withMemoryRebound(capacity)

example.swift

var systemInfo = utsname()
uname(&systemInfo)

// capacity = 1
let modelIdentifier1 = withUnsafePointer(to: &systemInfo.machine) {
    $0.withMemoryRebound(to: CChar.self, capacity: 1) { ptr in
        String(cString: ptr)
    }
}

// actual capacity
let modelIdentifier2 = withUnsafePointer(to: &systemInfo.machine) {
    $0.withMemoryRebound(to: CChar.self, capacity: MemoryLayout.size(ofValue: $0)) { ptr in
        String(cString: ptr)
    }
}