Fixes UDF format bug and ncdump bug

libdispatch/dinfermodel.c

@@ -1573,6 +1573,7 @@ NC_interpret_magic_number(char* magic, NCmodel* model)
          {
              int formatx = (i <= 1) ? (NC_FORMATX_UDF0 + i) : (NC_FORMATX_UDF2 + i - 2);
              model->impl = formatx;
+             model->format = NC_FORMAT_NETCDF4;
              status = NC_NOERR;
              break;
          }

ncdump/CMakeLists.txt

@@ -208,6 +208,20 @@ endif()
     # Add this test by hand, as it is also called from a script.
     # Editing the script would break autotools compatibility.
     build_bin_test_no_prefix(tst_special_atts)
+    if(NETCDF_ENABLE_PLUGINS)
+      add_library(tst_udf_format_plugin MODULE tst_udf_format_plugin.c)
+      target_link_libraries(tst_udf_format_plugin netcdf ${ALL_TLL_LIBS})
+      set_target_properties(tst_udf_format_plugin PROPERTIES PREFIX "")
+      if(WIN32)
+        set_target_properties(tst_udf_format_plugin PROPERTIES
+          LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+          LIBRARY_OUTPUT_DIRECTORY_DEBUG ${CMAKE_CURRENT_BINARY_DIR}
+          LIBRARY_OUTPUT_DIRECTORY_RELEASE ${CMAKE_CURRENT_BINARY_DIR}
+          RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+          RUNTIME_OUTPUT_DIRECTORY_DEBUG ${CMAKE_CURRENT_BINARY_DIR}
+          RUNTIME_OUTPUT_DIRECTORY_RELEASE ${CMAKE_CURRENT_BINARY_DIR})
+      endif()
+    endif()
   endif(USE_HDF5)
 
   # Base tests
@@ -262,6 +276,9 @@ endif()
 
   if(USE_HDF5)
     add_sh_test(ncdump tst_formatx4)
+    if(NETCDF_ENABLE_PLUGINS)
+      add_sh_test(ncdump tst_udf_format)
+    endif()
     add_sh_test(ncdump_sh tst_fillbug)
     add_sh_test(ncdump_shell tst_h_scalar)
     add_sh_test(ncdump tst_mud)

ncdump/Makefile.am

@@ -122,15 +122,26 @@ tst_vlen_data tst_comp tst_comp2 tst_nans tst_special_atts	\
 tst_unicode tst_fillbug tst_compress tst_chunking tst_h_scalar  \
 tst_enum_undef
 
+if NETCDF_ENABLE_PLUGINS
+check_LTLIBRARIES = tst_udf_format_plugin.la
+tst_udf_format_plugin_la_SOURCES = tst_udf_format_plugin.c
+tst_udf_format_plugin_la_LDFLAGS = -module -avoid-version -shared -no-undefined -rpath /tmp
+tst_udf_format_plugin_la_LIBADD = ${top_builddir}/liblib/libnetcdf.la
+endif # NETCDF_ENABLE_PLUGINS
+
 check_PROGRAMS += tst_vlen_demo
 
 # Tests for netCDF-4 behavior.
 TESTS += tst_fileinfo.sh tst_hdf5_offset.sh tst_inttags4.sh		\
-tst_netcdf4.sh tst_fillbug.sh \
-tst_grp_spec.sh tst_mud.sh tst_h_scalar.sh tst_formatx4.sh		\
+tst_netcdf4.sh tst_fillbug.sh tst_grp_spec.sh tst_mud.sh \
+tst_h_scalar.sh tst_formatx4.sh		\
 run_utf8_nc4_tests.sh run_ncgen_nc4_tests.sh	\
 tst_ncgen4.sh test_scope.sh
 
+if NETCDF_ENABLE_PLUGINS
+TESTS += tst_udf_format.sh
+endif # NETCDF_ENABLE_PLUGINS
+
 # Record interscript dependencies so parallel builds work.
 tst_nccopy4.log: run_ncgen_tests.log tst_output.log tst_ncgen4.log	\
                  tst_fillbug.log tst_netcdf4_4.log tst_h_scalar.log tst_netcdf4.log
@@ -220,7 +231,8 @@ ref_rcapi.txt ref_tst_enum_undef.cdl tst_calendars_nc4.cdl ref_times_nc4.cdl ref
 EXTRA_DIST += L512.bin
 
