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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