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Error handling

Error handling is the process of anticipating and responding to errors that might occur in your application. 4D provides a comprehensive support for catching and reporting errors at runtime, as well as for investigating their conditions.

Error handling meets two main needs:

• finding out and fixing potential errors and bugs in your code during the development phase,
• catching and recovering from unexpected errors in deployed applications; in particular, you can replace system error dialogs (disk full, missing file, etc.) with you own interface.

It is highly recommended to install an error-handling method on 4D Server, for all code running on the server. This method would avoid unexpected dialog boxes to be displayed on the server machine, and could log errors in a dedicated file for further analyses.

Error or status

Many 4D class functions, such as entity.save() or transporter.send(), return a status object. This object is used to store "predictable" errors in the runtime context, e.g. invalid password, locked entity, etc., that do not stop program execution. This category of errors can be handled by regular code.

Other "unpredictable" errors include disk write error, network failure, or in general any unexpected interruption. This category of errors generates exceptions and needs to be handled through an error-handling method.

Installing an error-handling method

In 4D, all errors can be catched and handled in a specific project method, the error-handling (or error-catching) method.

This project method is installed for the current process and will be automatically called for any error that occurs in the process, in interpreted or compiled mode. To install this project method, you just need to call the ON ERR CALL command with the project method name as parameter. For example:

ON ERR CALL("IO_ERRORS") //Installs the error-handling method

To stop catching errors and give back hand to 4D, call ON ERR CALL with an empty string:

ON ERR CALL("") //gives back control to 4D

The Method called on error command allows to know the name of the method installed by ON ERR CALL for the current process. It is particularly useful in the context of generic code because it enables you to temporarily change and then restore the error-catching method:

 $methCurrent:=Method called on error
 ON ERR CALL("NewMethod")
  //If the document cannot be opened, an error is generated
 $ref:=Open document("MyDocument")
  //Reinstallation of previous method
 ON ERR CALL($methCurrent)

Scope and components

You can define a single error-catching method for the whole application or different methods per application module. However, only one method can be installed per process.

An error-handling method installed by the ON ERR CALL command only applies to the running application. In the case of an error generated by a component, the ON ERR CALL error-handling method of the host application is not called, and vice versa.

Handling errors within the method

Within the custom error method, you have access to several information that will help you identifying the error:

• dedicated system variables(*):

  • Error (longint): error code
  • Error method (text): name of the method that triggered the error
  • Error line (longint): line number in the method that triggered the error
  • Error formula (text): formula of the 4D code (raw text) which is at the origin of the error.

(*) 4D automatically maintains a number of variables called system variables, meeting different needs. See the 4D Language Reference manual.

• the GET LAST ERROR STACK command that returns information about the current stack of errors of the 4D application.
• the Get call chain command that returns a collection of objects describing each step of the method call chain within the current process.

Example

Here is a simple error-handling system:

//installing the error handling method
 ON ERR CALL("errorMethod")
 //... executing code
 ON ERR CALL("") //giving control back to 4D

// errorMethod project method
 If(Error#1006) //this is not a user interruption
    ALERT("The error "+String(Error)+" occurred". The code in question is: \""+Error formula+"\"")
 End if

Using an empty error-handling method

If you mainly want the standard error dialog box to be hidden, you can install an empty error-handling method. The Error system variable can be tested in any method, i.e. outside of the error-handling method:

ON ERR CALL("emptyMethod") //emptyMethod exists but is empty
$doc:=Open document( "myFile.txt")
If (Error=-43)
    ALERT("File not found.")
End if
ON ERR CALL("")