 EXTRA_DIST += tst_ctests.sh ref_ctest_small_3.c ref_ctest_small_4.c \
-	ref_ctest_special_atts_4.c test_ncdump.sh
+	ref_ctest_special_atts_4.c test_ncdump.sh tst_udf_format.sh \
+	tst_udf_format_plugin.c
 
 EXTRA_DIST += testpathcvt.sh ref_pathcvt.txt
 
@@ -255,7 +267,8 @@ tst_roman_szip_unlim.cdl tst_perdimpspecs.nc tmppds.*                   \
 keyword1.nc keyword2.nc keyword3.nc keyword4.nc                         \
 tmp_keyword1.cdl tmp_keyword2.cdl tmp_keyword3.cdl tmp_keyword4.cdl     \
 type_*.nc copy_type_*.cdl                                               \
-scope_*.nc copy_scope_*.cdl keyword5.nc tst_enum_undef.cdl tst_times_nc4.cdl
+scope_*.nc copy_scope_*.cdl keyword5.nc tst_enum_undef.cdl tst_times_nc4.cdl \
+	tst_udf_format.ncrc tst_udf_format.udf tst_udf_format.out tst_udf_format.err
 
 # Remove directories
 clean-local:

ncdump/ncdump.c

@@ -276,7 +276,10 @@ kind_string(int kind)
     case NC_FORMAT_NETCDF4_CLASSIC:
 	return "netCDF-4 classic model";
     default:
-       error("unrecognized file format: %d", kind);
+        if(kind == NC_FORMATX_UDF0 || kind == NC_FORMATX_UDF1
+           || (kind >= NC_FORMATX_UDF2 && kind <= NC_FORMATX_UDF9))
+            return "user-defined format";
+        error("unrecognized file format: %d", kind);
 	return "unrecognized";
     }
 }
@@ -315,8 +318,13 @@ kind_string_extended(int kind, int mode)
 	snprintf(text,sizeof(text),"%s mode=%08x", "unknown",mode);
 	break;
     default:
-	error("unrecognized extended format: %d",kind);
-	snprintf(text,sizeof(text),"%s mode=%08x", "unrecognized",mode);
+        if(kind == NC_FORMATX_UDF0 || kind == NC_FORMATX_UDF1
+           || (kind >= NC_FORMATX_UDF2 && kind <= NC_FORMATX_UDF9))
+            snprintf(text,sizeof(text),"%s mode=%08x", "user-defined format",mode);
+        else {
+            error("unrecognized extended format: %d",kind);
+            snprintf(text,sizeof(text),"%s mode=%08x", "unrecognized",mode);
+        }
 	break;
     }
     return text;

ncdump/tst_udf_format.sh

@@ -0,0 +1,75 @@
+#!/bin/sh
+#
+# This is part of the netCDF package. Copyright 2026 University
+# Corporation for Atmospheric Research/Unidata. See COPYRIGHT file
+# for conditions of use.
+#
+# Test for netCDF-C issue #3411. ncdump could open a UDF file and
+# print its metadata, but then rejected the UDF extended format code
+# while generating the virtual _Format attribute or processing -K.
+#
+# The test autoloads tst_udf_format_plugin through an RC file and opens
+# a fixture recognized by its UDF magic number. It verifies that -hs
+# prints the generic user-defined _Format value and -K prints the
+# generic extended format name with its mode.
+#
+# Author: Edward Hartnett, 2026-07-21
+
+if test "x$srcdir" = x ; then srcdir=`pwd`; fi
+. ../test_common.sh
+
+for directory in . .libs; do
+    for extension in so dylib dll; do
+        if test -f $directory/tst_udf_format_plugin.$extension; then
+            plugin=`pwd`/$directory/tst_udf_format_plugin.$extension
+            break 2
+        fi
+    done
+done
+
+if test -z "$plugin" || test ! -f "$plugin"; then
+    echo "*** Fail: UDF format test plugin was not built"
+    echo "Working directory: `pwd`"
+    echo "Plugin candidates:"
+    find . -maxdepth 2 -type f -name 'tst_udf_format_plugin*' -print
+    echo "Current directory contents:"
+    ls -la
+    if test -d .libs; then
+        echo ".libs contents:"
+        ls -la .libs
+    fi
+    exit 1
+fi
+
+cat > tst_udf_format.ncrc <<EOF
+NETCDF.UDF0.LIBRARY=$plugin
+NETCDF.UDF0.INIT=tst_udf_format_init
+NETCDF.UDF0.MAGIC=TUSF
+EOF
+printf TUSF00000 > tst_udf_format.udf
+
+if ! NCRCENV_RC=`pwd`/tst_udf_format.ncrc ${NCDUMP} -hs tst_udf_format.udf > tst_udf_format.out 2> tst_udf_format.err; then
+    cat tst_udf_format.err
+    rm -f tst_udf_format.ncrc tst_udf_format.udf tst_udf_format.out tst_udf_format.err
+    exit 1
+fi
+
+if ! grep ':_Format = "user-defined format" ;' tst_udf_format.out; then
+    cat tst_udf_format.out
+    rm -f tst_udf_format.ncrc tst_udf_format.udf tst_udf_format.out tst_udf_format.err
+    exit 1
+fi
+
+if ! NCRCENV_RC=`pwd`/tst_udf_format.ncrc ${NCDUMP} -K tst_udf_format.udf > tst_udf_format.out 2> tst_udf_format.err; then
+    cat tst_udf_format.err
+    rm -f tst_udf_format.ncrc tst_udf_format.udf tst_udf_format.out tst_udf_format.err
+    exit 1
+fi
+
+if ! grep 'user-defined format mode=' tst_udf_format.out; then
+    cat tst_udf_format.out
+    rm -f tst_udf_format.ncrc tst_udf_format.udf tst_udf_format.out tst_udf_format.err
+    exit 1
+fi
+
+rm -f tst_udf_format.ncrc tst_udf_format.udf tst_udf_format.out tst_udf_format.err

