diff --git a/.github/workflows/run_tests_osx.yml b/.github/workflows/run_tests_osx.yml
index 11864a801..21932fdb4 100644
--- a/.github/workflows/run_tests_osx.yml
+++ b/.github/workflows/run_tests_osx.yml
@@ -21,14 +21,14 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
         ###
         # libhdf5
         ###
       - name: Cache libhdf5-${{ runner.os }}-${{ matrix.hdf5 }}
         id: cache-hdf5-osx
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -61,7 +61,7 @@ jobs:
         use_nczarr: [ nczarr_off, nczarr_on ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
 ###
 # Set Environmental Variables
@@ -93,7 +93,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf-osx
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -167,7 +167,7 @@ jobs:
         use_nczarr: [ nczarr_off, nczarr_on ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
 ###
 # Set Environmental Variables
@@ -199,7 +199,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf5-osx
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -260,7 +260,7 @@ jobs:
         hdf5: [ 1.12.2 ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
 ###
 # Set Environmental Variables
@@ -277,7 +277,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf-osx
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -342,7 +342,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
 ###
 # Set Environmental Variables
@@ -357,7 +357,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf5-osx
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
diff --git a/.github/workflows/run_tests_ubuntu.yml b/.github/workflows/run_tests_ubuntu.yml
index 9df8b7048..816367990 100644
--- a/.github/workflows/run_tests_ubuntu.yml
+++ b/.github/workflows/run_tests_ubuntu.yml
@@ -18,7 +18,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -29,7 +29,7 @@ jobs:
         ###
       - name: Cache libhdf5-${{ matrix.hdf5 }}
         id: cache-hdf5
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -67,7 +67,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -78,7 +78,7 @@ jobs:
         ###
       - name: Cache libhdf5-parallel-${{ matrix.hdf5 }}
         id: cache-hdf5
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-parallel-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -128,7 +128,7 @@ jobs:
         hdf5: [ 1.12.2 ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -149,7 +149,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -208,7 +208,7 @@ jobs:
         hdf5: [ 1.12.2 ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -224,7 +224,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-parallel-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -289,7 +289,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -308,7 +308,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf5
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -369,7 +369,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -388,7 +388,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf5
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-parallel-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -448,7 +448,7 @@ jobs:
         use_nczarr: [ nczarr_off, nczarr_on ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -482,7 +482,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
@@ -501,31 +501,37 @@ jobs:
 
       - name: Configure
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure ${ENABLE_HDF5} ${ENABLE_DAP} ${ENABLE_NCZARR}
+        run: |
+          current_directory="$(pwd)"
+          mkdir ../build
+          cd ../build && CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} "${current_directory}/configure" ${ENABLE_HDF5} ${ENABLE_DAP} ${ENABLE_NCZARR}
         if: ${{ success() }}
 
       - name: Look at config.log if error
         shell: bash -l {0}
-        run: cat config.log
+        run: cd ../build && cat config.log
         if: ${{ failure() }}
 
       - name: Print Summary
         shell: bash -l {0}
-        run: cat libnetcdf.settings
+        run: cd ../build && cat libnetcdf.settings
 
       - name: Build Library and Utilities
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} make -j
+        run: |
+          cd ../build && CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} make -j
         if: ${{ success() }}
 
       - name: Build Tests
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} make check TESTS="" -j
+        run: |
+          cd ../build && CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} make check TESTS="" -j
         if: ${{ success() }}
 
       - name: Run Tests
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} make check -j
+        run: |
+          cd ../build && CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} make check -j
         if: ${{ success() }}
 
   nc-cmake:
@@ -541,7 +547,7 @@ jobs:
         use_nczarr: [ nczarr_off, nczarr_on ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install System dependencies
         shell: bash -l {0}
@@ -575,7 +581,7 @@ jobs:
 
       - name: Fetch HDF Cache
         id: cache-hdf5
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/environments/${{ matrix.hdf5 }}
           key: hdf5-${{ runner.os }}-${{ matrix.hdf5 }}
diff --git a/.github/workflows/run_tests_win_cygwin.yml b/.github/workflows/run_tests_win_cygwin.yml
index d3e4f5c88..bfd642f50 100644
--- a/.github/workflows/run_tests_win_cygwin.yml
+++ b/.github/workflows/run_tests_win_cygwin.yml
@@ -20,7 +20,7 @@ jobs:
       - name: Fix line endings
         run: git config --global core.autocrlf input
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - uses: cygwin/cygwin-install-action@v2
         with:
diff --git a/.github/workflows/run_tests_win_mingw.yml b/.github/workflows/run_tests_win_mingw.yml
index d87212859..11f066be6 100644
--- a/.github/workflows/run_tests_win_mingw.yml
+++ b/.github/workflows/run_tests_win_mingw.yml
@@ -22,7 +22,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: msys2/setup-msys2@v2
         with:
           msystem: MINGW64
diff --git a/docs/CMakeLists.txt b/docs/CMakeLists.txt
index 6a310a0f2..119d23eec 100644
--- a/docs/CMakeLists.txt
+++ b/docs/CMakeLists.txt
@@ -95,6 +95,6 @@ obsolete/fan_utils.html bestpractices.md filters.md indexing.md
 inmemory.md DAP2.dox FAQ.md
 known_problems.md
 COPYRIGHT.dox user_defined_formats.md DAP4.md DAP4.dox
-testserver.dox byterange.dox filters.md nczarr.md auth.md quantize.md)
+testserver.dox byterange.md filters.md nczarr.md auth.md quantize.md)
 
