Support dynamical loading of LZF filter plugin

Author: petercolberg

lzf/README.txt

@@ -15,8 +15,14 @@ is released under the BSD license (see LICENSE.txt for details).
 Using the filter from HDF5
 --------------------------
 
-There is exactly one new public function declared in lzf_filter.h, with
-the following signature:
+With HDF5 version 1.8.11 or later the filter can be loaded dynamically by the
+HDF5 library.  The filter needs to be compiled as a plugin as described below
+that is placed in the default plugin path /usr/local/hdf5/lib/plugin/.  The
+plugin path can be overridden with the environment variable HDF5_PLUGIN_PATH.
+
+With older HDF5 versions, or when statically linking the filter to your program,
+the filter must be registered manually. There is exactly one new public function
+declared in lzf_filter.h, with the following signature:
 
     int register_lzf(void)
 
@@ -38,17 +44,23 @@ version of the LZF compression library.  Since the filter is stateless, it's
 recommended to statically link the entire thing into your program; for
 example:
 
-    $ gcc -O2 -lhdf5 lzf/*.c lzf_filter.c myprog.c -o myprog
+    $ gcc -O2 lzf/*.c lzf_filter.c myprog.c -lhdf5 -o myprog
 
 It can also be built as a shared library, although you will have to install
 the resulting library somewhere the runtime linker can find it:
 
-    $ gcc -O2 -lhdf5 -fPIC -shared lzf/*.c lzf_filter.c -o liblzf_filter.so
+    $ gcc -O2 -fPIC -shared lzf/*.c lzf_filter.c -lhdf5 -o liblzf_filter.so
 
 A similar procedure should be used for building C++ code.  As in these
 examples, using option -O1 or higher is strongly recommended for increased
 performance.
 
+With HDF5 version 1.8.11 or later the filter can be dynamically loaded as a
+plugin.  The filter is built as a shared library that is *not* linked against
+the HDF5 library:
+
+    $ gcc -O2 -fPIC -shared lzf/*.c lzf_filter.c -o liblzf_filter.so
+
 
 Contact
 -------

lzf/lzf_filter.c

@@ -63,32 +63,44 @@ size_t lzf_filter(unsigned flags, size_t cd_nelmts,
 
 herr_t lzf_set_local(hid_t dcpl, hid_t type, hid_t space);
 
+#if H5PY_H5Z_NEWCLS
+static const H5Z_class_t filter_class = {
+    H5Z_CLASS_T_VERS,
+    (H5Z_filter_t)(H5PY_FILTER_LZF),
+    1, 1,
+    "lzf",
+    NULL,
+    (H5Z_set_local_func_t)(lzf_set_local),
+    (H5Z_func_t)(lzf_filter)
+};
+#else
+static const H5Z_class_t filter_class = {
+    (H5Z_filter_t)(H5PY_FILTER_LZF),
+    "lzf",
+    NULL,
+    (H5Z_set_local_func_t)(lzf_set_local),
+    (H5Z_func_t)(lzf_filter)
+};
+#endif
+
+/* Support dynamical loading of LZF filter plugin */
+#if defined(H5_VERSION_GE)
+#if H5_VERSION_GE(1, 8, 11)
+
+#include "H5PLextern.h"
+
+H5PL_type_t H5PLget_plugin_type(void){ return H5PL_TYPE_FILTER; }
+
+const void *H5PLget_plugin_info(void){ return &filter_class; }
+
+#endif
+#endif
 
 /* Try to register the filter, passing on the HDF5 return value */
 int register_lzf(void){
 
     int retval;
 
-#if H5PY_H5Z_NEWCLS
-    H5Z_class_t filter_class = {
-        H5Z_CLASS_T_VERS,
-        (H5Z_filter_t)(H5PY_FILTER_LZF),
-        1, 1,
-        "lzf",
-        NULL,
-        (H5Z_set_local_func_t)(lzf_set_local),
-        (H5Z_func_t)(lzf_filter)
-    };
-#else
-    H5Z_class_t filter_class = {
-        (H5Z_filter_t)(H5PY_FILTER_LZF),
-        "lzf",
-        NULL,
-        (H5Z_set_local_func_t)(lzf_set_local),
-        (H5Z_func_t)(lzf_filter)
-    };
-#endif
-
     retval = H5Zregister(&filter_class);
     if(retval<0){
         PUSH_ERR("register_lzf", H5E_CANTREGISTER, "Can't register LZF filter");