how to check file is updated or not without using modified time

how to check file is updated or not without using modified time  in cocoa for mac-osx objective-c

How to get checkSum of a file........

#import "NSData+MD5.h"

@interface NSData(MD5)

 - (NSString *)MD5;

@end

#import <CommonCrypto/CommonDigest.h>

 @implementation NSData(MD5)

- (NSString*)MD5

{

  // Create byte array of unsigned chars

  unsigned char md5Buffer[CC_MD5_DIGEST_LENGTH];

// Create 16 byte MD5 hash value, store in buffer

  CC_MD5(self.bytes, self.length, md5Buffer);

    

// Convert unsigned char buffer to NSString of hex values

  NSMutableString *output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];

  for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) 

[output appendFormat:@"%02x",md5Buffer[i]];

  return output;

}

@end

Call from Own class: 

+(NSString *)getCheckSum:(NSString *)filePath{

     NSData *nsData = [NSData dataWithContentsOfFile:filePath];

     if (nsData){

         return [nsData MD5];

     }

    else{

         return nil;

     }

}