 ADD_EXTRA_DIST("${CUR_EXTRA_DIST}")
diff --git a/docs/Doxyfile.in b/docs/Doxyfile.in
index 58064b113..35f1c8d3b 100644
--- a/docs/Doxyfile.in
+++ b/docs/Doxyfile.in
@@ -809,13 +809,10 @@ INPUT                  = @abs_top_srcdir@/docs/mainpage.dox \
                          @abs_top_srcdir@/RELEASE_NOTES.md \
                          @abs_top_srcdir@/docs/install-fortran.md \
                          @abs_top_srcdir@/docs/windows-binaries.md \
-                         @abs_top_srcdir@/docs/attribute_conventions.md \
-                         @abs_top_srcdir@/docs/file_format_specifications.md \
-                         @abs_top_srcdir@/docs/all-error-codes.md \
                          @abs_top_srcdir@/docs/inmemory.md \
                          @abs_top_srcdir@/docs/filter_quickstart.md \
                          @abs_top_srcdir@/docs/filters.md \
-                         @abs_top_srcdir@/docs/byterange.dox \
+                         @abs_top_srcdir@/docs/byterange.md \
                          @abs_top_srcdir@/docs/nczarr.md \
                          @abs_top_srcdir@/docs/notes.md \
                          @abs_top_srcdir@/docs/building-with-cmake.md \
@@ -829,6 +826,9 @@ INPUT                  = @abs_top_srcdir@/docs/mainpage.dox \
                          @abs_top_srcdir@/docs/indexing.dox \
                          @abs_top_srcdir@/docs/testserver.dox \
                          @abs_top_srcdir@/docs/quantize.md \
+                         @abs_top_srcdir@/docs/attribute_conventions.md \
+                         @abs_top_srcdir@/docs/file_format_specifications.md \
+                         @abs_top_srcdir@/docs/all-error-codes.md \
                          @abs_top_srcdir@/include/netcdf.h \
                          @abs_top_srcdir@/include/netcdf_mem.h \
                          @abs_top_srcdir@/include/netcdf_par.h \
diff --git a/docs/Doxyfile.user b/docs/Doxyfile.user
index c70f0f458..a6b85743c 100644
--- a/docs/Doxyfile.user
+++ b/docs/Doxyfile.user
@@ -748,7 +748,7 @@ INPUT = \
     ./docs/windows-binaries.md \
     ./docs/attribute_conventions.md \
     ./docs/file_format_specifications.md \
-    ./docs/byterange.dox \
+    ./docs/byterange.md \
     ./docs/inmemory.md \
     ./docs/auth.md \
     ./docs/filters.md \
diff --git a/docs/Makefile.am b/docs/Makefile.am
index 29be233d9..df5fac746 100644
--- a/docs/Makefile.am
+++ b/docs/Makefile.am
@@ -7,15 +7,13 @@
 # See netcdf-c/COPYRIGHT file for more info.
 
 # These files will be included with the dist.
-EXTRA_DIST = CMakeLists.txt  COPYRIGHT.md FAQ.md		\
-netcdf.m4 DoxygenLayout.xml Doxyfile.in footer.html		\
-mainpage.dox tutorial.dox architecture.dox			\
-groups.dox  indexing.dox  inmeminternal.dox testserver.dox	\
-byterange.dox							\
-windows-binaries.md dispatch.md building-with-cmake.md		\
-notes.md install-fortran.md credits.md auth.md filters.md	\
-obsolete/fan_utils.html inmemory.md known_problems.md		\
-nczarr.md quantize.md all-error-codes.md
+EXTRA_DIST = netcdf.m4 DoxygenLayout.xml Doxyfile.in footer.html	\
+mainpage.dox tutorial.dox architecture.dox internal.dox			\
+windows-binaries.md dispatch.md building-with-cmake.md CMakeLists.txt groups.dox	\
+notes.md install-fortran.md credits.md auth.md filters.md		\
+obsolete/fan_utils.html indexing.dox inmemory.md FAQ.md			\
+known_problems.md COPYRIGHT.md inmeminternal.dox testserver.dox		\
+byterange.md nczarr.md quantize.md all-error-codes.md
 
