尝试从指针读取字节数组时 JNA 内存访问无效

Question

Im trying to read a byte array from a Pointer using JNA and I keep getting:

Decompress with insz 11107, and outsize 65536
recieved 1
Decompression complete!
Decompress with insz 22112, and outsize 65536
recieved 1
Decompression complete!
Decompress with insz 22041, and outsize 65536
recieved 1
Decompression complete, final out size of 0!
Exception in thread "main" java.lang.Error: Invalid memory access
    at com.sun.jna.Native.read(Native Method)
    at com.sun.jna.Pointer.read(Pointer.java:149)
    at com.sun.jna.Pointer.getByteArray(Pointer.java:715)
    at me.TTARCHExtract.redo(TTARCHExtract.java:330)
    at me.TTARCHExtract.redo(TTARCHExtract.java:323)
    at me.TTARCHExtract.z_decompress(TTARCHExtract.java:313)

when I execute the code below:

public static final Pointer toPointer(byte[] array, int length){
        Memory ret = new Memory(length);
        ret.write(0, array, 0, length);
        return ret;
    }
//code starts here
private byte[] z_decompress(byte[] in,int insize,byte[] out,int outsize) {
        if(in==null)return null;
        System.out.println("Decompress with insz "+insize+", and outsize "+outsize);
        Pointer inptr = toPointer(in, insize);
        Pointer outptr = toPointer(out, outsize);
        ZStream deflate = new ZStream();
        ZStream z = new ZStream();
        TTARCHHelper.load();
        ZlibLibrary lib = TTARCHHelper.ZLIB_LIBRARY;
        this.initz(lib, z, 15);
        this.initz(lib, deflate, -15);
        return this.redo(insize, outsize, z, lib, inptr, outptr, true, deflate);
    }

    private byte[] redo(int insize,int outsize,ZStream z,ZlibLibrary lib,Pointer inptr,Pointer outptr,boolean first,ZStream deflate) {
        lib.inflateReset(z);
        z.next_in=inptr;
        z.next_out=outptr;
        z.avail_in=insize;
        z.avail_out=outsize;
        int out = lib.inflate(z, ZlibLibrary.Z_FINISH);
        if(!first)System.out.println("recieved "+out);
        if(out != ZlibLibrary.Z_STREAM_END) {
            if(first)return this.redo(insize, outsize, deflate, lib, inptr, outptr, false, null);
            System.out.println("Compressed zlib/deflate input at offset "
                    + ""+dgboff+" ("+insize+" > "+outsize+") is wrong or complete");
            System.exit(-1);
            return null;
        }
        System.out.println("Decompression complete!");
        return z.next_out.getByteArray(0, outsize);
    }

private void initz(ZlibLibrary lib,ZStream z, int w) {
        lib.inflateInit2_(z, w, lib.zlibVersion(), z.size());
    }

The getByteArray is where the error is happening. What could be causing this?

This error happens sometimes and with not all zlib input streams so Is it to do with the out size maybe being wrong?

Code is from C written project ttarchext

Answer 1

An Invalid Memory Access error in JNA is a sign that you are attempting to access memory which has not been allocated. In this case, the next_out pointer, with the full length of outsize . To debug this, you need to consult the API to see whether the function expects you to allocate the memory and pass it to the native function, or whether the native function itself will allocate the necessary memory. (In the latter case, the native code usually tells you how to free the memory when you're done with it.) For this API, the allocation is apparently done in the inflateInit2() call, so that's a hint toward the bug's root cause.

The output is instructive in that it shows that it succeeds once with a smaller insz but fails the second time with a larger insz . The difference is also evident in the stack trace for the crash, showing that the recursive call occurred in this second (larger input) case, but likely did not in the first case. (To confirm this in debugging, you should add some more output.)

For the recursive call, the only change is that instead of ZStream z , the third parameter is changed to Zstream deflate (where null is passed as the possible next value on the iteration.) While changing z to deflate seems right, I don't see where in the original code there should be a null . This seems to be intended to recurse as a "next" type of iteration until it's done. (This may not be the cause of the error but is suspicious.)

The only difference in the redo() call with the deflate argument instead of z is that deflate was called with a windowsize of -15 . This seems to be contrary to the documentation for inflateInit2_() which you have mapped:

The windowBits parameter shall be a base 2 logarithm of the maximum window size to use, and shall be a value between 8 and 15.

Since the original C code you're porting also used -15, this may be correct, but it's clear the different windowsize has an impact on the output.

I would suggest keeping deflate as the last argument of the recursive call instead of null , and adding more output statements to give you more insight on the values of the parameters as you recurse.

---

The other variable which could cause the error is the outsize value. This seems to imply the full outsize value is avialable to read, which may not be the case if you have reached the end of the allocation. It is possible outsize is a minimum size (perhaps the windowsize = 15 causes this to be true) the first time, but when recursing (the windowsize = -15 case) that can not be relied upon, and you should read fewer bytes from the output on the final iteration (reviewing the original source suggests z.total_out .)