Section 9.3 - Miscellaneous Text_IO Capabilities

Text_IO defines functions Current_Input, Current_Output, and Current_Error; each returns a File_Type. Current_Error is like Current_Output but should be used for error messages. Here's an example of printing an error message:

  Put(Current_Error, "Error - the widget file is missing.");

Procedures Set_Input, Set_Output, and Set_Error let you set the current input, output, and error files somewhere else. For example, if you want all error messages to go to file "error", do the following:

  Error_File : File_Type;
  -- ...
  Create(Error_File, Out_File, "error");
  Set_Error(Error_File);

Ada and the underlying operating system may delay sending output to the user to increase overall performance. This is called buffering. Usually buffering is a good idea, but sometimes you want the output to be displayed right away. In that case, use the Flush operation to immediately display everything sent to a given file (the default is Current_Output).

Sometimes you want to check the keyboard to see if a user has pressed a key, and if so, what that key was. The Ada 95 procedure to do that is called Get_Immediate. There are a few caveats with Get_Immediate:

• On some systems it's not possible to implement Get_Immediate; in that case Ada can't do it (obviously).
• On multiprocessing systems it's often not a good idea to continuously check if something has happened. This is called polling, and polling can slow the whole system down. If you plan to just wait if a key hasn't been pressed, use Get instead.

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