 # Turn off parallel builds in this directory.
 .NOTPARALLEL:
diff --git a/docs/byterange.dox b/docs/byterange.dox
deleted file mode 100644
index 82730a50e..000000000
--- a/docs/byterange.dox
+++ /dev/null
@@ -1,156 +0,0 @@
-/**
-@if INTERNAL
-
-@page byterange Remote Dataset Access Using HTTP Byte Ranges
-
-\tableofcontents
-
-<!-- Note that this file has the .dox extension, but is mostly markdown -->
-<!-- Begin MarkDown -->
-
-# Introduction {#byterange_intro}
-
-Suppose that you have the URL to a remote dataset
-which is a normal netcdf-3 or netcdf-4 file.
-
-The netCDF-c library now supports read-only access to such
-datasets using the HTTP byte range capability [], assuming that
-the remote server supports byte-range access.
-
-Two examples:
-
-1. An Amazon S3 object containing a netcdf classic file.
-   - location: "https://remotetest.unidata.ucar.edu/thredds/fileServer/testdata/2004050300_eta_211.nc#mode=bytes" 
-2. A Thredds Server dataset supporting the Thredds HTTPServer protocol.
-   and containing a netcdf enhanced file.
-   - location: "http://noaa-goes16.s3.amazonaws.com/ABI-L1b-RadC/2017/059/03/OR_ABI-L1b-RadC-M3C13_G16_s20170590337505_e20170590340289_c20170590340316.nc#mode=bytes"
-
-Other remote servers may also provide byte-range access in a similar form.
-
-It is important to note that this is not intended as a true
-production capability because it is believed that this kind of access
-can be quite slow. In addition, the byte-range IO drivers do not
-currently do any sort of optimization or caching.
-
-# Configuration {#byterange_config}
-
-This capability is enabled using the option *--enable-byterange* option
-to the *./configure* command for Automake. For Cmake, the option flag is
-*-DENABLE_BYTERANGE=true*.
-
-This capability requires access to *libcurl*, and an error will occur
-if byterange is enabled, but no *libcurl* could not be located.
-In this, it is similar to the DAP2 and DAP4 capabilities.
-
-Note also that here, the term "http" is often used as a synonym for *byterange*.
-
-# Run-time Usage {#byterange_url}
-
-In order to use this capability at run-time, with *ncdump* for
-example, it is necessary to provide a URL pointing to the basic
-dataset to be accessed. The URL must be annotated to tell the
-netcdf-c library that byte-range access should be used. This is
-indicated by appending the phrase ````#mode=bytes````
-to the end of the URL.
-The two examples above show how this will look.
-
-In order to determine the kind of file being accessed, the
-netcdf-c library will read what is called the "magic number"
-from the beginning of the remote dataset. This magic number
-is a specific set of bytes that indicates the kind of file:
-classic, enhanced, cdf5, etc. 
-
-# Architecture {#byterange_arch}
-
-Internally, this capability is implemented with three files:
-
-1. libdispatch/dhttp.c -- wrap libcurl operations.
-2. libsrc/httpio.c -- provide byte-range reading to the netcdf-3 dispatcher.
-3. libhdf5/H5FDhttp.c -- provide byte-range reading to the netcdf-4 dispatcher.
-
-Both *httpio.c* and *H5FDhttp.c* are adapters that use *dhttp.c*
-to do the work. Testing for the magic number is also carried out
-by using the *dhttp.c* code.
-
-## NetCDF Classic Access
-
-The netcdf-3 code in the directory *libsrc* is built using
-a secondary dispatch mechanism called *ncio*. This allows the
-netcdf-3 code be independent of the lowest level IO access mechanisms.
-This is how in-memory and mmap based access is implemented.
-The file *httpio.c* is the dispatcher used to provide byte-range
-IO for the netcdf-3 code.
-
-Note that *httpio.c* is mostly just an
-adapter between the *ncio* API and the *dhttp.c* code.
-
-## NetCDF Enhanced Access
-
-Similar to the netcdf-3 code, the HDF5 library
-provides a secondary dispatch mechanism *H5FD*. This allows the
-HDF5 code to be independent of the lowest level IO access mechanisms.
-The netcdf-4 code in libhdf5 is built on the HDF5 library, so
