Question

I have a function in c that adds a row to a table. The function takes as arguments various orderings of ints, floats, and strings by using an ellipsis add_row(int arg1, int arg2, ...) and parses this information based on how the columns are set up.

I need to call this function from Java and am using JNI. I’m not sure what the best way to do this is especially with Java’s stricter typing. I’ve considered a few solutions but none of them seem any good / I’m not sure how to implement them: passing everything as strings, passing a jobjectArray, or passing cell values one at a time.

Any help is much appreciated.

Thanks,

Ben

Answer 1

This is less a problem with Java and JNI and more a problem of how to call a var args function in C with a dynamic list of arguments.
See Calling a C function with a varargs argument dynamically which suggests having two versions of the var args function (although I think the convention is more to allow passthrough of an existing va_list, rather than for constructing one (which seems to be quite involved)).

The JNI bit should be just to define a Java native method with object array arguments which will have a C equivalent receiving the array. Use the JNI API to convert the values into the C equivalents (ints and ANSI strings) then load them into the var args structure and call your vadd_row() function.

Java:

package mypackage;
public class MyClass {
    ...
    public native void addRow(Object[] args);
    ...
}

C:

void vadd_row(int arg1, int arg2, va_list argp) {
    ... your function ...
}

void add_row(int arg1, int arg2, ...) {
    va_list argp;

    va_start(argp, arg2);
    vadd_row(int arg1, int arg2, argp);
    va_end(argp);
}


JNIEXPORT void JNICALL mypackage_MyClass_addRow(JNIEnv *env, jobject this, jint arg1, jint arg2, jobjectArray jarg_array) {
    va_list argp;
    /* need to construct argp, see link below for hints[1]; go through each element
       of the java array, get the object; convert to primitive value or ANSI string,
       then encode it into the va_list */
    vadd_row((int)arg1, (int)arg2, argp);
}

[1] https://bbs.archlinux.org/viewtopic.php?pid=238721

Is it worth the hassle?

Consider just writing a simpler C function that receives the arguments in an array, then create a wrapper that uses var args if needed.