Trouble with byte array marshaling in ObjC/C++ to C# in IOS - c++

Question

** This is still unsolved **
I'm trying to call an ObjC/C++ function code from C#. I've done my best to follow different example code, the latest being mostly from:
http://msdn.microsoft.com/en-us/library/ms146631(v=VS.80).aspx
This is for an iPhone/MonoTouch environment, so I'm not sure I've done everything I should. The bytes appear to be ok in the ObjC/C++ function, but the byte array I get back into C# ends up containing 0 0 0 0 0 0 etc.
** Update **
Corrected for loop initializer, and now its giving a EXC_BAD_ACCESS signal on the *returnbytes[i] = bytes[i]; line.
C# code:
[DllImport ("__Internal")]
private static extern int _getjpeg(string url,ref IntPtr thebytes);
void somefunction(string image_id) {
int maxsize = 50000;
byte[] thebytes = new byte[maxsize];
IntPtr byteptr = Marshal.AllocHGlobal(maxsize);
int imagesize = _getjpeg(image_id,ref byteptr);
Debug.Log("Getting _picturesize()... "+ image_id);
int picsize = _picturesize();
Marshal.Copy(byteptr,thebytes,0,picsize);
var texture = new Texture2D(1,1);
string bytedebug = "";
for (int i=5000 ; i < 5020 ; i++)
bytedebug+=thebytes[i] + " ";
Debug.Log("Bytes length is "+imagesize);
Debug.Log("Bytes content is "+bytedebug);
}
C++/ObjC code:
int _getjpeg(const char* url,unsigned char** returnbytes) {
ALAsset* asset = [_pictures objectForKey:[NSString stringWithUTF8String:url]];
if(asset != NULL)
NSLog(#"_getjpeg() found URL: %#",[NSString stringWithUTF8String: url]);
else {
NSLog(#"_getjpeg() could not find URL: %#",[NSString stringWithUTF8String: url]);
return NULL;
}
UIImage *image = [UIImage imageWithCGImage: [asset thumbnail]];
NSData* pictureData = UIImageJPEGRepresentation (image, 1.0);
picturesize = (int)[pictureData length];
unsigned char* bytes = (unsigned char*)[pictureData bytes];
// This test does not give EXC_BAD_ACCESS
*returnbytes[5] = (unsigned int)3;
// updated below initializer in below for loop according to Eikos suggestion
for(int i=0 ; i < picturesize ; i++) {
// below lines gives EXC_BAD_ACCESS
*returnbytes[i] = bytes[i];
}
NSString* debugstr = [NSString string];
for(int i=5000; i < 5020 ; i++) {
unsigned char byteint = bytes[i];
debugstr = [debugstr stringByAppendingString:[NSString stringWithFormat:#"%i ",byteint]];
}
NSLog(#"bytes %s",[debugstr UTF8String]);
return picturesize;
}
Thanks

Answer 1

Keep in mind that the JPGRepresentation is probably not exactly the same as you put into it, so the length may differ.
In
for(int i;i < picturesize;i++) {
// *** Not sure I'm doing this correctly ***
*returnbytes[i] = bytes[i];
}
you forget to initialize i, so it might start with a random value which is bigger than picturesize, so the loop won't run at all.

Answer 2

You want unsigned char*, not **. You are passing a pointer in that is already allocated. A ** is for when you are passing in a pointer to variable that is itself a pointer to data: i.e. when the callee will allocate the memory and the caller wants to know about it.
Just pass in unsigned char* and then use
returnbytes[i] = bytes[i];
Alternatively, allocate in the calee and use an out, not a ref.