-it indirectly inherits the H5FD mechanism.
-
-The file *H5FDhttp.c* implements the H5FD dispatcher API
-and provides byte-range IO for the netcdf-4 code
-(and for the HDF5 library as a side effect).
-
-Note that *H5FDhttp.c* is mostly just an
-adapter between the *H5FD* API and the *dhttp.c* code.
-
-# The dhttp.c Code {#byterange_dhttp}
-
-The core of all this is *dhttp.c* (and its header
-*include/nchttp.c*). It is a wrapper over *libcurl*
-and so exposes the libcurl handles -- albeit as _void*_.
-
-The API for *dhttp.c* consists of the following procedures:
-- int nc_http_open(const char* objecturl, void** curlp, fileoffset_t* filelenp);
-- int nc_http_read(void* curl, const char* url, fileoffset_t start, fileoffset_t count, NCbytes* buf);
-- int nc_http_close(void* curl);
-- typedef long long fileoffset_t;
-
-The type *fileoffset_t* is used to avoid use of *off_t* or *off64_t*
-which are too volatile. It is intended to be represent file lengths
-and offsets.
-
-## nc_http_open
-The *nc_http_open* procedure creates a *Curl* handle and returns it
-in the *curlp* argument. It also obtains and searches the headers
-looking for two headers:
-
-1. "Accept-Ranges: bytes" -- to verify that byte-range access is supported.
-2. "Content-Length: ..." -- to obtain the size of the remote dataset.
-
-The dataset length is returned in the *filelenp* argument.
-
-## nc_http_read
-
-The *nc_http_read* procedure reads a specified set of contiguous bytes
-as specified by the *start* and *count* arguments. It takes the *Curl*
-handle produced by *nc_http_open* to indicate the server from which to read.
-
-The *buf* argument is a pointer to an instance of type *NCbytes*, which
-is a dynamically expandable byte vector (see the file *include/ncbytes.h*).
-
-This procedure reads *count* bytes from the remote dataset starting at
-the offset *start* position. The bytes are stored in *buf*.
-
-## nc_http_close
-
-The *nc_http_close* function closes the *Curl* handle and does any
-necessary cleanup.
-
-# Point of Contact {#byterange_poc}
-
-__Author__: Dennis Heimbigner<br>
-__Email__: dmh at ucar dot edu<br>
-__Initial Version__: 12/30/2018<br>
-__Last Revised__: 12/30/2018
-
-<!-- End MarkDown -->
-
-@endif
-
-*/
diff --git a/libdispatch/dfile.c b/libdispatch/dfile.c
index a53be4789..7dba910f3 100644
--- a/libdispatch/dfile.c
+++ b/libdispatch/dfile.c
@@ -125,8 +125,6 @@ int
 nc_def_user_format(int mode_flag, NC_Dispatch *dispatch_table, char *magic_number)
 {
     /* Check inputs. */
-    if (mode_flag != NC_UDF0 && mode_flag != NC_UDF1)
-        return NC_EINVAL;
     if (!dispatch_table)
         return NC_EINVAL;
     if (magic_number && strlen(magic_number) > NC_MAX_MAGIC_NUMBER_LEN)
@@ -135,21 +133,29 @@ nc_def_user_format(int mode_flag, NC_Dispatch *dispatch_table, char *magic_numbe
     /* Check the version of the dispatch table provided. */
     if (dispatch_table->dispatch_version != NC_DISPATCH_VERSION)
         return NC_EINVAL;
-
+    /* user defined magic numbers not allowed with netcdf3 modes */ 
+    if (magic_number && (fIsSet(mode_flag, NC_64BIT_OFFSET) ||
+                         fIsSet(mode_flag, NC_64BIT_DATA) ||
+                        (fIsSet(mode_flag, NC_CLASSIC_MODEL) &&
+                        !fIsSet(mode_flag, NC_NETCDF4))))
+        return NC_EINVAL;
     /* Retain a pointer to the dispatch_table and a copy of the magic
      * number, if one was provided. */
-    switch(mode_flag)
+    if (fIsSet(mode_flag,NC_UDF0))
     {
-    case NC_UDF0:
         UDF0_dispatch_table = dispatch_table;
         if (magic_number)
             strncpy(UDF0_magic_number, magic_number, NC_MAX_MAGIC_NUMBER_LEN);
-        break;
-    case NC_UDF1:
+    }
+    else if(fIsSet(mode_flag, NC_UDF1))
+    {
         UDF1_dispatch_table = dispatch_table;
         if (magic_number)
             strncpy(UDF1_magic_number, magic_number, NC_MAX_MAGIC_NUMBER_LEN);
-        break;
+    }
+    else
+    {
+        return NC_EINVAL;
     }
 