ncdump/tst_udf_format_plugin.c

@@ -0,0 +1,169 @@
+/* This is part of the netCDF package. Copyright 2026 University
+   Corporation for Atmospheric Research/Unidata. See COPYRIGHT file
+   for conditions of use.
+
+   Minimal self-registering UDF0 plugin for tst_udf_format.sh. It
+   provides an empty root dataset and reports NC_FORMATX_UDF0 through
+   both format inquiry functions.
+
+   This plugin reproduces netCDF-C issue #3411 without depending on
+   an external UDF implementation. Before the fix, ncdump could read
+   the UDF dataset but rejected extended format code 8 while generating
+   its virtual _Format attribute or processing -K output.
+
+   Author: Edward Hartnett, 2026-07-21
+*/
+
+#include "config.h"
+#include <string.h>
+#include "netcdf.h"
+#include "netcdf_dispatch.h"
+
+static NC_Dispatch dispatcher;
+
+static int
+plugin_open(const char *path, int mode, int basepe, size_t *chunksizehintp,
+            void *parameters, const NC_Dispatch *table, int ncid)
+{
+    return NC_NOERR;
+}
+
+static int
+plugin_close(int ncid, void *dispatchdata)
+{
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_format(int ncid, int *formatp)
+{
+    if (formatp)
+        *formatp = NC_FORMATX_UDF0;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_format_extended(int ncid, int *formatp, int *modep)
+{
+    if (formatp)
+        *formatp = NC_FORMATX_UDF0;
+    if (modep)
+        *modep = NC_NOWRITE;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq(int ncid, int *ndimsp, int *nvarsp, int *nattsp, int *unlimdimidp)
+{
+    if (ndimsp)
+        *ndimsp = 0;
+    if (nvarsp)
+        *nvarsp = 0;
+    if (nattsp)
+        *nattsp = 0;
+    if (unlimdimidp)
+        *unlimdimidp = -1;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_att(int ncid, int varid, const char *name, nc_type *xtypep,
+               size_t *lenp)
+{
+    return NC_ENOTATT;
+}
+
+static int
+plugin_inq_typeids(int ncid, int *ntypes, int *typeids)
+{
+    if (ntypes)
+        *ntypes = 0;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_grps(int ncid, int *numgrps, int *ncids)
+{
+    if (numgrps)
+        *numgrps = 0;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_grp_parent(int ncid, int *parent_ncid)
+{
+    return NC_ENOGRP;
+}
+
+static int
+plugin_inq_ncid(int ncid, const char *name, int *grp_ncid)
+{
+    if (name != NULL)
+        return NC_ENOGRP;
+    if (grp_ncid)
+        *grp_ncid = ncid;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_dimids(int ncid, int *ndims, int *dimids, int include_parents)
+{
+    if (ndims)
+        *ndims = 0;
+    return NC_NOERR;
+}
+
+static int
+plugin_inq_unlimdims(int ncid, int *nunlimdimsp, int *unlimdimidsp)
+{
+    if (nunlimdimsp)
+        *nunlimdimsp = 0;
+    return NC_NOERR;
+}
+
+#if defined(_MSC_VER)
+#define PLUGIN_EXPORT __declspec(dllexport)
+#elif defined(__GNUC__) && __GNUC__ >= 4
+#define PLUGIN_EXPORT __attribute__((visibility("default")))
+#else
+#define PLUGIN_EXPORT
+#endif
+
+PLUGIN_EXPORT NC_Dispatch *
+tst_udf_format_init(void)
+{
+    memset(&dispatcher, 0, sizeof(dispatcher));
+    dispatcher.model = NC_FORMATX_UDF0;
+    dispatcher.dispatch_version = NC_DISPATCH_VERSION;
+    dispatcher.create = NC_RO_create;
+    dispatcher.open = plugin_open;
+    dispatcher.redef = NC_RO_redef;
+    dispatcher._enddef = NC_RO__enddef;
+    dispatcher.sync = NC_RO_sync;
+    dispatcher.close = plugin_close;
+    dispatcher.set_fill = NC_RO_set_fill;
+    dispatcher.inq_format = plugin_inq_format;
+    dispatcher.inq_format_extended = plugin_inq_format_extended;
+    dispatcher.inq = plugin_inq;
+    dispatcher.inq_att = plugin_inq_att;
+    dispatcher.inq_grps = plugin_inq_grps;
+    dispatcher.inq_ncid = plugin_inq_ncid;
+    dispatcher.inq_grp_parent = plugin_inq_grp_parent;
+    dispatcher.inq_dimids = plugin_inq_dimids;
+    dispatcher.inq_unlimdims = plugin_inq_unlimdims;
+    dispatcher.inq_typeids = plugin_inq_typeids;
+    dispatcher.rename_att = NC_RO_rename_att;
+    dispatcher.del_att = NC_RO_del_att;
+    dispatcher.put_att = NC_RO_put_att;
+    dispatcher.def_dim = NC_RO_def_dim;
+    dispatcher.rename_dim = NC_RO_rename_dim;
+    dispatcher.def_var = NC_RO_def_var;
+    dispatcher.rename_var = NC_RO_rename_var;
+    dispatcher.put_vara = NC_RO_put_vara;
+    dispatcher.var_par_access = NC_NOTNC4_var_par_access;
+    dispatcher.def_var_fill = NC_RO_def_var_fill;
+    dispatcher.inq_var_filter_ids = NC_NOOP_inq_var_filter_ids;
+    dispatcher.inq_var_filter_info = NC_NOOP_inq_var_filter_info;
+    dispatcher.inq_filter_avail = NC_NOOP_inq_filter_avail;
+    return &dispatcher;
+}

