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objective-cuiimagensdataunsigned-char

When converting NSData to unsigned char*, I get the wrong result

I would like to convert an UIImage into unsigned char*. First I convert UIImage into NSData, then NSData to unsigned char*. I use the following method to convert NSData to unsigned char*:

unsigned char *binaryData = (unsigned char *)[imageData bytes];

The problem is that there are many many bytes in imageData (which is an NSData), but after the method, the binaryData is just 0x75e8000. I have no idea why. Could anybody help me out there? And can you suggest me one way to correctly convert NSData to unsigned char*?

Thanks in advance!!!

Solution

  • I don't think you understand what a C array is. It is represented as a pointer into the start of the array. Your 0x75e8000 is the start of the array. But the array contains no information about its length; how much more data there may be is unknown.

    Also, you may not understand what the bytes method is. The bytes method does not "convert" anything, nor does it generate a copy; it merely provides a pointer into the NSData itself. As long as the NSData exists, the bytes points at it, but treats it as a const void* (which you are casting to an unsigned char*), suitable for handing to a C function.

    So, the length of the C array, which you would need to tell any C function that uses this information, must be obtained separately; it is the length of the NSData object. And if you want to extract the data, as a copy, into a separate C array, then call getBytes:length:.