Add strict mode for netCDF4 IOSP

cdm/core/src/main/java/ucar/nc2/util/xml/RuntimeConfigParser.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998-2025 John Caron and University Corporation for Atmospheric Research/Unidata
+ * Copyright (c) 1998-2026 John Caron and University Corporation for Atmospheric Research/Unidata
  * See LICENSE for license information.
  */
 
@@ -282,20 +282,27 @@ public static void read(org.jdom2.Element root, StringBuilder errlog) {
           String name = elem.getChildText("libraryName");
           errlog.append(
               String.format("Netcdf4Clibrary from NJ22CONFIG: libraryPath = '%s', libraryName = '%s' \n", path, name));
+
+          Element useForReadingElm = elem.getChild("useForReading");
+          boolean useForReading = Boolean.parseBoolean(useForReadingElm.getText());
+          errlog.append(String.format("    useForReading = '%s' \n", useForReading));
+
+          boolean strict = Boolean.parseBoolean(useForReadingElm.getAttributeValue("strict"));
+          errlog.append(String.format("    strictReader = '%s' \n", strict));
+
           if (path != null && name != null) {
             // reflection is used to decouple optional jars
             try {
               Class<?> netcdfClibraryClass =
                   RuntimeConfigParser.class.getClassLoader().loadClass(netcdfClibraryClassName);
-              Method method = netcdfClibraryClass.getMethod("setLibraryNameAndPath", String.class, String.class);
-              method.invoke(null, path, name); // static method has null for object
+              Method method =
+                  netcdfClibraryClass.getMethod("setLibraryNameAndPath", String.class, String.class, boolean.class);
+              method.invoke(null, path, name, strict); // static method has null for object
             } catch (Throwable e) {
               errlog.append(netcdfClibraryClassName + " is not on classpath\n");
             }
           }
 
-          boolean useForReading = Boolean.parseBoolean(elem.getChildText("useForReading"));
-          errlog.append(String.format("    useForReading = '%s' \n", useForReading));
           if (useForReading) {
             try {
               // Registers Nc4Iosp in front of all the other IOSPs already registered in NetcdfFile.<clinit>().

docs/src/site/pages/netcdfJava_tutorial/runtime/runtimeloading.md

@@ -1,6 +1,6 @@
 ---
 title: Runtime loading
-last_updated: 2025-08-15
+last_updated: 2026-06-16
 sidebar: netcdfJavaTutorial_sidebar
 permalink: runtime_loading.html
 toc: false
@@ -122,7 +122,7 @@ The configuration file looks like this:
   <Netcdf4Clibrary>{% raw %}{% annotation 9 %}{% endraw %}
     <libraryPath>/usr/local/lib</libraryPath>
     <libraryName>netcdf</libraryName>
-    <useForReading>false</useForReading>
+    <useForReading strict="false">false</useForReading>
   </Netcdf4Clibrary>
 </runtimeConfig>
 {% endhighlight_with_annotations %}
@@ -138,8 +138,10 @@ The configuration file looks like this:
 * {% annotation 9 %} Configure how the [NetCDF-4 C library](netcdf4_c_library.html) is discovered and used.
     * `libraryPath`: The directory in which the native library is installed.
     * `libraryName`: The name of the native library. This will be used to locate the proper `.DLL`, `.SO`, or `.DYLIB` file within the `libraryPath` directory.
-    * `useForReading`: By default, the native library is only used for writing NetCDF-4 files; a pure-Java layer is responsible for reading them. 
-    However, if this property is set to `true`, then it will be used for reading NetCDF-4 (and HDF5) files as well.
+    * `useForReading`: By default, the native library is only used for writing NetCDF-4 files; a pure-Java layer is responsible for reading them.
+    However, if this property is set to `true`, then the native library will be used for reading.
+    When enabled, all HDF5 files will be read by the native netCDF-C library.
+    However, if the strict attribute on the `useForReading` element is set to `true`, then the netCDF-C library will only be used to read in files that are highly likely to be netCDF-4 files (vanilla HDF5 files will still be read by the pure-Java layer).
 
 There are several ways pass the Runtime Configuration XML to the CDM library. From your application, you can pass a `java.io.InputStream` (or JDOM element) to 
 `ucar.nc2.util.xml.RuntimeConfigParser`, as in the following examples:

netcdf4/src/main/java/ucar/nc2/ffi/netcdf/NetcdfClibrary.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998-2021 John Caron and University Corporation for Atmospheric Research/Unidata
+ * Copyright (c) 1998-2026 John Caron and University Corporation for Atmospheric Research/Unidata
  * See LICENSE for license information.
  */
 
@@ -32,6 +32,8 @@ public class NetcdfClibrary {
   // Track if already tested for library presence.
   private static Boolean isClibraryPresent;
 
+  private static boolean strictRead = false;
+
   /**
    * Set the path and name of the netcdf c library.
    * <p>
@@ -45,7 +47,25 @@ public class NetcdfClibrary {
    * @param lib_name library name, may be null. If null, will use "netcdf".
    */
   public static void setLibraryNameAndPath(@Nullable String jna_path, @Nullable String lib_name) {
+    setLibraryNameAndPath(jna_path, lib_name, false);
+  }
 