     return NC_NOERR;
@@ -175,23 +181,23 @@ int
 nc_inq_user_format(int mode_flag, NC_Dispatch **dispatch_table, char *magic_number)
 {
     /* Check inputs. */
-    if (mode_flag != NC_UDF0 && mode_flag != NC_UDF1)
-        return NC_EINVAL;
-
-    switch(mode_flag)
+    if (fIsSet(mode_flag,NC_UDF0))
     {
-    case NC_UDF0:
         if (dispatch_table)
             *dispatch_table = UDF0_dispatch_table;
         if (magic_number)
             strncpy(magic_number, UDF0_magic_number, NC_MAX_MAGIC_NUMBER_LEN);
-        break;
-    case NC_UDF1:
+    }
+    else if(fIsSet(mode_flag,NC_UDF1))
+    {
         if (dispatch_table)
             *dispatch_table = UDF1_dispatch_table;
         if (magic_number)
             strncpy(magic_number, UDF1_magic_number, NC_MAX_MAGIC_NUMBER_LEN);
-        break;
+    }
+    else
+    {
+        return NC_EINVAL;
     }
 
     return NC_NOERR;
diff --git a/libdispatch/dinfermodel.c b/libdispatch/dinfermodel.c
index ff3e8e977..5bfd27747 100644
--- a/libdispatch/dinfermodel.c
+++ b/libdispatch/dinfermodel.c
@@ -118,8 +118,8 @@ static struct FORMATMODES {
 {"classic",NC_FORMATX_NC3,0}, /* ditto */
 {"netcdf-4",NC_FORMATX_NC4,NC_FORMAT_NETCDF4},
 {"enhanced",NC_FORMATX_NC4,NC_FORMAT_NETCDF4},
-{"udf0",NC_FORMATX_UDF0,NC_FORMAT_NETCDF4},
-{"udf1",NC_FORMATX_UDF1,NC_FORMAT_NETCDF4},
+{"udf0",NC_FORMATX_UDF0,0},
+{"udf1",NC_FORMATX_UDF1,0},
 {"nczarr",NC_FORMATX_NCZARR,NC_FORMAT_NETCDF4},
 {"zarr",NC_FORMATX_NCZARR,NC_FORMAT_NETCDF4},
 {"bytes",NC_FORMATX_NC4,NC_FORMAT_NETCDF4}, /* temporary until 3 vs 4 is determined */
@@ -182,8 +182,8 @@ static struct Readable {
 {NC_FORMATX_PNETCDF,1},
 {NC_FORMATX_DAP2,0},
 {NC_FORMATX_DAP4,0},
-{NC_FORMATX_UDF0,0},
-{NC_FORMATX_UDF1,0},
+{NC_FORMATX_UDF0,1},
+{NC_FORMATX_UDF1,1},
 {NC_FORMATX_NCZARR,0}, /* eventually make readable */
 {0,0},
 };
@@ -762,13 +762,31 @@ NC_omodeinfer(int useparallel, int cmode, NCmodel* model)
      * use some of the other flags, like NC_NETCDF4, so we must first
      * check NC_UDF0 and NC_UDF1 before checking for any other
      * flag. */
-    if(fIsSet(cmode,(NC_UDF0|NC_UDF1))) {
-	model->format = NC_FORMAT_NETCDF4;
-        if(fIsSet(cmode,NC_UDF0)) {
+    if(fIsSet(cmode, NC_UDF0)  || fIsSet(cmode, NC_UDF1))
+    {
+        if(fIsSet(cmode, NC_UDF0))
+        {
 	    model->impl = NC_FORMATX_UDF0;
 	} else {
 	    model->impl = NC_FORMATX_UDF1;
 	}
+        if(fIsSet(cmode,NC_64BIT_OFFSET)) 
+        {
+            model->format = NC_FORMAT_64BIT_OFFSET;
+        }
+        else if(fIsSet(cmode,NC_64BIT_DATA))
+        {
+            model->format = NC_FORMAT_64BIT_DATA;
+        }
+        else if(fIsSet(cmode,NC_NETCDF4))
+        {
+            if(fIsSet(cmode,NC_CLASSIC_MODEL))
+                model->format = NC_FORMAT_NETCDF4_CLASSIC;
+            else
+                model->format = NC_FORMAT_NETCDF4;
+        }
+        if(! model->format)
+            model->format = NC_FORMAT_CLASSIC;
 	goto done;
     }
 
@@ -981,8 +999,6 @@ NC_infermodel(const char* path, int* omodep, int iscreate, int useparallel, void
     case NC_FORMATX_NC4:
     case NC_FORMATX_NC_HDF4:
     case NC_FORMATX_DAP4:
-    case NC_FORMATX_UDF0:
-    case NC_FORMATX_UDF1:
     case NC_FORMATX_NCZARR:
 	omode |= NC_NETCDF4;
 	if(model->format == NC_FORMAT_NETCDF4_CLASSIC)
@@ -1001,6 +1017,17 @@ NC_infermodel(const char* path, int* omodep, int iscreate, int useparallel, void
     case NC_FORMATX_DAP2:
 	omode &= ~(NC_NETCDF4|NC_64BIT_OFFSET|NC_64BIT_DATA|NC_CLASSIC_MODEL);
 	break;
+    case NC_FORMATX_UDF0:
+    case NC_FORMATX_UDF1:
+        if(model->format == NC_FORMAT_64BIT_OFFSET) 
+            omode |= NC_64BIT_OFFSET;
+        else if(model->format == NC_FORMAT_64BIT_DATA)
+            omode |= NC_64BIT_DATA;
+        else if(model->format == NC_FORMAT_NETCDF4)  
+            omode |= NC_NETCDF4;
+        else if(model->format == NC_FORMAT_NETCDF4_CLASSIC)  
+            omode |= NC_NETCDF4|NC_CLASSIC_MODEL;
+        break;
     default:
 	{stat = NC_ENOTNC; goto done;}
     }
@@ -1513,23 +1540,10 @@ static int
 NC_interpret_magic_number(char* magic, NCmodel* model)
 {
     int status = NC_NOERR;
+    int tmpimpl = 0;
     /* Look at the magic number */
-#ifdef USE_NETCDF4
-    if (strlen(UDF0_magic_number) && !strncmp(UDF0_magic_number, magic,
-                                              strlen(UDF0_magic_number)))
-    {
-	model->impl = NC_FORMATX_UDF0;
-	model->format = NC_FORMAT_NETCDF4;
-	goto done;
-    }
-    if (strlen(UDF1_magic_number) && !strncmp(UDF1_magic_number, magic,
-                                              strlen(UDF1_magic_number)))
-    {
-	model->impl = NC_FORMATX_UDF1;
-	model->format = NC_FORMAT_NETCDF4;
-	goto done;
-    }
-#endif /* USE_NETCDF4 */
+    if(model->impl == NC_FORMATX_UDF0 || model->impl == NC_FORMATX_UDF1)
+        tmpimpl = model->impl;
 
