Question

I am writing a text-editor and I would need to store a few pieces of information (generally just a few strings; the storage needn’t be particularly durable) with each file the app saves (without that being part of the text-file as other apps might read it and the info is only specific to my app).

How would I go about this?

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More info: I have a NSDocument set up and I would like to simply store a NSString instance variable as a per file meta-datum. Based on the answers below I’ve come up with this, which is currently buggy and causes the program to crash on startup:

#import <sys/xattr.h>
@interface MyDocument : NSDocument {
  NSString *metadatum;
}

@implementation MyDocument 

- (BOOL)writeToURL:(NSURL *)url ofType:(NSString *)type error:(NSError **)err
{
  BOOL output = [super writeToURL:url ofType:type error:err];
  if(!setxattr([[url path] cStringUsingEncoding:NSUTF8StringEncoding], 
               "eu.gampleman.xattrs.style", 
               [metadatum cStringUsingEncoding:NSUTF8StringEncoding], 
               sizeof(char) * [styleName length], 0, 0)) 
  {
      NSLog(@"Write failure");
  }
  return output;
}

- (BOOL)readFromURL:(NSURL *)url ofType:(NSString *)type error:(NSError **)err {
  char *output;
  ssize_t bytes = getxattr([[url path] cStringUsingEncoding:NSUTF8StringEncoding],
                           "eu.gampleman.xattrs.style", &output, 1024, 0, 0);
  if (bytes > 0) {
    metadatum = [[NSString alloc] initWithBytes:output length:bytes 
          encoding:NSUTF8StringEncoding]; // <- crashes here with "EXC_BAD_ACCESS"
  }
  return [super readFromURL:url ofType:type error: err];
}

// ...
// fairly standard -dataOfType:error: and 
// -readFromData:ofType:error: implementations

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PS: If your answer is really good (with sample code, etc.), I will award a 100rep bounty.

Answer 1

Use extended attributes. See setxattr().

Here’s a sample call to write a string:

NSData* encodedString = [theString dataUsingEncoding:NSUTF8StringEncoding];
int rc = setxattr("/path/to/your/file", "com.yourcompany.yourapp.yourattributename", [encodedString bytes], [encodedString length], 0, 0);
if (rc)
    /* handle error */;

To read a string:

ssize_t len = getxattr("/path/to/your/file", "com.yourcompany.yourapp.yourattributename", NULL, 0, 0, 0);
if (len < 0)
    /* handle error */;
NSMutableData* data = [NSMutableData dataWithLength:len];
len = getxattr("/path/to/your/file", "com.yourcompany.yourapp.yourattributename", [data mutableBytes], len, 0, 0);
NSString* string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

PS: Don’t you have to set the bounty on the question before it’s answered?