How to obtain array of pixels from a .png image?

Question

I want to get the color of each pixel of a .png image. Its size is 320*480.

But each time I use 'malloc' or 'calloc' to get memory, I get only NULL.

Here is the code:

CGImageRef imageRef = image.CGImage;
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));

NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;

CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                             bitsPerComponent, bytesPerRow, colorSpace,
                                             kCGImageAlphaNoneSkipLast);
CGColorSpaceRelease(colorSpace);
NSLog(@"width height %d %d", height, width);

CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);

//The values in Variables view is as follows
 rawData    unsigned char * 0x1324b000
   *rawData   unsigned char   '\0'

Is it because the size m asking for is huge?

I tired obtaining memory with both 'malloc' and 'calloc' for small values , but all in vain. I also used 'memset()' when malloc was used. But all i get in 'rawdata' is NULL.

CGImage also in not nil. Width and height to have correct values 320, 480 respectively.

Please help.

Thanks in advance.