+  /**
+   * Set the path and name of the netcdf c library.
+   * <p>
+   * Must be called prior to calling {@link #isLibraryPresent() isLibraryPresent}
+   * or {@link #getForeignFunctionInterface() getForeignFunctionInterface}, as
+   * the C library can only be successfully loaded once.
+   *
+   * @param jna_path path to shared libraries, may be null. If null, will look for system property
+   *        "jna.library.path", then environment variable "JNA_PATH". If set, will set
+   *        the environment variable "JNA_PATH".
+   * @param lib_name library name, may be null. If null, will use "netcdf".
+   * @param strict if true, only attempt to read files through the netCDF-C library that are likely
+   *        netCDF-4 files, not just any HDF5 file.
+   *
+   */
+  public static void setLibraryNameAndPath(@Nullable String jna_path, @Nullable String lib_name, boolean strict) {
     if (nc4 != null) {
       log.warn("netCDF-C library already set, ignoring.");
       return;
@@ -72,6 +92,7 @@ public static void setLibraryNameAndPath(@Nullable String jna_path, @Nullable St
 
     libName = lib_name;
     jnaPath = jna_path;
+    strictRead = strict;
 
     if ((isClibraryPresent == null || !isClibraryPresent) && jnaPath != null) {
       // call load to retry loading, but this time with jnaPath set
@@ -145,6 +166,10 @@ public static synchronized void shutdown() {
     }
   }
 
+  public static boolean isStrictRead() {
+    return strictRead;
+  }
+
   private static Nc4prototypes load() {
     if (nc4 == null) {
       if (jnaPath == null) {

netcdf4/src/main/java/ucar/nc2/jni/netcdf/Nc4Iosp.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998-2025 University Corporation for Atmospheric Research/Unidata
+ * Copyright (c) 1998-2026 University Corporation for Atmospheric Research/Unidata
  * See LICENSE for license information.
  */
 
@@ -132,6 +132,36 @@ public static void useHdfEos(boolean val) {
 
   public static void setDebugFlags(DebugFlags flags) {}
 
+  private static boolean isLikelyNetcdf4(RandomAccessFile raf) {
+    boolean likelyNc4 = false;
+    // Check if likely netCDF-4 file (to prevent reading vanilla HDF5 through the netCDF-C library)
+    // See https://github.com/Unidata/netcdf-c/discussions/3085
+    // Note: netCDF-4 files written outside the netCDF-C library may not pass these checks.
+    IntByReference test_ncid = new IntByReference();
+    int ret = nc4.nc_open(raf.getLocation(), NC_NOWRITE, test_ncid);
+    SizeTByReference lenp = new SizeTByReference();
+    // Does _NCProperties global attribute exist?
+    ret = nc4.nc_inq_attlen(test_ncid.getValue(), NC_GLOBAL, "_NCCCCC", lenp);
+    // ret = nc4.nc_inq_attlen(test_ncid.getValue(), NC_GLOBAL, CDM.NCPROPERTIES, lenp);
+    likelyNc4 = ret == NC_NOERR;
+    if (!likelyNc4) {
+      // Does _IsNetcdf4 global attribute exist, and is its value not 0?
+      // note: newer versions of netCDF-C include the _NCProperties check as part of determining
+      // the value of this attribute, but not all, so we need both checks.
+      lenp = new SizeTByReference();
+      ret = nc4.nc_inq_attlen(test_ncid.getValue(), NC_GLOBAL, CDM.ISNETCDF4, lenp);
+      if (ret == NC_NOERR && lenp.getValue().longValue() == 1) {
+        int[] isnc4 = new int[1];
+        ret = nc4.nc_get_att_int(test_ncid.getValue(), NC_GLOBAL, CDM.ISNETCDF4, isnc4);
+        likelyNc4 = ret == NC_NOERR && isnc4[0] != 0;
+      }
+    }
+    if (!likelyNc4) {
+      log.debug("May not be a netCDF-4 file: {}; falling back to HDF5 IOSP.", raf.getLocation());
+    }
+    return likelyNc4;
+  }
+
   //////////////////////////////////////////////////
   // Instance Variables
 
@@ -177,24 +207,32 @@ public void setChunker(Nc4Chunking chunker) {
   @Override
   public boolean isValidFile(RandomAccessFile raf) throws IOException {
     int format = NCheader.checkFileType(raf);
-    boolean valid = false;
+    boolean validCheck1 = false;
     switch (format) {
       case NCheader.NC_FORMAT_NETCDF4:
       case NCheader.NC_FORMAT_64BIT_DATA:
-        valid = true;
+        validCheck1 = true;
         break;
       default:
         break;// everything else is invalid
     }
-    if (valid) {
-      if (isClibraryPresent()) {
-        return true;
+
+    boolean validCheck2 = false;
+    if (!validCheck1) {
+      log.debug("File cannot be opened by Nc4Iosp: {}", raf.getLocation());
+    } else if (!isClibraryPresent()) {
+      log.debug("File appears to be valid but netCDF-C isn't installed: {}", raf.getLocation());
+    } else {
+      // file appears to be valid and netCDF-c is present
+      if (NetcdfClibrary.isStrictRead()) {
+        // strictly limit to reading files that are very likely netcdf4
+        validCheck2 = isLikelyNetcdf4(raf);
       } else {
-        log.debug("File is valid but the NetCDF-4 native library isn't installed: {}", raf.getLocation());
+        // try to read all HDF5 files through the netCDF-C library
+        validCheck2 = true;
       }
     }
-
-    return false;
+    return validCheck2;
   }
 
   // 2016-06-06 note: Once netcdf-c v4.4.1 is released, we should be able to return much better information from