GEOS 3.4 Requirements and Improvements

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Maximum Version, Minimum Version, Packaging and Linking, Oh My !

The GEOS library and C interface are designed in such a way that a client can link to a generic C Interface lib (libgeos_c), which then calls a second generic library linked by name (libgeos), then finally the actual current version of libgeos. With this setup, multiple versions of GEOS can easily sit side by side. PostGIS uses this convention to link to GEOS.

strk> PostGIS links against the geos_c library, which have had

the _same_ SONAME since the very beginning

When does it make sense to declare a MINIMUM version ?

when entry points are called that are only present in newer versions and above

When does it make sense to declare a MAXIMUM version ?

when behavior changes in an incompatible way in a new version, either API breakage or perhaps the results of a common operation like noding

Minimum Version

For a long while, features and fixes were just added to the newest GEOS, so any client app would just get the newest available GEOS and all is well.

However, to take advantage of new functionality, clients have to call entry points explicitly. So a *minimum* version has to be declared.

With PostGIS 2.1, if new GEOS functionality is required, then the *minimum* version might be 3.4.

Maximum Version

With GEOS 3.4, there is a change in noding algorithm, which could have an effect on output. So a client app's results may be different by upgrading to GEOS 3.4. This change may be undesirable, so a *maximum* version could be declared, saying in effect, if you want the same reliable output, you must not go above v3.3x.

Packaging -- IANAP (I am not a packager ;-)

/ -> GDAL -> PostGIS

GEOS-> | -> PostGIS

\ -> QGis

GDAL is unique in that it is designed to be an extensible collection of drivers. Since the tools are command line oriented and open source, I suspect a substantial number of GDAL installs are built from source.

Not so with PostGIS. I suspect that the vast majority of PostGIS installs are by official installers and packages *only*.

PostGIS now requires GDAL (for raster). What collection of GDAL drivers are included by an installer or package of PostGIS that provides GDAL ? and what GEOS version? Opinions vary on this situation. User surveys and use case based testing are one option to understand field experience by users.

Misc Background

GDAL

GDAL is a suite of tools, including gdal raster tools, and ogr vector tools long ago GEOS was optional to build GDAL, and was used only in ogr vector tools in practice, ogr tools have required GEOS for some time (years)

gdal the raster tool now uses GEOS in one place, quoting from #gdal

EvenR> alg/gdalcutline.cpp needs GEOS for blend distance support

QGis 2.0

is coming out very soon. If there was a time to make a change, this is a good time to do that

Linkage

For example on a work XUbuntu Linux machine, I get the following output:

$ldd /usr/lib/postgresql/9.2/lib/postgis-2.0.so | grep geos

libgeos_c.so.1 => /usr/local/lib/libgeos_c.so.1 (0x00007f73b246c000) libgeos-3.3.9dev.so => /usr/local/lib/libgeos-3.3.9dev.so (0x00007f73b1569000)

$ldd which qgis | grep geos

.... error output about gdal .so versioning

libgeos_c.so.1 => /usr/local/lib/libgeos_c.so.1 (0x00007fd19145d000) libgeos-3.3.9dev.so => /usr/local/lib/libgeos-3.3.9dev.so (0x00007fd18da38000)

$ldd which ogrinfo | grep geos

libgeos_c.so.1 => /usr/local/lib/libgeos_c.so.1 (0x00007fd19145d000) libgeos-3.3.9dev.so => /usr/local/lib/libgeos-3.3.9dev.so (0x00007fd18da38000)

$ldd which gdalinfo | grep geos

libgeos_c.so.1 => /usr/local/lib/libgeos_c.so.1 (0x00007f0d60454000) libgeos-3.3.9dev.so => /usr/local/lib/libgeos-3.3.9dev.so (0x00007f0d5ca2f000)

actual libgeos, has naming like this..

libgeos.so -> libgeos-3.3.9dev.so

My own geos libs on XUbuntu at the moment:

dbb@xps-8300:~$ ls -l /usr/local/lib/*geos* -rwxr-xr-x 1 root root 18178990 Oct 1 2012 /usr/local/lib/libgeos-3.3.6dev.so -rwxr-xr-x 1 root root 18199357 Jan 22 12:07 /usr/local/lib/libgeos-3.3.7.so -rwxr-xr-x 1 root root 18183679 Mar 8 10:58 /usr/local/lib/libgeos-3.3.9dev.so -rw-r--r-- 1 root root 49492572 Mar 8 10:58 /usr/local/lib/libgeos.a -rw-r--r-- 1 root root 2194792 Mar 8 10:58 /usr/local/lib/libgeos_c.a -rwxr-xr-x 1 root root 979 Mar 8 10:58 /usr/local/lib/libgeos_c.la lrwxrwxrwx 1 root root 18 Mar 8 10:58 /usr/local/lib/libgeos_c.so -> libgeos_c.so.1.7.9 lrwxrwxrwx 1 root root 18 Mar 8 10:58 /usr/local/lib/libgeos_c.so.1 -> libgeos_c.so.1.7.9 -rwxr-xr-x 1 root root 1030668 Oct 1 2012 /usr/local/lib/libgeos_c.so.1.7.6 -rwxr-xr-x 1 root root 1030668 Jan 22 12:07 /usr/local/lib/libgeos_c.so.1.7.7 -rwxr-xr-x 1 root root 1029963 Mar 8 10:58 /usr/local/lib/libgeos_c.so.1.7.9 -rwxr-xr-x 1 root root 956 Mar 8 10:58 /usr/local/lib/libgeos.la lrwxrwxrwx 1 root root 19 Mar 8 10:58 /usr/local/lib/libgeos.so -> libgeos-3.3.9dev.so