     /* Use the complete magic number string for HDF5 */
     if(memcmp(magic,HDF5_SIGNATURE,sizeof(HDF5_SIGNATURE))==0) {
@@ -1561,10 +1575,29 @@ NC_interpret_magic_number(char* magic, NCmodel* model)
 	}
      }
      /* No match  */
-     status = NC_ENOTNC;
+     if (!tmpimpl) 
+         status = NC_ENOTNC;         
+
      goto done;
 
 done:
+     /* if model->impl was UDF0 or UDF1 on entry, make it so on exit */
+     if(tmpimpl)
+         model->impl = tmpimpl;
+     /* if this is a UDF magic_number update the model->impl */
+     if (strlen(UDF0_magic_number) && !strncmp(UDF0_magic_number, magic,
+                                               strlen(UDF0_magic_number)))
+     {
+         model->impl = NC_FORMATX_UDF0;
+         status = NC_NOERR;
+     }
+     if (strlen(UDF1_magic_number) && !strncmp(UDF1_magic_number, magic,
+                                               strlen(UDF1_magic_number)))
+     {
+         model->impl = NC_FORMATX_UDF1;
+         status = NC_NOERR;
+     }    
+
      return check(status);
 }
 
diff --git a/libdispatch/dvar.c b/libdispatch/dvar.c
index 4e78285c5..f42f2b6aa 100644
--- a/libdispatch/dvar.c
+++ b/libdispatch/dvar.c
@@ -1303,6 +1303,9 @@ NC_check_nulls(int ncid, int varid, const size_t *start, size_t **count,
    pointer back to this function, when you're done with the data, and
    it will free the string memory.
 
+   WARNING: This does not free the data vector itself, only
+   the strings to which it points.
+
    @param len The number of character arrays in the array.
    @param data The pointer to the data array.
 
diff --git a/nc_test4/tst_udf.c b/nc_test4/tst_udf.c
index 47fb9590d..887e603d8 100644
--- a/nc_test4/tst_udf.c
+++ b/nc_test4/tst_udf.c
@@ -307,7 +307,7 @@ main(int argc, char **argv)
              * priority. If NC_NETCDF4 flag were given priority, then
              * nc_abort() will not return TEST_VAL_42, but instead will
              * return 0. */
-            if (nc_open(FILE_NAME, mode[i]|NC_NETCDF4, &ncid)) ERR;
+            if (nc_open(FILE_NAME, mode[i], &ncid)) ERR;
             if (nc_inq_format(ncid, NULL) != TEST_VAL_42) ERR;
             if (nc_inq_format_extended(ncid, NULL, NULL) != TEST_VAL_42) ERR;
             if (nc_abort(ncid) != TEST_VAL_42) ERR;
@@ -336,6 +336,7 @@ main(int argc, char **argv)
         for (i = 0; i < NUM_UDFS; i++)
         {
             /* Add our test user defined format. */
+            mode[i] = mode[i]|NC_NETCDF4;
             if (nc_def_user_format(mode[i], &tst_dispatcher, magic_number)) ERR;
 
             /* Check that our user-defined format has been added. */
@@ -360,6 +361,7 @@ main(int argc, char **argv)
     printf("*** testing bad version causes dispatch table to be rejected...");
     {
         int i;
+        char magic_number[5] = "1111";
 
         /* Test all available user-defined format slots. */
         for (i = 0; i < NUM_UDFS; i++)
@@ -367,6 +369,9 @@ main(int argc, char **argv)
             /* Make sure our bad version format is rejected. */
             if (nc_def_user_format(mode[i], &tst_dispatcher_bad_version,
                                    NULL) != NC_EINVAL) ERR;
+            /* Make sure defining a magic number with netcdf3 is rejected. */
+            if (nc_def_user_format(NC_CLASSIC_MODEL, &tst_dispatcher, 
+                                   magic_number) != NC_EINVAL) ERR;
         }
     }
     SUMMARIZE_ERR;
diff --git a/ncdump/CMakeLists.txt b/ncdump/CMakeLists.txt
index 39610f803..00447e9bf 100644
--- a/ncdump/CMakeLists.txt
+++ b/ncdump/CMakeLists.txt
@@ -263,6 +263,7 @@ endif()
     add_sh_test(ncdump tst_ncgen4)
     add_sh_test(ncdump tst_netcdf4_4)
     add_sh_test(ncdump tst_nccopy4)
+    add_sh_test(ncdump tst_calendars_nc4)
 
