Revision history for ProPgErrorHandling


Revision [23982]

Last edited on 2020-06-28 14:58:05 by fxm [added paragraph ('On [Local] Error Goto' statement use)]
Additions:
//'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'//).
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 ##[[KeyPgDdfberr|__FB_ERR__]]##, in order not to penalize the execution speed (if not error checking option is used).
Deletions:
//'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'//).
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 ##[[KeyPgDdfberr|__FB_ERR__]]##, in order not to penalize the execution speed (if not error checking option is used).


Revision [23981]

Edited on 2020-06-28 08:13:16 by fxm [added paragraph ('On [Local] Error Goto' statement use)]
Additions:
{{fbdoc item="section" value="'On [Local] Error Goto' statement use"}}
//'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 ##[[KeyPgError|Error]]## statement is used.
The error checking for built-in statements is only enabled if the program is compiled with one of the //[[CompilerOpte|-e]]//, //[[CompilerOptex|-ex]]// or //[[CompilerOptexx|-exx]]// options. Otherwise, no jump will be performed, but the command will still consume processor time.
When triggered by the only ##[[KeyPgError|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 ##[[KeyPgLocal|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 ##[[KeyPgLocal|Local]]## clause is ignored by the compiler, and the error handler can be either in the scope of the same procedure the //'On [Local] Error'// is in, or in the main part of the module (if defined before the procedure).
Exception when //[[CompilerOptgen|-gen gcc]]// is used (or for the fbc 64-bit): the ##[[KeyPgLocal|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 //[[CompilerOptlang|-lang fb]]// dialect and //[[CompilerOptexx|-exx]]// option, because to test programs with option //[[CompilerOptexx|-exx]]// for debugging purposes is a great helping:
**""-""** Avoid to use the statement ##[[KeyPgResume|Resume]]## or ##[[KeyPgResumenext|Resume Next]]##, because it is not at all supported by //[[CompilerOptlang|-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 //[[CompilerOpte|-e]]//, //[[CompilerOptex|-ex]]//, //[[CompilerOptexx|-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 ##[[KeyPgDdfberr|__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):
{{fbdoc item="filename" value="examples/manual/proguide/errors/on-error2.bas"}}%%(freebasic)
#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"
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"
' - 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"
error_handler:
Print "file not found (processed by 'On Error Goto')"
Print "QB-like error handling 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"
error_handler:
Print "file not found (processed by 'On Error Goto')"
Print "QB-like error handling 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').


Revision [23980]

Edited on 2020-06-28 04:19:50 by fxm [formatted for future update (addition)]
Additions:
{{fbdoc item="filename" value="examples/manual/proguide/errors/err.bas"}}%%(freebasic)
%%
{{fbdoc item="filename" value="examples/manual/proguide/errors/result.bas"}}%%(freebasic)
%%
{{fbdoc item="filename" value="examples/manual/proguide/errors/on-error.bas"}}%%(freebasic)
%%
%%
%%
{{fbdoc item="filename" value="examples/manual/proguide/errors/on-local.bas"}}%%(freebasic)
%%
Deletions:
{{fbdoc item="filename" value="examples/manual/proguide/errors/err.bas"}}%%(freebasic)
%%
{{fbdoc item="filename" value="examples/manual/proguide/errors/result.bas"}}%%(freebasic)
%%
{{fbdoc item="filename" value="examples/manual/proguide/errors/on-error.bas"}}%%(freebasic)
%%
%%
%%
::c::
{{fbdoc item="filename" value="examples/manual/proguide/errors/on-local.bas"}}%%(freebasic)
%%


Revision [22310]

Edited on 2018-05-14 04:14:10 by fxm [Added -exx option in text]
Additions:
- If compiled with //[[CompilerOpte|-e]]//, //[[CompilerOptex|-ex]]// or //[[CompilerOptexx|-exx]]// options, ""FreeBASIC"" uses QB-like error handling.
If the //[[CompilerOpte|-e]]//, //[[CompilerOptex|-ex]]// or //[[CompilerOptexx|-exx]]// switch is used at compile time, the program is expected to have a QB-like error handler enabled. If no handler processes the error, the program stops with an error.
With (//[[CompilerOptex|-ex]]// or //[[CompilerOptexx|-exx]]//) and //[[CompilerOptlang|-lang qb]]// dialect only, the error routine can end by using ##[[KeyPgResume|Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext|Resume Next]]## (continues at the next instruction) .
Deletions:
- If compiled with //[[CompilerOpte|-e]]// or //[[CompilerOptex|-ex]]// options, ""FreeBASIC"" uses QB-like error handling.
If the //[[CompilerOpte|-e]]// or //[[CompilerOptex|-ex]]// switch is used at compile time, the program is expected to have a QB-like error handler enabled. If no handler processes the error, the program stops with an error.
With //[[CompilerOptex|-ex]]// and //[[CompilerOptlang|-lang qb]]// dialect only, the error routine can end by using ##[[KeyPgResume|Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext|Resume Next]]## (continues at the next instruction) .


