Function open

Asynchronous file open. Exclusive mode ensures that path is newly created. Mode defaults to 0644

flags can be:

• 'r' - Open file for reading. An exception occurs if the file does not exist.
• 'r+' - Open file for reading and writing. An exception occurs if the file does not exist.
• 'rs' - Open file for reading in synchronous mode. Instructs the filesystem to not cache writes.
• 'rs+' - Open file for reading and writing, and opens the file in synchronous mode.
• 'w' - Open file for writing. The file is created (if it does not exist) or truncated (if it exists).
• 'wx' - Like 'w' but opens the file in exclusive mode.
• 'w+' - Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).
• 'wx+' - Like 'w+' but opens the file in exclusive mode.
• 'a' - Open file for appending. The file is created if it does not exist.
• 'ax' - Like 'a' but opens the file in exclusive mode.
• 'a+' - Open file for reading and appending. The file is created if it does not exist.
• 'ax+' - Like 'a+' but opens the file in exclusive mode.