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?

Matt Gallagher · Accepted Answer · 2014-06-05T04:30:20.137

63

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:length: 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.

edited Jun 05 '14 at 04:30

answered Apr 07 '09 at 06:39

Matt Gallagher

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Byte byteData = malloc(len); pops up a warning saying "initialization makes interger from pointer without a cast". And byteData turns out to be a byte variable, not a byte array. Any ideas about this? – Chilly Zhong Apr 07 '09 at 14:53
There was a small typo in the code, it should work now as matt intended. – Jason Coco Apr 07 '09 at 15:30
Yes. The second parameter of memcpy should be [data bytes]. – Chilly Zhong Apr 10 '09 at 01:09
Thanks, iphony, I've fixed the memcpy 2nd parameter. – Matt Gallagher Apr 12 '09 at 01:16
Also, in case you didn't spot it yourself... Byte byteData = malloc(len); doesn't work because byteData must be declared as a pointer, i.e. Byte *byteData. – Matt Gallagher Apr 12 '09 at 01:19
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@matt Gallagher :It works fine,but this conversion make some delay. how to reduce that... any idea????? – Vasu Ashok Jun 04 '11 at 06:41
1

`Byte byteData[len]` will work with Clang and GCC. This is valid C (C supports variable-length arrays) and therefore valid Objective-C as well. – zneak Mar 21 '13 at 18:37
My tests show that only the first byte of data will be copied to byteData: byteData[1], byteData[2], etc. contain 0; – Elise van Looij Feb 19 '18 at 19:38
When I used it, it became a Lost Device. What is the reason? – Jungwon Jun 17 '22 at 04:49

score 43 · Answer 2 · answered Dec 03 '10 at 20:09

43

You could also just use the bytes where they are, casting them to the type you need.

unsigned char *bytePtr = (unsigned char *)[data bytes];

answered Dec 03 '10 at 20:09

stevex

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Doing this, I get a "Variable-sized object may not be initialized" error – Żabojad Aug 23 '17 at 15:03
1

Again, tests show that only the first byte of data will be copied to bytePtr: bytePtr[1], bytePtr[2], etc. contain 0; – Elise van Looij Feb 19 '18 at 19:41

score 16 · Answer 3 · answered Apr 21 '14 at 19:03

Already answered, but to generalize to help other readers:

    //Here:   NSData * fileData;
    uint8_t * bytePtr = (uint8_t  * )[fileData bytes];

    // Here, For getting individual bytes from fileData, uint8_t is used.
    // You may choose any other data type per your need, eg. uint16, int32, char, uchar, ... .
    // Make sure, fileData has atleast number of bytes that a single byte chunk would need. eg. for int32, fileData length must be > 4 bytes. Makes sense ?

    // Now, if you want to access whole data (fileData) as an array of uint8_t
    NSInteger totalData = [fileData length] / sizeof(uint8_t);

    for (int i = 0 ; i < totalData; i ++)
    {
        NSLog(@"data byte chunk : %x", bytePtr[i]);
    }

score 10 · Answer 4 · answered Apr 07 '09 at 04:05

10

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.

answered Apr 07 '09 at 04:05

Nicholas Riley

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After I add [data getBytes:byteData length:len]; the byteData turns out to be invalid. What might be the problem? – Chilly Zhong Apr 07 '09 at 04:20
Try printing the NSData and seeing if it matches; then check the length and make sure it looks like what you're expecting. If the data is a string, you can also try -[NSString stringWithContentsOfFile]. – Nicholas Riley Apr 07 '09 at 04:41
I find out the problem: the length shouldn't be a variable. It should be Byte byteData[255]; – Chilly Zhong Apr 07 '09 at 06:30
1

Too much Java for me recently. :-) Glad you got it figured out. – Nicholas Riley Apr 07 '09 at 11:39

score 2 · Answer 5 · answered Apr 07 '09 at 04:07

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.

answered Apr 07 '09 at 04:07

Kendall Helmstetter Gelner

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The byteData is invalid. What might be the problem? – Chilly Zhong Apr 07 '09 at 04:32
@ChillyZhong try to specify the the byteData size for example `uint8_t *cipherBuffer[16]` then remove the `&` and NSLog(@"%s", (char *)byteData); – Mazen Kasser May 30 '16 at 08:02

score -1 · Answer 6 · answered Feb 01 '18 at 16:51

-1

Here's what I believe is the Swift equivalent:

if let data = NSData(contentsOfFile: filePath) {
   let length = data.length
   let byteData = malloc(length)
   memcmp(byteData, data.bytes, length)
}

answered Feb 01 '18 at 16:51

jervine10

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How to convert NSData to byte array in iPhone?

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