Revision [21209]

Edited on 2016-03-13 12:43:54 by fxm [Formatting]

No Differences

Revision [20635]

Edited on 2016-02-10 16:12:45 by DkLwikki [Update link format]
Additions:
- By default the program does nothing with the errors - they are silently ignored and code continues. In this case code should process possible errors in the next line by using the ##[[KeyPgErr|Err]]## function.
- If compiled with //[[CompilerOpte|-e]]// or //[[CompilerOptex|-ex]]// options, ""FreeBASIC"" uses QB-like error handling.
Some IO functions such as ##[[KeyPgOpen|Open]]## and ##[[KeyPgPutfileio|Put #]]##... can be used in function form, returning an error number or zero if successful.
If the //[[CompilerOpte|-e]]// or //[[CompilerOptex|-ex]]// switch is used at compile time, the program is expected to have a QB-like error handler enabled. If no handler processes the error, the program stops with an error.
##[[KeyPgOnerror|On Error]]## sets an error handling routine which the program will jump to when an error is found. ##[[KeyPgOnerror|On Error]] [[KeyPgGoto|Goto]] 0## disables the error handling.
The error handler routine can be at the end of the program, as in QB. The ##[[KeyPgLocal|On Local Error]]## statement allows the setting of a local error handler routine at the end of the same ##[[KeyPgSub|Sub]]## or ##[[KeyPgFunction|Function]]## in which the error occurs.
" in function " & *Erfn & _
" on line " & Erl
If the //[[CompilerOpte|-e]]// switch is used (whatever the //[[CompilerOptlang|-lang]]// dialect), the error handler must terminate the program.
With //[[CompilerOptex|-ex]]// and //[[CompilerOptlang|-lang qb]]// dialect only, the error routine can end by using ##[[KeyPgResume|Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext|Resume Next]]## (continues at the next instruction) .
See [[TblRuntimeErrors|Runtime Error Codes]] for a listing of runtime error numbers and their associated meaning.
No user error code range is defined. If ##[[KeyPgError|Error]]## is used to set an error code it is wise to use high values to avoid collisions with the list of built-in error codes. (This built-in list may be expanded later.)
- [[CatPgError|Error Handling Functions]]
- [[TblRuntimeErrors|Runtime Error Codes]]
Deletions:
- By default the program does nothing with the errors - they are silently ignored and code continues. In this case code should process possible errors in the next line by using the ##[[KeyPgErr Err]]## function.
- If compiled with //[[CompilerOpte -e]]// or //[[CompilerOptex -ex]]// options, ""FreeBASIC"" uses QB-like error handling.
Some IO functions such as ##[[KeyPgOpen Open]]## and ##[[KeyPgPutfileio Put #]]##... can be used in function form, returning an error number or zero if successful.
If the //[[CompilerOpte -e]]// or //[[CompilerOptex -ex]]// switch is used at compile time, the program is expected to have a QB-like error handler enabled. If no handler processes the error, the program stops with an error.
##[[KeyPgOnerror On Error]]## sets an error handling routine which the program will jump to when an error is found. ##[[KeyPgOnerror On Error]] [[KeyPgGoto Goto]] 0## disables the error handling.
The error handler routine can be at the end of the program, as in QB. The ##[[KeyPgLocal On Local Error]]## statement allows the setting of a local error handler routine at the end of the same ##[[KeyPgSub Sub]]## or ##[[KeyPgFunction Function]]## in which the error occurs.
" in function " & *Erfn & _
" on line " & Erl
If the //[[CompilerOpte -e]]// switch is used (whatever the //[[CompilerOptlang -lang]]// dialect), the error handler must terminate the program.
With //[[CompilerOptex -ex]]// and //[[CompilerOptlang -lang qb]]// dialect only, the error routine can end by using ##[[KeyPgResume Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext Resume Next]]## (continues at the next instruction) .
See [[TblRuntimeErrors Runtime Error Codes]] for a listing of runtime error numbers and their associated meaning.
No user error code range is defined. If ##[[KeyPgError Error]]## is used to set an error code it is wise to use high values to avoid collisions with the list of built-in error codes. (This built-in list may be expanded later.)
- [[CatPgError Error Handling Functions]]
- [[TblRuntimeErrors Runtime Error Codes]]


Revision [16869]

Edited on 2013-06-22 18:16:02 by DkLwikki [Fix On Error example, var init is not supported in lang qb]
Additions:
Dim e as integer
Deletions:
Dim as integer e = Err


Revision [15161]

Edited on 2011-08-07 17:07:16 by FxMwikki [Fix On Error example, var init is not supported in lang qb]
Additions:
If the //[[CompilerOpte -e]]// switch is used (whatever the //[[CompilerOptlang -lang]]// dialect), the error handler must terminate the program.
With //[[CompilerOptex -ex]]// and //[[CompilerOptlang -lang qb]]// dialect only, the error routine can end by using ##[[KeyPgResume Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext Resume Next]]## (continues at the next instruction) .
Deletions:
If the //[[CompilerOpte -e]]// switch is used the error handler must terminate the program.
With //[[CompilerOptex -ex]]// the error routine can end by using ##[[KeyPgResume Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext Resume Next]]## (continues at the next instruction) .


