How to Use 'On [Local] Error Goto' statement in FB

Post by **fxm** » May 14, 2018 14:51

'On [Local] Error Goto label' causes a program jump to a specified label as soon as an error occurs. Such errors can be triggered by built-in statements such as 'Open', 'Get', 'Put', or when the 'Error' statement is used.
The error checking for built-in statements is only enabled if the program is compiled with one of the '-e', '-ex' or '-exx' options. Otherwise, no jump will be performed, but the command will still consume processor time.
When triggered by the only 'Error' statement, 'On [Local] Error Goto label' remains always working even when none of these compile options are used.
'On Error Goto 0' deactivates the current error handler. If an error occurs, FreeBASIC will not jump.

The optional 'Local' keyword (authorized only inside Sub/Function) was intended to be used to define an error handler only in the same procedure the 'On Local Error' is in (for compatibility with PDS 7.1 and VB Dos 1.0 for example). In this case, FreeBASIC should have searched for the label in the current procedure only.
But presently, the 'Local' clause is ignored by the compiler, and the error handler should be in the scope of the same procedure the 'On [Local] Error' is in.
Exception when '-gen gcc' is used (or for the fbc 64-bit): the 'Local' clause seems to be rightly taken into account, but except inside a local scope!

'On [Local] Error Goto label' is not the best way to catch errors from a built-in procedure when a syntax in the form of a function is available. The return error code can be directly tested (using the returned error code from function inhibits the QuickBASIC-like error checking and statement 'On Error Goto').

It is advisable to try to write programs compatible with the '-lang fb' dialect and '-exx' option, because to test programs with option '-exx' for debugging purposes is a great helping:

Avoid to use the statement 'Resume [Next]', because it is not at all supported by '-lang fb' (compilation error).
On the other hand, sometimes when it is useful (when no form of a function is available), use the statement 'On [Local] Error Goto', because it runs with any option among '-e', '-ex', '-exx'. Otherwise (no error checking option), 'On [Local] Error Goto' is inactive (without compilation error), but it consumes CPU time.

Accordingly and if necessary, usually write QuickBasic-like error handling ('On [Local] Error Goto label' ..... 'label:' .....), with conditional assembly directive depending on the value of '__FB_ERR__', in order not to penalize the execution speed (if not error checking option is used).

The behavior of statement 'On Error Goto' (-lang fb) regarding compilation options (none, -e/-ex/-exx) is enlighten with the following program including several examples (4), to be compiled with or without error checking option (4*2=8 tests):

Code: Select all

#DEFINE Config 1
'#DEFINE Config 2
'#DEFINE Config 3
'#DEFINE Config 4



#IF Config = 1 '-----------------------------------------------------------

Open "does_not_exist" For Input As #1

Print "main end"
Sleep
System

' - with compiler option 'none' :
'     console output :
'       'main end'
'
' - with compiler option '-e' or '-ex' or '-exx' :
'     console output :
'       'Aborting due to runtime error 2 (file not found) at line 10 of .....'

#ENDIF '-------------------------------------------------------------------



#IF Config = 2 '-----------------------------------------------------------

Dim As Integer Result = Open("does_not_exist" For Input As #1)
If Result <> 0 Then
  Print "error code returned: " & Result
  Print "file not found (processed by 'Result = Open(.....)')"
End If

Print "main end"
Sleep
End

' - with compiler option 'none' or '-e' or '-ex' or '-exx' :
'     console output :
'       'error code returned: 2'
'       'file not found (processed by 'Result = Open(.....)')'
'       'main end'

#ENDIF '-------------------------------------------------------------------



#IF Config = 3 '-----------------------------------------------------------

On Error Goto Error_Handler
Open "does_not_exist" For Input As #1

Print "main end"
Sleep
End

error_handler:
Print "file not found (processed by 'On Error Goto')"
On Error Goto 0
Print "QB-like error handling end"
Sleep
End

' - with compiler option 'none' :
'     console output :
'       'main end'
'
' - with compiler option '-e' or '-ex' or '-exx' :
'     console output :
'       'file not found (processed by 'On Error Goto')'
'       'QB-like error handling end'

#ENDIF '-------------------------------------------------------------------



#IF Config = 4 '-----------------------------------------------------------

On Error Goto error_handler
Dim As Integer Result = Open("does_not_exist" For Input As #1)
If Result <> 0 Then
  Print "error code returned: " & Result
  Print "file not found (processed by 'Result = Open(.....)')"
End If

Print "main end"
Sleep
End

error_handler:
Print "file not found (processed by 'On Error Goto')"
On Error Goto 0
Print "QB-like error handling end"
Sleep
End

' - with compiler option 'none' or '-e' or '-ex' or '-exx' :
'     console output :
'       'error code returned: 2'
'       'file not found (processed by 'Result = Open(.....)')'
'       'main end'

#ENDIF '-------------------------------------------------------------------

The Config=2 and Config=4 sections highlight that when FB function is called using explicitly its returned error code (-lang fb dialect), 'On Error Goto' is by-passed whatever the error checking level ('none', '-e', '-ex', '-exx').