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# Error Handling

Octave includes several functions for printing error and warning messages. When you write functions that need to take special action when they encounter abnormal conditions, you should print the error messages using the functions described in this chapter.

Built-in Function: error (template, ...)
  The `error` function formats the optional arguments under the control of the template string template using the same rules as the `printf` family of functions (see section Formatted Output). The resulting message is prefixed by the string ``error: '` and printed on the `stderr` stream.

  Calling `error` also sets Octave's internal error state such that control will return to the top level without evaluating any more commands. This is useful for aborting from functions or scripts.

  If the error message does not end with a new line character, Octave will print a traceback of all the function calls leading to the error. For example, given the following function definitions:

      function f () g () end
      function g () h () end
      function h () nargin == 1 || error ("nargin != 1"); end

  calling the function `f` will result in a list of messages that can help you to quickly locate the exact location of the error:

      f ()
      error: nargin != 1
      error: evaluating index expression near line 1, column 30
      error: evaluating binary operator `||' near line 1, column 27
      error: called from `h'
      error: called from `g'
      error: called from `f'

  If the error message ends in a new line character, Octave will print the message but will not display any traceback messages as it returns control to the top level. For example, modifying the error message in the previous example to end in a new line causes Octave to only print a single message:

      function h () nargin == 1 || error ("nargin != 1\n"); end
      f ()
      error: nargin != 1

Built-in Variable: error_text
  This variable contains the text of error messages that would have been printed in the body of the most recent `unwind_protect` or `try` statement or the try part of the most recent call to the `eval` function. Outside of the `unwind_protect` and `try` statements or the `eval` function, or if no error has occurred within them, the value of `error_text` is guaranteed to be the empty string.

  Note that the message does not include the first ``error: '` prefix, so that it may easily be passed to the `error` function without additional processing(5).

  See section The `try` Statement and section The `unwind_protect` Statement.

Built-in Variable: beep_on_error
  If the value of `beep_on_error` is nonzero, Octave will try to ring your terminal's bell before printing an error message. The default value is 0.

Built-in Function: warning (msg)
  Print a warning message msg prefixed by the string ``warning: '`. After printing the warning message, Octave will continue to execute commands. You should use this function should when you want to notify the user of an unusual condition, but only when it makes sense for your program to go on.

Built-in Function: usage (msg)
  Print the message msg, prefixed by the string ``usage: '`, and set Octave's internal error state such that control will return to the top level without evaluating any more commands. This is useful for aborting from functions.

  After `usage` is evaluated, Octave will print a traceback of all the function calls leading to the usage message.

  You should use this function for reporting problems errors that result from an improper call to a function, such as calling a function with an incorrect number of arguments, or with arguments of the wrong type. For example, most functions distributed with Octave begin with code like this

      if (nargin != 2)
        usage ("foo (a, b)");
      endif

  to check for the proper number of arguments.

The following pair of functions are of limited usefulness, and may be removed from future versions of Octave.

Function File: perror (name, num)
  Print the error message for function name corresponding to the error number num. This function is intended to be used to print useful error messages for those functions that return numeric error codes.

Function File: strerror (name, num)
  Return the text of an error message for function name corresponding to the error number num. This function is intended to be used to print useful error messages for those functions that return numeric error codes.

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