Syntax errors - Converting Java to C
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Thread: Syntax errors - Converting Java to C

  1. #1
    Join Date
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    Syntax errors - Converting Java to C

    Hi there,

    I'm trying to perform some image processing using the Android NDK. I have a snippet of Java code that converts a YUV bitmap to RGB format, however, I need to convert it to C.

    This is what I have so far:

    Code:
    JNIEXPORT void JNICALL Java_com_twothreetwo_zoomplus_ZoomPlus_YUVtoRGB(JNIEnv * env, jobject obj, jintArray rgb, jbyteArray yuv420sp, jint width, jint height)
    {
    	int             sz;
    	int             i;
    	int             j;
    	int             Y;
    	int             Cr = 0;
    	int             Cb = 0;
    	int             pixPtr = 0;
    	int             jDiv2 = 0;
    	int             R = 0;
    	int             G = 0;
    	int             B = 0;
    	int             cOff;
    
    	sz = width * height;
    	 //if(out == null) throw new NullPointerException("buffer 'out' is null");
    	 //if(out.length < sz) throw new IllegalArgumentException("buffer 'out' size " + out.length + " < minimum " + sz);
    	 //if(fg == null) throw new NullPointerException("buffer 'fg' is null");
    	 //if(fg.length < sz) throw new IllegalArgumentException("buffer 'fg' size " + fg.length + " < minimum " + sz * 3/ 2);
    	 for(j = 0; j < height; j++) {
    			 pixPtr = j * width;
    			 jDiv2 = j >> 1;
    			 for(i = 0; i < width; i++) {
    					 Y = yuv420sp[pixPtr]; if(Y < 0) Y += 255;
    					 if((i & 0x1) != 1) {
    							 cOff = sz + jDiv2 * width + (i >> 1) * 2;
    							 Cb = yuv420sp[cOff];
    							 if(Cb < 0) Cb += 127; else Cb -= 128;
    							 Cr = yuv420sp[cOff + 1];
    							 if(Cr < 0) Cr += 127; else Cr -= 128;
    					 }
    					 R = Y + Cr + (Cr >> 2) + (Cr >> 3) + (Cr >> 5);
    					 if(R < 0) R = 0; else if(R > 255) R = 255;
    					 G = Y - (Cb >> 2) + (Cb >> 4) + (Cb >> 5) - (Cr >> 1) + (Cr >> 3) + (Cr >> 4) + (Cr >> 5);
    					 if(G < 0) G = 0; else if(G > 255) G = 255;
    					 B = Y + Cb + (Cb >> 1) + (Cb >> 2) + (Cb >> 6);
    					 if(B < 0) B = 0; else if(B > 255) B = 255;
    					 rgb[pixPtr++] = 0xff000000 + (B << 16) + (G << 8) + R;
    			 }
    	 }
    
    }
    created from the Java version:

    Code:
    public static void decodeYUV(int[] out, byte[] fg, int width, int height) throws NullPointerException, IllegalArgumentException 
         { 
        		         final int sz = width * height; 
        		         if(out == null) throw new NullPointerException("buffer 'out' is null"); 
        		         if(out.length < sz) throw new IllegalArgumentException("buffer 'out' size " + out.length + " < minimum " + sz); 
        		         if(fg == null) throw new NullPointerException("buffer 'fg' is null"); 
        		         if(fg.length < sz) throw new IllegalArgumentException("buffer 'fg' size " + fg.length + " < minimum " + sz * 3/ 2); 
        		         int i, j; 
        		         int Y, Cr = 0, Cb = 0; 
        		         for(j = 0; j < height; j++) { 
        		                 int pixPtr = j * width; 
        		                 final int jDiv2 = j >> 1; 
        		                 for(i = 0; i < width; i++) { 
        		                         Y = fg[pixPtr]; if(Y < 0) Y += 255; 
        		                         if((i & 0x1) != 1) { 
        		                                 final int cOff = sz + jDiv2 * width + (i >> 1) * 2; 
        		                                 Cb = fg[cOff]; 
        		                                 if(Cb < 0) Cb += 127; else Cb -= 128; 
        		                                 Cr = fg[cOff + 1]; 
        		                                 if(Cr < 0) Cr += 127; else Cr -= 128; 
        		                         } 
        		                         int R = Y + Cr + (Cr >> 2) + (Cr >> 3) + (Cr >> 5); 
        		                         if(R < 0) R = 0; else if(R > 255) R = 255; 
        		                         int G = Y - (Cb >> 2) + (Cb >> 4) + (Cb >> 5) - (Cr >> 1) + (Cr >> 
        		 3) + (Cr >> 4) + (Cr >> 5); 
        		                         if(G < 0) G = 0; else if(G > 255) G = 255; 
        		                         int B = Y + Cb + (Cb >> 1) + (Cb >> 2) + (Cb >> 6); 
        		                         if(B < 0) B = 0; else if(B > 255) B = 255; 
        		                         out[pixPtr++] = 0xff000000 + (B << 16) + (G << 8) + R; 
        		                 } 
        		         } 
        		 }
    However I'm getting the following errors:

