Errors

Exception and Error Handling

There are two ways to handle error conditions in Ctalk. You can simply print an error or warning message in your code. An error message formats the text and data that you provide, the same as in a printf statement, and then exits the program. Here is an example.

_error ("Program exited with code %d.\n", result_code);

A _warning message is similar, but it prints the message and continues processing. See errorfuncs.

The other way to handle errors is with exceptions. This is the method you need to use if an error occurs within a method, and the program needs either to print a warning message, or exit.

There are two methods of class Exception that handle exceptions in application programs: pending and handle. There are also other API functions, but they are mostly used internally to translate exceptions into events that application programs can use.

These two methods are generally used together. The method pending, if it returns TRUE, signals that there is an exception pending. Then the function handle handles the event by executing an exception handler.

Generally, events simply issue error messages. It is up to you to determine how the program should handle the exception: by exiting, trying the procedure again, ignoring the condition, or some other procedure. Here is an example.

Exception new e;
...
inputStream openOn fileArg;
if (e pending) {
   e handle;
  exit (1);
}
if (inputStream isDir) {
  printf ("Input is a directory.\n");
  exit (1);
}

This is simply the way that Ctalk notifies the application if the method openOn (class ReadFileStream) encountered an error while opening the file named by its argument, fileArg.

You should note that the program also checks whether the input is actually a directory, because opening a directory as if it were a file does not necessary cause an error condition. The isDir (class FileStream) method is one portable way to check if the input path is actually a directory.

The method openOn, like other methods, raises an exception if necessary. It does this with raiseException (class SystemErrnoException).

Ctalk handles most stdio error codes in this manner. A program that uses the ReadFileStream and WriteFileStream classes should rarely need to use the C library’s errno macro, but it is still available if applications need to check for other errors from the C libraries.

The Ctalk library also provides exceptions for missing arguments, undefined methods, and other error conditions. The file, include/except.h, contains the definitions of Ctalk’s compile and run time exceptions.