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Error handling

Syntax errors

Syntax errors are those which are reported when translating a procedure. (Other errors can occur while you're running a program.) The OPL translator will return you to the line where the first syntax error is detected.

All programming languages are very particular about the way commands and functions are used, especially in the way program statements are laid out. Below are a number of errors which are easy to make in OPL. The incorrect statements are in bold and the correct versions are on the right.

Punctuation errors

Omitting the colon between statements on a multi-statement line:

Incorrect				Correct
a$="text" PRINT a$		a$="text" :PRINT a$

Omitting the space before the colon between statements : Incorrect Correct a$=b$:PRINT a$ a$=b$ :PRINT a$

Omitting the colon after a called procedure name: Incorrect Correct

proc2 proc2:

Using only 1 colon after a label in GOTO/ONERR/VECTOR (instead of 0 or 2): Incorrect Correct GOTO below: GOTO below . . below:: below::

Structure errors

The DO...UNTIL, WHILE...ENDWH and IF...ENDIF structures can produce a `Structure fault' error if used incorrectly:

VECTOR...ENDV can also produce a `Structure fault' error if used incorrectly.

Errors in running procedures

OPL may display an error message and stop a running program if certain `error' conditions occur. This may happen because:

Unless you include statements which can handle such errors when they occur, OPL will use its own error handling mechanism. The program will stop and an error message be displayed. The first line gives the names of the procedure in which the error occurred, and the module this procedure is in. The second line is the `error message' one of the messages listed at the end of this chapter. If appropriate, you will also see a list of variable names or procedure names causing the error.

If you were editing the module with the Program editor and you ran it from there, you would also be taken back to editing the OPL module, with the cursor at the line where the error occurred.

Error handling functions and commands

To prevent your program being stopped by OPL when an error occurs, include statements in your program which anticipate possible errors and take appropriate action. The following error handling facilities are available in OPL:

These facilities put you in control and must be used carefully.


You should design the error handling of a program in the same way as the program itself. OPL works best when programs are built up from procedures, and you should design your error handling on the same basis. Each procedure should normally contain its own local error handling:


The error handling statements can then be appropriate to the procedure. For example, a procedure which performs a calculation would have one type of error handling, but another procedure which offers a set of choices would have another.



The TRAP command immediately precedes any of these commands, separated from it by a space for example:


If an error occurs in the execution of the command, the program does not stop, and the next line of the program executes as if there had been no error. Normally you would use ERR on the line after the TRAP to find out what the error was.


When INPUT is used without TRAP and a text string is entered when a number is required, the display just scrolls up and a "?" is shown, prompting for another entry. With TRAP in front of INPUT, you can handle bad entries yourself:

This example uses the ERR function, described next.

ERR and ERR$

When an error occurs in a program, check what number the error was, with the ERR function:


If ERR returns zero, there was no error. The value returned by ERR is the number of the last error which occurred it changes when a new error occurs. TRAP sets ERR to zero if no error occurred. Check the number it returns against the error messages listed at the end of this Chapter.

The ERR$ function gives you the message for error number e%:


You can also use ERR and ERR$ in conjunction:


This returns the error message for the most recent error.


The lines below anticipate that error number -101 (File already open) may occur. If it does, an appropriate message is displayed.

TRAP OPEN "main",A,a$
IF e% REM Checks for an error
  IF e%=-101
    PRINT "file is already open!"
    PRINT ERR$(e%)
The inner IF...ENDIF structure displays either the message in quotes if the error was number -101, or the standard error message for any other error.

TRAP INPUT/EDIT and the Esc key

If in response to a "TRAP INPUT" or "TRAP EDIT" statement, the Esc key is pressed while no text is on the input/edit line, the `Escape key pressed' error (number -114) will be raised. (This error will only be raised if the INPUT or EDIT has been trapped. Otherwise, the Esc key still leaves you editing.)

You can use this feature to enable someone to press the Esc key to escape from editing or inputing a value. For example:


ONERR sets up an error handler. This means that, whenever an error occurs in the procedure containing ONERR, the program will jump to a specified label instead of stopping in the normal way. This error handler is active until an ONERR OFF statement.

You specify the label after the word ONERR.

The label itself can then occur anywhere in the same procedure even above the ONERR statement. After the label should come the statements handling whatever error may have caused the program to jump there. For example, you could just have the statement "PRINT ERR$(ERR)" to display the message for whatever error occurred.

All statements after the ONERR command, including those in procedures called by the procedure containing the ONERR, are protected by the ONERR, until the ONERR OFF instruction is given.



If an error occurs in the lines between "ONERR errHand" and "ONERR OFF", the program jumps to the label "errHand::" where a message is displayed.

Always cancel ONERR with ONERR OFF immediately after the label.

When to use ONERR OFF

You could protect the whole of your program with a single ONERR. However, it's often easier to manage a set of procedures which each have their own ONERR...ONERR OFF handlers, each covering their own procedure. Secondly, an endless loop may occur if all errors feed back to the same single label.

