使用纯文本文件存储任意元数据 [英] Storing arbitrary metadata with a plain-text file

问题描述

我正在编写一个文本编辑器，我需要存储几个信息（通常只有几个字符串;存储不需要特别耐用）与应用程序保存的每个文件（不包括文本文件，因为其他应用程序可能会读取它，并且该信息仅特定于我的应用程序）。

我该如何解决？

---

更多信息：我有一个NSDocument设置，我想简单地存储一个NSString实例变量作为每个文件元数据。根据下面的答案我想出了这是目前的bug，并导致程序崩溃在启动时：

  #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 withEXC_BAD_ACCESS
} 
 return [super readFromURL：url ofType：type error：err]; 
} 
 
 // ... 
 //相当标准-dataOfType：error：和
 // -readFromData：ofType：error：实现
  
 
 
 
 
 
 PS：如果你的答案真的很好。），我将授予100rep奖励。
解决方案
使用扩展属性。请参见 setxattr（）。
 
 
 
以下是调用写入字符串的示例：
  NSData * encodedString = [theString dataUsingEncoding：NSUTF8StringEncoding]; 
 int rc = setxattr（/ path / to / your / file，com.yourcompany.yourapp.yourattributename，[encodedString bytes]，[encodedString length]，0，0）; 
 if（rc）
 / *句柄错误* /; 
  
要读取字符串：
  ssize_t len = getxattr（/ path / to / your / file，com.yourcompany.yourapp.yourattributename，NULL，0，0，0）; 
 if（len< 0）
 / *句柄错误* /; 
 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：你不必在问题回答之前设置赏金？
 
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? 



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




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

                        
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