How to convert NSData to byte array in iPhone?

Question

I want to convert NSData to a byte array, so I write the following code:

NSData *data = [NSData dataWithContentsOfFile:filePath];
int len = [data length];
Byte byteData[len];
byteData = [data bytes];

But the last line of code pops up an error saying "incompatible types in assignment". What is the correct way to convert the data to byte array then?

Answer 1

The signature of -[NSData bytes] is - (const void *)bytes. You can't assign a pointer to an array on the stack. If you want to copy the buffer managed by the NSData object into the array, use -[NSData getBytes:]. If you want to do it without copying, then don't allocate an array; just declare a pointer variable and let NSData manage the memory for you.

Answer 2

That's because the return type for [data bytes] is a void* c-style array, not a Uint8 (which is what Byte is a typedef for).

The error is because you are trying to set an allocated array when the return is a pointer type, what you are looking for is the getBytes:length: call which would look like:

[data getBytes:&byteData length:len];

Which fills the array you have allocated with data from the NSData object.

Answer 3

You can't declare an array using a variable so Byte byteData[len]; won't work. If you want to copy the data from a pointer, you also need to memcpy (which will go through the data pointed to by the pointer and copy each byte up to a specified length).

Try:

NSData *data = [NSData dataWithContentsOfFile:filePath];
NSUInteger len = [data length];
Byte *byteData = (Byte*)malloc(len);
memcpy(byteData, [data bytes], len);

This code will dynamically allocate the array to the correct size (you must free(byteData) when you're done) and copy the bytes into it.

You could also use getBytes:len: as indicated by others if you want to use a fixed length array. This avoids malloc/free but is less extensible and more prone to buffer overflow issues so I rarely ever use it.