     SET_TESTS_PROPERTIES(ncdump_tst_nccopy4 PROPERTIES DEPENDS "ncdump_run_ncgen_tests;ncdump_tst_output;ncdump_tst_ncgen4;ncdump_sh_tst_fillbug;ncdump_tst_netcdf4_4;ncdump_tst_h_scalar;tst_comp;tst_comp2;tst_nans;tst_opaque_data;tst_create_files;tst_special_atts")
     SET_TESTS_PROPERTIES(ncdump_tst_nccopy5 PROPERTIES DEPENDS "ncdump_tst_nccopy4")
diff --git a/ncdump/Makefile.am b/ncdump/Makefile.am
index e601cdca4..6407a755b 100644
--- a/ncdump/Makefile.am
+++ b/ncdump/Makefile.am
@@ -151,7 +151,7 @@ TESTS += tst_output.sh
 TESTS += tst_nccopy3.sh
 if USE_HDF5
 TESTS += run_back_comp_tests.sh tst_netcdf4_4.sh
-TESTS += tst_nccopy4.sh tst_nccopy5.sh
+TESTS += tst_nccopy4.sh tst_nccopy5.sh tst_calendars_nc4.sh
 endif
 endif
 endif
@@ -177,7 +177,7 @@ ref_tst_noncoord.cdl ref_tst_compounds2.nc ref_tst_compounds2.cdl	\
 ref_tst_compounds3.nc ref_tst_compounds3.cdl ref_tst_compounds4.nc	\
 ref_tst_compounds4.cdl ref_tst_group_data_v23.cdl tst_mslp.cdl		\
 tst_bug321.cdl ref_tst_format_att.cdl ref_tst_format_att_64.cdl		\
-tst_nccopy3.sh tst_nccopy4.sh tst_nccopy5.sh				\
+tst_nccopy3.sh tst_nccopy4.sh tst_nccopy5.sh tst_calendars_nc4.sh	\
 ref_nc_test_netcdf4_4_0.nc run_back_comp_tests.sh			\
 ref_nc_test_netcdf4.cdl ref_tst_special_atts3.cdl tst_brecs.cdl		\
 ref_tst_grp_spec0.cdl ref_tst_grp_spec.cdl tst_grp_spec.sh		\
@@ -205,7 +205,7 @@ test_keywords.sh ref_keyword1.cdl ref_keyword2.cdl ref_keyword3.cdl ref_keyword4
 ref_tst_nofilters.cdl test_scope.sh  \
 test_rcmerge.sh ref_rcmerge1.txt ref_rcmerge2.txt ref_rcmerge3.txt \
 scope_ancestor_only.cdl scope_ancestor_subgroup.cdl scope_group_only.cdl scope_preorder.cdl \
-ref_rcapi.txt ref_tst_enum_undef.cdl
+ref_rcapi.txt ref_tst_enum_undef.cdl tst_calendars_nc4.cdl ref_times_nc4.cdl
 
 # The L512.bin file is file containing exactly 512 bytes each of value 0.
 # It is used for creating hdf5 files with varying offsets for testing.
@@ -247,7 +247,7 @@ 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
+scope_*.nc copy_scope_*.cdl keyword5.nc tst_enum_undef.cdl tst_times_nc4.cdl
 
 # Remove directories
 clean-local:
diff --git a/ncdump/nctime0.c b/ncdump/nctime0.c
index 461a79800..f7b3d2478 100644
--- a/ncdump/nctime0.c
+++ b/ncdump/nctime0.c
@@ -79,13 +79,18 @@ calendar_type(int ncid, int varid) {
     int ncals = (sizeof calmap)/(sizeof calmap[0]);
     ctype = cdMixed;  /* default mixed Gregorian/Julian ala udunits */
     stat = nc_inq_att(ncid, varid, CF_CAL_ATT_NAME, &catt.type, &catt.len);
-    if(stat == NC_NOERR && catt.type == NC_CHAR && catt.len > 0) {
-	char *calstr = (char *)emalloc(catt.len + 1);
+    if(stat == NC_NOERR && (catt.type == NC_CHAR || catt.type == NC_STRING) && catt.len > 0) {
+	char *calstr;
+	size_t cf_cal_att_name_len = strlen(CF_CAL_ATT_NAME);
+	strncpy(catt.name, CF_CAL_ATT_NAME, cf_cal_att_name_len);
+	catt.name[cf_cal_att_name_len] = '\0';
+	catt.tinfo = get_typeinfo(catt.type);
+	nc_get_att_single_string(ncid, varid, &catt, &calstr);
+
 	int itype;
-	NC_CHECK(nc_get_att(ncid, varid, CF_CAL_ATT_NAME, calstr));	
-	calstr[catt.len] = '\0';
+	int calstr_len = strlen(calstr);
 	for(itype = 0; itype < ncals; itype++) {
-	    if(strncasecmp(calstr, calmap[itype].attname, catt.len) == 0) {
+	    if(strncasecmp(calstr, calmap[itype].attname, calstr_len) == 0) {
 		ctype = calmap[itype].type;
 		break;
 	    }
@@ -204,10 +209,11 @@ get_timeinfo(int ncid1, int varid1, ncvar_t *vp) {
 
