CODEWITHSUNDEEP
iosobjective-c
Levi
Levi

Reputation: 7343

Write extended file attributes

I am trying to write some data (the length of the data is 367 bytes) in the header of a file using the following code:

const char *attrName = [kAttributeForKey UTF8String];
const char *path = [filePath fileSystemRepresentation];

const uint8_t *myDataBytes = (const uint8_t*)[myData bytes];
int result = setxattr(path, attrName, myDataBytes, sizeof(myDataBytes), 0, 0);

When I try to read it, the result is different:

const char *attrName = [kAttributeForKey UTF8String];
const char *path = [filePath fileSystemRepresentation];

int bufferLength = getxattr(path, attrName, NULL, 0, 0, 0);
char *buffer = malloc(bufferLength);
getxattr(path, attrName, buffer, bufferLength, 0, 0);

NSData *myData = [[NSData alloc] initWithBytes:buffer length:bufferLength];  
free(buffer);

Could someone tell me how can I make this work? Thanks in advance.

Upvotes: 0

Views: 3412

Answers (3)

Peter
Peter

Reputation: 2165

Here's a convenient NSFileManager category that gets and sets an NSString as a file's extended attribute.

+ (NSString *)xattrStringValueForKey:(NSString *)key atURL:(NSURL *)URL
{
    NSString *value = nil;
    const char *keyName = key.UTF8String;
    const char *filePath = URL.fileSystemRepresentation;

    ssize_t bufferSize = getxattr(filePath, keyName, NULL, 0, 0, 0);

    if (bufferSize != -1) {
        char *buffer = malloc(bufferSize+1);

        if (buffer) {
            getxattr(filePath, keyName, buffer, bufferSize, 0, 0);
            buffer[bufferSize] = '\0';
            value = [NSString stringWithUTF8String:buffer];
            free(buffer);
        }
    }
    return value;
}

+ (BOOL)setXAttrStringValue:(NSString *)value forKey:(NSString *)key atURL:(NSURL *)URL
{
    int failed = setxattr(URL.fileSystemRepresentation, key.UTF8String, value.UTF8String, value.length, 0, 0);
    return (failed == 0);
}

Upvotes: 5

rmaddy
rmaddy

Reputation: 318804

The problem is with your call to setxattr. The sizeof call can't be used. You want:

int result = setxattr(path, attrName, myDataBytes, [myData length], 0, 0);

The call to sizeof(myDataBytes) will return the size of the pointer, not the length of the data.

Upvotes: 3

art-divin
art-divin

Reputation: 1645

Read the section "Getting and Setting attributes" here.

For your example here is some basic approach, maybe it'll help:

NSFileManager *fm = [NSFileManager defaultManager];

NSURL *path;
/*
 * You can set the following attributes: NSFileBusy, NSFileCreationDate, 
   NSFileExtensionHidden, NSFileGroupOwnerAccountID, NSFileGroupOwnerAccountName, 
   NSFileHFSCreatorCode, NSFileHFSTypeCode, NSFileImmutable, NSFileModificationDate,
   NSFileOwnerAccountID, NSFileOwnerAccountName, NSFilePosixPermissions
 */
[fm setAttributes:@{ NSFileOwnerAccountName : @"name" } ofItemAtPath:path error:nil];

Upvotes: 0

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