How to write binary file in objective C?

Question

I want to write binary data to a file and add that as an email attachment.

  MFMailComposeViewController *picker = [[MFMailComposeViewController alloc] init];
  picker.mailComposeDelegate = self;
  NSData *data = [[self.samples componentsJoinedByString:@"\n"] dataUsingEncoding:NSUTF8StringEncoding];
  [picker addAttachmentData:data mimeType:@"application/octet-stream" fileName:filename];

addAttachmentData expects an NSData * to be passed (although I'd prefer a byte array). samples is an NSMutableArray *. I'm getting extra formatting in the file that I do NOT want however, like angle brackets and whitespace:

...
<17700000 16c0f710 c78a2660 1570f960 c78926>
<17710060 17d0f800 c98826d0 2160f990 cb8d26>
...

Where are these coming from? How can I truly write just binary data?

matt · Accepted Answer · 2015-10-04 06:40:18Z

4

Calm down. NSData is a byte array — no more, no less. What you're seeing is just a figment of what the logging process does. Try this:

NSArray <NSString*>* arr = @[@"Mannie", @"Moe", @"Jack"];
NSData *data = 
    [[arr componentsJoinedByString:@"\n"] dataUsingEncoding:NSUTF8StringEncoding];
NSLog(@"%@", data);

The console outputs:

<4d616e6e 69650a4d 6f650a4a 61636b>

It's just NSLog making things neater. The bytes are just the 2-digit hex number values; there are no angle brackets and spaces in the data.

If you really don't believe me, print it this way:

char buffer[100];
[data getBytes:buffer length:data.length];
for (NSUInteger i = 0; i < data.length; i++) {
    NSLog(@"%02x", buffer[i]);
}

The output is just the character bytes:

4d
61
6e
6e
69
65
0a
4d
6f
65
0a
4a
61
63
6b

edited Oct 4, 2015 at 6:40

answered Oct 4, 2015 at 6:33

matt

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2 Comments

tarabyte Over a year ago

Wow you have an incredible reputation matt. Hmm, I'm reading the sent attachment as a binary file in python and am seeing 0x3C = 60 = '<' as the initial byte. When I open the file in sublime, I see the same thing.

matt Over a year ago

Okay, but now you've sort of changed the question, haven't you? It seemed to me that question was "What are these extra things in my NSData?" And I showed you how to prove to yourself there are no extra things. How an octet stream is represented in an email is another matter.

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