For example, the diagram below shows how an error handler is left active by mistake. Two completely different errors cause a jump to the same label, and cause an inappropriate explanatory message to be displayed. In this example an endless loop is created because "next:" is called repeatedly: (picture)

Multiple ONERRs

You can have more than one ONERR in a procedure, but only the most recent ONERR is active. Any errors cause a jump to the label for the most recent ONERR.

ONERR OFF disables all ONERRs in the current procedure. But if there are other ONERRs in procedures above this procedure (calling procedures) these ONERRs are not disabled.


TRAP has priority over ONERR. In other words, an error from a command used with TRAP will not cause a jump to the error handler specified with ONERR.


The RAISE command generates an error, in the same way that OPL raises errors whenever it meets certain conditions which it recognises as unacceptable (for example, when invalid arguments are passed to a function). Once an error has been raised, either by OPL or by the RAISE command, the error-handling mechanism currently in use takes effect the program will stop and report a message, or if you've used ONERR the program will jump to the ONERR label.

There are two reasons for using RAISE:

In this case, you would RAISE one of the standard error numbers. You could handle this yourself with ONERR, or let OPL handle it in the normal way.

In this case, you would RAISE a new error number. With ONERR on, RAISE would go to the ONERR label, where you would have code to interpret your new error numbers. You could then display appropriate messages.

You can use any positive number (from 0 to 127) as a new error code. Do not use any of the numbers in the list that follows.

You may also find RAISE useful for testing your error handling.


This uses RAISE to raise the `Invalid arguments' error not the `divide by zero' error, since the former is the more appropriate message.

Error messages

These are the numbers of the errors which OPL can raise, and the message associated with them:

Number	Message:
-1 		General failure
-2 		Invalid arguments
-3 		O/S error
-4 		Service not supported
-5		Underflow (number too small)
-6		Overflow (number too large)
-7		Out of range
-8		Divide by zero
-9		In use (eg serial port being used by another program)
-10		No system memory
-13		Process table full/Too many processes
-14		Resource already open
-15		Resource not open
-16		Invalid image/device file
-17		No receiver
-18		Device table full
-19		File system not found (eg if you unplug cable to PC)
-20		Failed to start
-21		Font not loaded
-22		Too wide (dialogs)
-23		Too many items (dialogs)
-24		Batteries too low for digital audio
-25		Batteries too low to write to Flash

File and device errors -32 File already exists -33 File does not exist -34 Write failed -35 Read failed -36 End of file (when you try to read past end of file) -37 Disk full -38 Invalid name -39 Access denied (eg to a protected file on PC) -40 File or device in use -41 Device does not exist -42 Directory does not exist -43 Record too large -44 Read only file -45 Invalid I/O request -46 I/O operation pending -47 Invalid volume (corrupt disk) -48 I/O cancelled

-50 Disconnected -51 Connected -52 Too many retries -53 Line failure -54 Inactivity timeout -55 Incorrect parity -56 Serial frame (usually because Baud setting is wrong) -57 Serial overrun (usually because Handshaking is wrong) -58 Cannot connect to remote modem -59 Remote modem busy -60 No answer from remote modem -61 Number is black listed (you may try a number only a certain number of times; wait a while and try again) -62 Not ready -63 Unknown media (corrupt SSD) -64 Root directory full (on any device, the root directory has a maximum amount of memory allocated to it) -65 Write protected -66 Media is corrupt -67 User abandoned -68 Erase pack failure -69 Wrong file type

Translator errors -70 Missing " -71 String too long -72 Unexpected name -73 Name too long -74 Logical device must be A-D -75 Bad field name -76 Bad number -77 Syntax error -78 Illegal character -79 Function argument error -80 Type mismatch -81 Missing label -82 Duplicate name -83 Declaration error -84 Bad array size -85 Structure fault -86 Missing endp -87 Syntax Error -88 Mismatched ( or ) -89 Bad field list -90 Too complex -91 Missing , -92 Variables too large -93 Bad assignment -94 Bad array index -95 Inconsistent procedure arguments

OPL specific errors -96 Illegal Opcode (corrupt module translate again) -97 Wrong number of arguments (to a function or parameters to a procedure) -98 Undefined externals (a variable has been encountered which hasn't been declared) -99 Procedure not found -100 Field not found -101 File already open -102 File not open -103 Record too big (data file contains record too big for OPL) -104 Module already loaded (when trying to LOADM) -105 Maximum modules loaded (when trying to LOADM) -106 Module does not exist (when trying to LOADM) -107 Incompatible translator version (OPL file needs retranslation) -108 Module not loaded (when trying to UNLOADM) -109 Bad file type (data file header wrong or corrupt) -110 Type violation (passing wrong type to parameter) -111 Subscript or dimension error (out of range in array) -112 String too long -113 Device already open (when trying to LOPEN) -114 Escape key pressed -115 Incompatible runtime version -116 ODB file(s) not closed -117 Maximum drawables open (maximum 8 windows and/or bitmaps allowed) -118 Drawable not open -119 Invalid Window (window operation attempted on a bitmap) -120 Screen access denied (when run from Calculator)

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