1. This procedure opens a file for reading or writing. If the operation succeeds, depending on the opening mode, the file becomes the active input, output or error stream. The procedures write and writeln are used to write data to the default output stream and the functions read, readln, and fskipline are used to read data from the default input stream. Error messages are sent to the error stream.

2. The behavior of this function in case of an IO error ( i.e. the file cannot be opened) is directed by the control parameter ioctrl: if the value of this parameter is `false' (default value), the interpreter stops. Otherwise, the interpreter ignores the error and continues. The error status of an IO operation is stored in the control parameter iostatus which is 0 when the last operation has been executed successfully. Note that this parameter is automatically reset once its value has been read using the function getparam. The behavior of IO operations after an unhandled error is not defined.

3. The binary mode disables character encoding conversion (see section Character encoding of text files).