src/jdk.jextract/share/classes/com/sun/tools/jextract/HeaderResolver.java

*** 36,49 ****
--- 36,51 ----
      // The folder path mapping to package name
      private final Map<Path, String> pkgMap = new LinkedHashMap<>();
      // The header file parsed
      private final Map<Path, HeaderFile> headerMap = new LinkedHashMap<>();
      private final Log log;
+     private final Path builtinHeader;
  
      public HeaderResolver(Context ctx) {
          this.log = ctx.log;
          usePackageForFolder(Context.getBuiltinHeadersDir(), "clang_support");
+         this.builtinHeader = Context.getBuiltinHeaderFile();
          ctx.sources.stream()
                  .map(Path::getParent)
                  .forEach(p -> usePackageForFolder(p, ctx.options.targetPackage));
          ctx.options.pkgMappings.forEach(this::usePackageForFolder);
      }
*** 142,149 ****
--- 144,155 ----
          final HeaderPath hp = resolveHeaderPath(header);
          return new HeaderFile(this, header, hp.pkg, hp.headerCls, hp.forwarderCls);
      }
  
      public HeaderFile headerFor(Path path) {
+         if (path == null) {
+             path = builtinHeader;
+         }
+ 
          return headerMap.computeIfAbsent(path.normalize().toAbsolutePath(), this::getHeaderFile);
      }
  }