     /* time variables must have appropriate units attribute or be a bounds variable */
     nc_status = nc_inq_att(ncid, varid, "units", &uatt.type, &uatt.len);
-    if(nc_status == NC_NOERR && uatt.type == NC_CHAR) { /* TODO: NC_STRING? */
-	units = emalloc(uatt.len + 1);
-	NC_CHECK(nc_get_att(ncid, varid, "units", units));
-	units[uatt.len] = '\0';
+    if(nc_status == NC_NOERR && (uatt.type == NC_CHAR || uatt.type == NC_STRING)) {
+	strncpy(uatt.name, "units", 5);
+	uatt.name[5] = '\0';
+	uatt.tinfo = get_typeinfo(uatt.type);
+	nc_get_att_single_string(ncid, varid, &uatt, &units);
 	if(!is_valid_time_unit(units)) {
 	    free(units);
 	    return;
diff --git a/ncdump/utils.c b/ncdump/utils.c
index f2b79863a..263ae9f0a 100644
--- a/ncdump/utils.c
+++ b/ncdump/utils.c
@@ -13,6 +13,7 @@
 #include <assert.h>
 #include <ctype.h>
 #include "utils.h"
+#include "nccomps.h"
 #ifndef isascii
 EXTERNL int isascii(int c);
 #endif
@@ -959,3 +960,39 @@ done:
 }
 #endif
 
+/*********************************************************************************/
+void nc_get_att_single_string(const int ncid, const int varid,
+                              const struct ncatt_t *att, char **str_out) {
+    if (att->type == NC_CHAR) {
+        // NC_CHAR type attribute
+        // Use a call to nc_get_att_text which expects to output the attribute value
+        // into a char * pointing to allocated memory. The number of bytes to allocate
+        // is the attribute length (which is the number of elements in a vector, 1 for
+        // scalar) times the size of each element in bytes. The attribute length is
+        // held in att->len, and the attribute element size is in att->tinfo->size.
+        *str_out = emalloc((att->len + 1) * att->tinfo->size);
+	(*str_out)[att->len] = '\0';
+        NC_CHECK(nc_get_att_text(ncid, varid, att->name, *str_out));
+    } else if (att->type == NC_STRING) {
+        // NC_STRING type attribute
+        // Use a call to nc_get_att_string which expects to output the attribute value
+        // into a vector of char pointers, where each entry points to allocated memory.
+        // The vector of char pointers needs to be allocated to the length (number of strings)
+        // times the size of each entry (size of a char *).
+        char **att_strings = emalloc((att->len + 1) * att->tinfo->size);
+        NC_CHECK(nc_get_att_string(ncid, varid, att->name, att_strings));
+        // str_out needs to be allocated to a size large enough to hold the string that
+        // the first pointer in att_strings is pointing to.
+        size_t att_str_len = strlen(att_strings[0]);
+        *str_out = emalloc((att_str_len + 1) * att->tinfo->size);
+	(*str_out)[att_str_len] = '\0';
+        strncpy(*str_out, att_strings[0], att_str_len);
+        nc_free_string(att->len, att_strings); /* Warning: does not free att_strings */
+	free(att_strings);
+    } else {
+        fprintf(stderr,"nc_get_att_single_string: unknown attribute type: %d\n", att->type);
+        fprintf(stderr,"                          must use one of:  NC_CHAR, NC_STRING\n");
+        fflush(stderr); fflush(stdout);
+        exit(2);
+    }
+}
diff --git a/ncdump/utils.h b/ncdump/utils.h
index dc2a7aee2..cd413e39a 100644
--- a/ncdump/utils.h
+++ b/ncdump/utils.h
@@ -10,6 +10,8 @@
 
 #include "config.h"
 
+struct ncatt_t;
+
 #ifndef NCSTREQ
 #define	NCSTREQ(a, b)	(*(a) == *(b) && strcmp((a), (b)) == 0)
 #endif
@@ -181,6 +183,16 @@ extern int nc_next_giter(ncgiter_t *iterp, int *grpid);
 extern void nc_free_giter(ncgiter_t *iterp);
 extern int getrootid(int grpid);
 
+/*
+ * Get attribute value for a single string value from either of NC_CHAR or NC_STRING types.
+ * This routine assumes that the attribute holds a single string value. If there are more
+ * than one string, subequent strings after the first one will be ignored.
+ *
+ * The caller is responsible for allocating and freeing memory for the str_out parameter.
+ */
+extern void nc_get_att_single_string(const int ncid, const int varid,
+                                     const struct ncatt_t *att, char **str_out);
+
 #ifdef __cplusplus
 }
 #endif