Revision [14778]

Edited on 2010-08-20 08:41:35 by GaLeon [Changed QuickBasic to QuickBASIC.]
Additions:
{{fbdoc item="section" value="QuickBASIC-like error handling"}}
Deletions:
{{fbdoc item="section" value="QuickBasic-like error handling"}}


Revision [14131]

Edited on 2009-04-26 22:03:56 by CountingPine [Remove direct "Print Err"s; minor changes]
Additions:
""FreeBASIC"" can handle the errors in the following ways:
- By default the program does nothing with the errors - they are silently ignored and code continues. In this case code should process possible errors in the next line by using the ##[[KeyPgErr Err]]## function.
- If compiled with //[[CompilerOpte -e]]// or //[[CompilerOptex -ex]]// options, ""FreeBASIC"" uses QB-like error handling.
- **Future** OOP versions of ""FreeBASIC"" may have a java-like TRY..CATCH...FINALLY exception handler implemented.
**NOTE:** The following information is valid unless the error produces an OS General Protection Fault (for example if the program writes outside the process memory area). In these cases the OS will immediately stop the program and issue an error: nothing can avoid it from inside ""FreeBASIC"".
The default ""FreeBASIC"" behavior is to set the ERR variable and continue.
Dim as integer e
e = Err
Print e
(The example program supposes there is no ##xzxwz.zwz## file). The program does not stop; it sets the ERR variable and continues. The error can be processed in the next line.
Some IO functions such as ##[[KeyPgOpen Open]]## and ##[[KeyPgPutfileio Put #]]##... can be used in function form, returning an error number or zero if successful.
If the //[[CompilerOpte -e]]// or //[[CompilerOptex -ex]]// switch is used at compile time, the program is expected to have a QB-like error handler enabled. If no handler processes the error, the program stops with an error.
Notice: if QB-Like error handling is used, the programmer should be prepared to handle all error conditions.
Dim as integer e = Err
Print e
##[[KeyPgOnerror On Error]]## sets an error handling routine which the program will jump to when an error is found. ##[[KeyPgOnerror On Error]] [[KeyPgGoto Goto]] 0## disables the error handling.
The error handler routine can be at the end of the program, as in QB. The ##[[KeyPgLocal On Local Error]]## statement allows the setting of a local error handler routine at the end of the same ##[[KeyPgSub Sub]]## or ##[[KeyPgFunction Function]]## in which the error occurs.
Dim errmsg As String
errmsg = "Error " & Err & _
" in function " & *Erfn & _
" on line " & Erl
Print errmsg
If the //[[CompilerOpte -e]]// switch is used the error handler must terminate the program.
With //[[CompilerOptex -ex]]// the error routine can end by using ##[[KeyPgResume Resume]]## (retries the statement that caused the error) or ##[[KeyPgResumenext Resume Next]]## (continues at the next instruction) .
Deletions:
Freebasic can handle the errors in the following ways:
- By default the program does nothing with the errors - they are silently ignored and code continues. In this case code should process possible errors in the next line by using the ERR function.
- If compiled with //[[CompilerOpte -e]]// or //[[CompilerOptex -ex]]// options, FreeBASIC uses QB-like error handling.
- **Future** OOP versions of FreeBASIC may have a java-like TRY..CATCH...FINALLY exception handler implemented.
**NOTE:** The following information is valid unless the error produces an OS General Protection Fault (for example if the program writes outside the process memory area). In these cases the OS will immediately stop the program and issue an error: nothing can avoid it from inside FreeBASIC.
The default FreeBASIC behavior is to set the ERR variable and continue.
Print Err
(The example program supposes there is no ##xzxwz.zwz## file). The program does not stop, it sets the ERR variable and continues. The error can be processed in the next line.
Some IO functions such as OPEN and PUT... can be used in FUNCTION form, returning an error number or zero if successful.
If the -e or -ex switch is used at compile time, the program is expected to have a QB-like error handler enabled. If no handler processes the error, the program stops with an error.
Notice: if QB-Like error handling is used, the programmer should be prepared to handle ALL error conditions. For example GETMOUSE issues an error if the cursor goes off the window.
Print Err
ON ERROR sets an error handling routine which the program will jump to when an error is found. ON ERROR GOTO 0 disables the error handling.
The error handler routine can be at the end of the program, as in QB. The ON LOCAL ERROR allows the setting of a local error handler routine at the end of the same SUB or FUNCTION in which the error occurs.
Print ("Error " & Err & " in function " & *Erfn & " on line " & Erl)
If the -e switch is used the error handler must terminate the program.
With -ex the error routine can end by using RESUME (retries the statement that caused the error) or RESUME NEXT (continues at the next instruction)


Revision [13545]

Edited on 2008-07-21 15:42:53 by JeffMarshall [added #lang or $lang to example]
Additions:
'$lang: "qb"


Revision [13237]

The oldest known version of this page was created on 2008-04-26 09:10:37 by JeffMarshall [added #lang or $lang to example]
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