Further, I have tried creating an empty proj-data folder available
to GDAL when GDALRegisterAll() is called. This did not change GDAL's
behavior. Here is the code I use for setting PROJ_LIB:

SetEnvironmentVariable("PROJ_LIB", "c:/path/to/proj-data/folder");
_putenv_s("PROJ_LIB", "c:/path/to/proj-data/folder"); // (See: GETENV_NOTE)

P.P.S Here is the GETENV_NOTE:
// GETENV_NOTE:
// _putenv_s() is used to ensure compatibility with getenv(). getenv()
operates on environment variables loaded into  the _environ global variable
(loaded at "process startup.")
// SetEnvironmentVariable() affects the environment variables set for this
process but does not change _environ. _putenv_s() updates _environ and
ensures compatibility w/
// genenv().

P.P.P.S I do know that there is a more current proj-data dataset available.
We may update to this dataset in the future. But unless that will fix the
current issue, I'd rather not do that now.


-- 
David Klaus
Carlson Software

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