6.8 – Input and Output Facilities
The I/O library provides two different styles for file manipulation. The first one uses implicit file handles; that is, there are operations to set a default input file and a default output file, and all input/output operations are over these default files. The second style uses explicit file handles.
When using implicit file handles, all operations are supplied by table io. When using explicit file handles, the operation io.open returns a file handle and then all operations are supplied as methods of the file handle.
The table io also provides three predefined file handles with their usual meanings from C: io.stdin, io.stdout, and io.stderr. The I/O library never closes these files.
Unless otherwise stated, all I/O functions return nil on failure (plus an error message as a second result and a system-dependent error code as a third result) and some value different from nil on success. On non-POSIX systems, the computation of the error message and error code in case of errors may be not thread safe, because they rely on the global C variable errno.