iOS how to store a NSDictionary or JSON in a file's xattr?

问题

I'm looking at extended file attributes for iOS and Mac files using setxattr command. From what I understand, I can store arbitrary data there, up to 128kb.

How can I write and read extended attributes as if I'm dealing with a dictionary, not dereferencing string pointers?

So far I have this code that attempts to set a single attribute.

NSString* filepath = [MyValueObject filepath]; 
const char *systemPath = [filepath fileSystemRepresentation];
const char *name = "special_value";
const char *value = "test string";

int result = setxattr(systemPath, name, &value, strlen(value), 0, 0);

If I need to store a small set of values (say 5 key-value pairs), I'm thinking of:

Creating a NSDictionary with my attributes
Converting the dictionary to JSON string
Converting the string to character pointer
Writing the string to the extended attributes
To read the attribute back, I would read back the string pointer
Convert to NSString
Convert to JSON object
Create a dictionary back
Retrieve a value from dictionary

Does this seem like the right approach? Is there's an easier way to store metadata in extended attributes ? Maybe there is a category on NSObject that handles the pointer operations for xattr?

回答1:

I found a Cocoanetics/DTFoundation that allows reading/writing arbitrary strings to xattr: Together with other posts, I was able to accomplish what I wanted - write/restore a dictionary

#import "Note+ExtendedAttribute.h"
#include <sys/xattr.h>


@implementation MyFile (ExtendedAttribute)

-(NSString*)dictionaryKey
{
    return @"mydictionary";
}

-(BOOL)writeExtendedAttributeDictionary:(NSDictionary*)dictionary
{
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dictionary
                                                       options:0
                                                         error:&error];
    if (! jsonData) {
        return NO;
    }

    NSString* jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];



    const char *filepath = [[self filepath] fileSystemRepresentation];

    const char *key = [[self dictionaryKey] UTF8String];
    const char *value = [jsonString UTF8String];

    int result = setxattr(filepath, key, value, strlen(value), 0, 0);

    if(result != 0)
    {
        return NO;
    }

    return YES;
}

Reading:

-(NSMutableDictionary*)readExtendedAttributeDictionary
{
    const char *attrName = [[self dictionaryKey] UTF8String];
    const char *filePath = [[self filepath] fileSystemRepresentation];

    // get size of needed buffer
    int bufferLength = getxattr(filePath, attrName, NULL, 0, 0, 0);

    if(bufferLength<=0)
    {
        return nil;
    }

    // make a buffer of sufficient length
    char *buffer = malloc(bufferLength);

    // now actually get the attribute string
    getxattr(filePath, attrName, buffer, bufferLength, 0, 0);

    // convert to NSString
    NSString *retString = [[NSString alloc] initWithBytes:buffer length:bufferLength encoding:NSUTF8StringEncoding];

    // release buffer
    free(buffer);

     NSData *data = [retString dataUsingEncoding:NSUTF8StringEncoding];
    if(data == nil || data.length == 0)
    {
        return nil;
    }

    NSError *error = nil;
    id json = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];

    if([json isKindOfClass:[NSDictionary class]])
    {
        return [NSMutableDictionary dictionaryWithDictionary:json];
    }

    if(error)
    {
        return nil;
    }


    return json;
}