java-native-access/jna · error · UnsupportedOperationException

Maximum argument count is

Error message

Maximum argument count is <MAX_NARGS>

What it means

The native invocation path has a hard limit MAX_NARGS on the number of arguments a function call may have. Java arrays larger than this cannot be marshaled, so JNA throws UnsupportedOperationException before any native code runs.

Solutions

1. Reduce the parameter count by bundling arguments into a JNA Structure passed by reference
2. Use a Pointer/pointer-array argument to pass bulk data instead of individual parameters
3. Change the native signature to take a struct or array if you control the native code

Example fix

// before
int f(int a1,int a2,...,int a300);
// after
class Args extends Structure { public int[] a = new int[300]; }
int f(Args args);

if (inArgs != null && inArgs.length > Function.MAX_NARGS) {
    throw new IllegalArgumentException("too many args: " + inArgs.length);
}

When it happens

Trigger: Invoking a native function whose mapped interface method declares more parameters than MAX_NARGS (typically 256), e.g. a generated binding with a huge parameter list or a variadic-style misuse.

Common situations: Code-generating large argument-pass-through functions; attempting to pass a big batch of scalars one-by-one instead of via a struct or array.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

• NOT IMPLEMENTED: getValue
• This pointer is opaque:
• Allocation size must be greater than zero
• Area is not polygonal: " + area
• Argument value 0x exceeds native capacity ( bytes) mask=0x

AI-assisted analysis of java-native-access/jna@d036ad9781 (2026-09-12). Data as JSON: /api/errors/ae325b5b511ee594. Report an issue: GitHub.

Appendix: source

Thrown at src/com/sun/jna/Function.java:332

     */
    public Object invoke(Class<?> returnType, Object[] inArgs, Map<String, ?> options) {
        Method invokingMethod = (Method)options.get(OPTION_INVOKING_METHOD);
        Class<?>[] paramTypes = invokingMethod != null ? invokingMethod.getParameterTypes() : null;
        return invoke(invokingMethod, paramTypes, returnType, inArgs, options);
    }

    /** Invoke the native function with the given arguments, returning the
     * native result as an Object. This method can be called if invoking method and parameter
     * types are already at hand. When calling {@link Function#invoke(Class, Object[], Map)},
     * the method has to be in the options under key {@link Function#OPTION_INVOKING_METHOD}.
     */
    Object invoke(Method invokingMethod, Class<?>[] paramTypes, Class<?> returnType, Object[] inArgs, Map<String, ?> options) {
        // Clone the argument array to obtain a scratch space for modified
        // types/values
        Object[] args = { };
        if (inArgs != null) {
            if (inArgs.length > MAX_NARGS) {
                throw new UnsupportedOperationException("Maximum argument count is " + MAX_NARGS);
            }
            args = new Object[inArgs.length];
            System.arraycopy(inArgs, 0, args, 0, args.length);
        }

        TypeMapper mapper = (TypeMapper)options.get(Library.OPTION_TYPE_MAPPER);
        boolean allowObjects = Boolean.TRUE.equals(options.get(Library.OPTION_ALLOW_OBJECTS));
        boolean isVarArgs = args.length > 0 && invokingMethod != null ? isVarArgs(invokingMethod) : false;
        int fixedArgs = args.length > 0 && invokingMethod != null ? fixedArgs(invokingMethod) : 0;
        for (int i=0; i < args.length; i++) {
            Class<?> paramType = invokingMethod != null
                ? (isVarArgs && i >= paramTypes.length-1
                   ? paramTypes[paramTypes.length-1].getComponentType()
                   : paramTypes[i])
                : null;
            args[i] = convertArgument(args, i, invokingMethod, mapper, allowObjects, paramType);
        }

View on GitHub (pinned to d036ad9781)