ios - Write extend file attributes swift example

Question

I am looking for a solution to add extended file attributes for a file in swift. I checked this link Write extended file attributes, but the solutions are in objective c and I need a solution for swift.

Answer 1

Here is a possible implementation in Swift 5 as an extension for URL, with methods to get, set, list, and remove extended attributes of a file. (Swift 2, 3, and 4 code can be found in the edit history.)

extension URL {

    /// Get extended attribute.
    func extendedAttribute(forName name: String) throws -> Data  {

        let data = try self.withUnsafeFileSystemRepresentation { fileSystemPath -> Data in

            // Determine attribute size:
            let length = getxattr(fileSystemPath, name, nil, 0, 0, 0)
            guard length >= 0 else { throw URL.posixError(errno) }

            // Create buffer with required size:
            var data = Data(count: length)

            // Retrieve attribute:
            let result =  data.withUnsafeMutableBytes { [count = data.count] in
                getxattr(fileSystemPath, name, $0.baseAddress, count, 0, 0)
            }
            guard result >= 0 else { throw URL.posixError(errno) }
            return data
        }
        return data
    }

    /// Set extended attribute.
    func setExtendedAttribute(data: Data, forName name: String) throws {

        try self.withUnsafeFileSystemRepresentation { fileSystemPath in
            let result = data.withUnsafeBytes {
                setxattr(fileSystemPath, name, $0.baseAddress, data.count, 0, 0)
            }
            guard result >= 0 else { throw URL.posixError(errno) }
        }
    }

    /// Remove extended attribute.
    func removeExtendedAttribute(forName name: String) throws {

        try self.withUnsafeFileSystemRepresentation { fileSystemPath in
            let result = removexattr(fileSystemPath, name, 0)
            guard result >= 0 else { throw URL.posixError(errno) }
        }
    }

    /// Get list of all extended attributes.
    func listExtendedAttributes() throws -> [String] {

        let list = try self.withUnsafeFileSystemRepresentation { fileSystemPath -> [String] in
            let length = listxattr(fileSystemPath, nil, 0, 0)
            guard length >= 0 else { throw URL.posixError(errno) }

            // Create buffer with required size:
            var namebuf = Array<CChar>(repeating: 0, count: length)

            // Retrieve attribute list:
            let result = listxattr(fileSystemPath, &namebuf, namebuf.count, 0)
            guard result >= 0 else { throw URL.posixError(errno) }

            // Extract attribute names:
            let list = namebuf.split(separator: 0).compactMap {
                $0.withUnsafeBufferPointer {
                    $0.withMemoryRebound(to: UInt8.self) {
                        String(bytes: $0, encoding: .utf8)
                    }
                }
            }
            return list
        }
        return list
    }

    /// Helper function to create an NSError from a Unix errno.
    private static func posixError(_ err: Int32) -> NSError {
        return NSError(domain: NSPOSIXErrorDomain, code: Int(err),
                       userInfo: [NSLocalizedDescriptionKey: String(cString: strerror(err))])
    }
}

Example usage:

let fileURL = URL(fileURLWithPath: "/path/to/file")

let attr1 = "com.myCompany.myAttribute"
let attr2 = "com.myCompany.otherAttribute"

let data1 = Data([1, 2, 3, 4])
let data2 = Data([5, 6, 7, 8, 9])

do {
    // Set attributes:
    try fileURL.setExtendedAttribute(data: data1, forName: attr1)
    try fileURL.setExtendedAttribute(data: data2, forName: attr2)

    // List attributes:
    let list = try fileURL.listExtendedAttributes()
    print(list)
    // ["com.myCompany.myAttribute", "com.myCompany.otherAttribute", "other"]

    let data1a = try fileURL.extendedAttribute(forName: attr1)
    print(data1a as NSData)
    // <01020304>

    // Remove attributes
    for attr in list {
        try fileURL.removeExtendedAttribute(forName: attr)
    }

} catch let error {
    print(error.localizedDescription)
}