    Code:
    apps/zoomplusndk/jni/zoomplusndk.c:53: warning: dereferencing 'void *' pointer
    apps/zoomplusndk/jni/zoomplusndk.c:53: error: void value not ignored as it ought to be
    apps/zoomplusndk/jni/zoomplusndk.c:56: warning: dereferencing 'void *' pointer
    apps/zoomplusndk/jni/zoomplusndk.c:56: error: void value not ignored as it ought to be
    apps/zoomplusndk/jni/zoomplusndk.c:58: warning: dereferencing 'void *' pointer
    apps/zoomplusndk/jni/zoomplusndk.c:58: error: void value not ignored as it ought to be
    apps/zoomplusndk/jni/zoomplusndk.c:67: warning: dereferencing 'void *' pointer
    apps/zoomplusndk/jni/zoomplusndk.c:67: error: invalid use of void expression
    Does anyone know why I'm getting these errors? As you can probably tell I'm not a C developer :/.

    Thank a lot!

    P.S line 53 is: Y = yuv420sp[pixPtr]; if(Y < 0) Y += 255;

  2. #2
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    Re: Syntax errors - Converting Java to C

    Quote Originally Posted by 232studios View Post
    Does anyone know why I'm getting these errors? As you can probably tell I'm not a C developer :/.
    Your code is not 'C' it is C++ as seen by you trying to throw an exception (even though it's commented out). The 'C' language has no "throw" syntax.

    You could have whittled your program down to something even a beginner would understand:
    Code:
    int main()
    {
        void *p;
        int x;
        x = *p;// error
    }
    You cannot aereference a void pointer to point to anything just like that. The reason being that a void pointer doesn't point to any particular type. In the example above, the void pointer p has no idea that x is an int, so the dereferencing will not work.

    The fix is to cast the void pointer to the appropriate type:
    Code:
    int main()
    {
        void *p[10];
        int x;
        x = *(int*)p;  
    }
    This cast the void* to an int*. Now the outer * dereferences the integer from p.

    So those variables you have in your code, something is declared as a void*, and you're doing the first code I posted above, which is illegal.
    Code:
    jbyteArray yuv420sp
    Since you are in a C++ forum, you should let persons reading this forum know what a "jbyteArray" is in terms of C++. Since this is where you say the error occurs, it is important to give this information to us.

    Regards,

    Paul McKenzie
    Last edited by Paul McKenzie; December 1st, 2011 at 11:47 AM.

  3. #3
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    2,413

    Re: Syntax errors - Converting Java to C

    Quote Originally Posted by 232studios View Post
    I need to convert it to C.
    Do it little by little.

    Start by making sure you can pass the parameters correctly. Then add code to the function in small increments and make sure you know what you're doing and that it works the same in both versions.

  4. #4
    Join Date
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    Re: Syntax errors - Converting Java to C

    Hi Paul,

    Thanks for your help. The code I'm trying to convert it to is C not C++. It probably looks a lot like C++ because it's being ported from Java (see original post) and the commented out lines are the original from Java. I posted both the original Java as well as my poor attempt to convert it to C as I thought it would help.

    The jbyteArray yuv420sp is just an array of bytes. It was auto generated when I created the .h (header?) file so I assumed it was ok. Should I convert it to an object instead?

    I've tried adding void * at the start of each yuv420sp[] but I'm still getting the same errors. e.g:

    Code:
    Y = (void *)yuv420sp[pixPtr];
    Am I implementing it incorrectly?

    Thanks a lot!!

  5. #5
    Join Date
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    1,162

    Re: Syntax errors - Converting Java to C

    writing c ( or c++ ) code without having at least a basic understanding of the language is simply masochistic ; even if you get your code to compile you could end up with a program not working as expected or, worse, showing erratical unreliable behavior.

