ON ERROR

Description

ON ERROR goto [branchLabel] provides a way to handle the event if your program encounters an error. Err and Err$ hold the error code number and the description. See Err.

Syntax

on error goto [errorHandler] - (cannot be a sub)

Hints

Here is a link to QBasic error codes. Several of these are supported by JB. Some do not apply. Other error codes have been added.

Here is a short list of common error codes:

3		RETURN without GOSUB
4		Read past end of data
8		Branch label not found
9		Subscript out of range
11		Division by zero
52		Bad file handle
53		OS Error: The system cannot find the file specified.
55		Error opening file
58		OS Error: Cannot create a file when that file already exists.
62		Input past end of file

Some errors do not have a numeric value and the value of Err will be zero.

Some errors could not be trapped (for example, pressing Ctrl+Beak)

In JB, error handler is local to the procedure.

If an error occurs in a user function or subroutine, Just BASIC will exit the current function or subroutine and continue to exit functions and subroutines until it finds ON ERROR handler.

In JB and RB, there is no support for RESUME.

In JB, there is no way to turn Error handling off. (so having it local in a sub/function might be wise).

In RB the handler must be in the main scope.

Example

An example trying to open a file that does not exist. The following will be printed to the mainwin:

Error string is "OS Error: The system cannot find the file specified."
Error number is 53

    on error goto [errorHandler]

    open "nofilename.txt" for input as #f
    close #f

    end

[errorHandler]
    print "Error string is " + chr$(34) + Err$ + chr$(34)
    print "Error number is ";Err
    end

Useful Procedures

Function that catches error on

loadbmp bmpName$, fName$

(because there are some BMPs JB cannot open)

function loadBmpErrChk(bmpName$, fName$)
    'returns 1 on success, 0 on failure

    on error goto [errHandler]

    loadbmp bmpName$, fName$
    loadBmpErrChk = 1
    exit function
[errHandler]
    loadBmpErrChk = 0
end function