Difference between revisions of "Error and Exception Handling"

Latest revision as of 15:10, 15 December 2009

Error and Exception Handling

When an error occurs in a running application, Recital will stop execution, display an error message and create an error.mem file, unless an alternative error handling procedure has been specified.

The error.mem file

The error.mem file includes the following information:

• The Recital Product version, release, platform and compile date information
• System information, such as maximum file size and maximum process stack
• Recital License information
• User and Node names and process ID
• The error number
• The error message
• The program line
• The program line number
• A stack trace of the programs and procedures called
• A list of all active public and private memory variables and classes
• Status of open tables, indexes and current records.
• Settings listing as per LIST STATUS

SET ERRORVERSION

The error.mem file will be named error.mem and will be overwritten by subsequent errors unless set errorversion is on. If set errorversion is on, multiple numbered error.mem files are created with the following naming format:

error0001.mem, error0002.mem, error0003.mem, ...

DB_ERRORDIR

The error.mem file(s) will be created in the directory that is current when the error occurs unless the DB_ERRORDIR environment variable is set. If set, DB_ERRORDIR points to a directory in which the error.mem file(s) will be created.

The ON ERROR statement

Alternative error handling procedures can be specified using the on error command. When there is an active on error setting, no error.mem file will be created and program execution willl not be halted. It is now up to the on error error handler to trap the information required in order to trace the error and to take appropriate action based on the error that has occurred.

on error [command]

SET ONERROR

The active on error behavior can be toggled on and off using the set onerror command.

set onerror on | off

Error Information Functions

Several functions are available to give information about errors:

• errno() - returns last operating system error number

numeric = errno()

• strerror() - returns last or specified operating system error description

character = strerror([error as numeric])

• error() - returns Recital error number

numeric = error()

• message() - returns Recital error message

character = message()

• message(1) - returns the line of code which caused the error

character = message(1)

• procline() - returns currently executing procedure line number

numeric = procline()

• procname() - returns currently executing procedure name

character = procname()

It is important to note that the procline() and procname() functions return information about the currently executing procedure. If these functions are called from within the error handling procedure, they will give information based on the error handling procedure itself and not the procedure in which the error occurred. They should, therefore, be specified as parameters to the error handler, e.g.

on error do MyErrProc with procname(), procline()

RETRY

The RETRY command can be used in conjunction with an on error error handler. It re-executes the command that was reported as being in error.

procedure checkexclu
    // Check for file is in use error
    if error() = 15
        nResponse = messagebox("File currently in use",5)
        if nResponse = 4
            sleep 1
            retry
        else
            cancel
        endif
    else
        messagebox("Exiting")
        cancel
    endif
return
on error do checkexclu
open database southwind shared
use example exclusive
on error

SAVE ERROR

The save error command allows for the creation of an equivalent of the error.mem file. It is given a '.mem' file extension.

procedure handle308
    if error() = 308
        // Access to PRIVATE member of object denied
        // Expected behavior: carry on
        return
    else
        messagebox("Unhandled error has occurred - exiting")
        save error to errlog
        cancel
    endif
endproc
class myclass
    protected prop1 = "Protected property"
    public procedure mymethod()
        echo prop1 + "\n"
    endproc
endclass
 
myobject = new myclass()
 
// Throws error 308 as prop1 is protected and can only be accessed by methods inside the class or sub-class
on error handle308()
echo myobject.prop1
// This will work as class method can access protected property
myobject.mymethod()
// Throws error 292 as prop2 does not exist
echo myobject.prop2
on error

The TRY...CATCH Statement

Summary