    Quote Originally Posted by 232studios View Post
    The jbyteArray yuv420sp is just an array of bytes
    indeed, I've never used JNI nor Java, but jbyteArray does not appear as an array of bytes and the compiler error that you showed us probably implies that all those "jX" things are just opaque handles. In fact, a quick google search gives this link explaining how you actually access those objects, like, for instance, the GetByteArrayElements() function.

    EDIT: BTW, are you converting Java to c code or interfacing Java to a c function ? I ask, because in the former case using JNI seems pointless ...
    Last edited by superbonzo; December 2nd, 2011 at 05:45 AM.

  6. #6
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    Re: Syntax errors - Converting Java to C

    Quote Originally Posted by 232studios View Post
    Thanks for your help. The code I'm trying to convert it to is C not C++. It probably looks a lot like C++ because it's being ported from Java (see original post) and the commented out lines are the original from Java. I posted both the original Java as well as my poor attempt to convert it to C as I thought it would help.
    As superbonzo pointed out, doing a line-by-line conversion from Java to C++ is not recommended, especially with the code you have.
    The jbyteArray yuv420sp is just an array of bytes.
    Please show the C++ definition of what a jbyteArray is -- don't describe it, actually show us how it's defined. We need the exact definition.
    It was auto generated when I created the .h (header?) file so I assumed it was ok. Should I convert it to an object instead?
    You don't seem to understand.

    There is no such thing in C++ as a "jbyteArray" or any of those "JNIxxx" types. Those are typedefs defined somewhere in this header file you generated. Those typedefs map to bonafide C++ types in some manner, and you are to show us what they are in their true C++ form.
    I've tried adding void * at the start of each yuv420sp[] but I'm still getting the same errors. e.g:
    Of course you get the errors, because you're doing exactly what I was saying was wrong. You cannot turn a pointer to a void to something concrete without casting it to a pointer to the concrete type -- that was the error all along, but you did nothing to solve it except repeated the error.

    The bottom line is to actually learn C++, so that you don't go down the road of creating code that is faulty. Unlike Java, when you make a mistake in C++ (other than syntax errors), there is no guarantee that the mistake will show up when you run the program. In other words, C++ has something Java doesn't have -- programs behaving in an undefined manner at runtime, all due to errors in programming.

    Regards,

    Paul McKenzie

  7. #7
    GCDEF is offline Elite Member Power Poster
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    Re: Syntax errors - Converting Java to C

    I'll add you're not doing yourself any favors with that writing style.

    if(G < 0) G = 0; else if(G > 255) G = 255;

    That's 5 separate lines of code there. Very hard to read and close to impossible to debug.

    Code:
    if(G < 0) 
        G = 0; 
    else 
    if(G > 255) 
        G = 255;
    Sure it takes up more space, but it's a whole lot easier to see what's happening and your debugger can step to each line so you can watch it run.

  8. #8
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    Re: Syntax errors - Converting Java to C

    Ok fair enough, this is trickier that I thought...although I'm still not sure why C++ is still being mentioned as I keep saying it's C I'm after?

    To get it to compile I added (unsigned char*) to each yuv420sp[pixPtr]...but as you've all mentioned this isn't enough to get it working so I'm looking at another way of doing what I need.

    Thanks for all your help!

  9. #9
    GCDEF is offline Elite Member Power Poster
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    Re: Syntax errors - Converting Java to C

    Quote Originally Posted by 232studios View Post
    Ok fair enough, this is trickier that I thought...although I'm still not sure why C++ is still being mentioned as I keep saying it's C I'm after?
    Just a quick glance at your code I see the words throw and new. Those are only valid in C++.

  10. #10
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    Re: Syntax errors - Converting Java to C

    Quote Originally Posted by 232studios View Post
    Ok fair enough, this is trickier that I thought...although I'm still not sure why C++ is still being mentioned as I keep saying it's C I'm after?
    I already mentioned that the code you had was C++ due to the calls to "new" and "throw". These keywords do not exist in the C language.

    Second, regardless of what we've stated so far, it makes no difference if it's C or C++. That doesn't change the fact that there is no such thing as a jbyteArray or JNIEnv or any of those proprietary types in C or C++.

    It also doesn't change the fact that bad coding (usually as a result of mismanaging pointers) in either C or C++ can produce programs that have undefined behaviour -- the program may work today and fail tomorrow, it could work on your machine but fail on another machine etc, even though the data given to the program is exactly the same. In Java, there is no such thing as "undefined behaviour" -- either the program has a logical bug and the program behaves the wrong way all the time, or the Java language itself has bounds checking and other such things, always ready to throw an exception if you do something wrong.

    Regards,

    Paul McKenzie

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