unit_test/CMakeLists.txt

@@ -19,6 +19,7 @@ ENDIF()
 
 SET(UNIT_TESTS test_ncuri)
 add_bin_test(unit_test test_ncuri)
+add_bin_test(unit_test tst_udf_infermodel)
 SET(UNIT_TESTS test_dauth)
 add_bin_test(unit_test test_dauth)
 

unit_test/Makefile.am

@@ -27,8 +27,8 @@ noinst_PROGRAMS =
 noinst_PROGRAMS += ncpluginpath
 ncpluginpath_SOURCES = ncpluginpath.c
 
-check_PROGRAMS += tst_nclist test_ncuri test_pathcvt test_dauth
-TESTS += tst_nclist test_ncuri run_pathcvt.sh test_dauth
+check_PROGRAMS += tst_nclist test_ncuri test_pathcvt test_dauth tst_udf_infermodel
+TESTS += tst_nclist test_ncuri run_pathcvt.sh test_dauth tst_udf_infermodel
 
 # Performance tests
 if BUILD_BENCHMARKS
@@ -75,7 +75,7 @@ EXTRA_DIST += ref_get.txt ref_set.txt
 EXTRA_DIST += ref_xget.txt ref_xset.txt
 EXTRA_DIST += ref_provparse.txt 
 EXTRA_DIST += reclaim_tests.baseline
-CLEANFILES = reclaim_tests*.txt reclaim_tests.nc reclaim_tests.dmp tmp_*.txt
+CLEANFILES = reclaim_tests*.txt reclaim_tests.nc reclaim_tests.dmp tmp_*.txt tst_udf_infermodel.udf
 
 # Remove directories
 clean-local: