Difference between revisions of "Goto"
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| − | [[ | + | <syntaxhighlight inline lang="pascal">goto</syntaxhighlight> is an unconditional jump to a previously declared [[Label|<syntaxhighlight inline lang="pascal">label</syntaxhighlight>]] (either before or after the <syntaxhighlight inline lang="pascal">goto</syntaxhighlight> command). |
| + | It is a [[Reserved words|reserved word]]. | ||
| + | |||
| + | Usage of <syntaxhighlight inline lang="pascal">goto</syntaxhighlight> in high-level programming languages such as Pascal is highly discredited, since control structures of all sorts are available. | ||
| + | |||
| + | The last situation a <syntaxhighlight inline lang="pascal">goto</syntaxhighlight> is agreed with bad grace to be a significant system error, where a “graceful exit” is better than causing a system breakdown. | ||
| + | |||
| + | As an example, here an excerpt from FPC’s code base {{gitlab|blob|FPC|release_3_2_0/rtl/inc/extres.inc#L324-376|<tt>rtl/inc/extres.inc</tt>}}]: | ||
| + | <syntaxhighlight lang="pascal" line start="324">procedure InitResources;</syntaxhighlight> | ||
| + | <syntaxhighlight lang="pascal" line start="328" highlight="1">label ExitErrMem, ExitErrFile, ExitNoErr; | ||
| + | begin</syntaxhighlight> | ||
| + | <syntaxhighlight lang="pascal" line start="339" highlight="2"> ResHeader:=GetMem(sizeof(TExtHeader)); | ||
| + | if ResHeader=nil then goto ExitErrFile;</syntaxhighlight> | ||
| + | <syntaxhighlight lang="pascal" line start="366" highlight="6"> goto ExitNoErr; | ||
| + | |||
| + | ExitErrMem: | ||
| + | FreeMem(ResHeader); | ||
| + | ResHeader:=nil; | ||
| + | ExitErrFile: | ||
| + | {$I-} | ||
| + | Close(fd); | ||
| + | {$I+} | ||
| + | ExitNoErr: | ||
| + | end;</syntaxhighlight> | ||
| + | According to the value of {{Doc|package=RTL|unit=system|identifier=returnnilifgrowheapfails|text=<syntaxhighlight inline lang="pascal">returnNilIfGrowHeapFails</syntaxhighlight>}} {{Doc|package=RTL|unit=system|identifier=getmem|text=<syntaxhighlight inline lang="pascal">getMem</syntaxhighlight>}} possibly may return [[Nil|<syntaxhighlight inline lang="pascal">nil</syntaxhighlight>]]. | ||
| + | Instead of placing ''everything'' in a “success”-branch, a couple <syntaxhighlight inline lang="pascal">goto</syntaxhighlight> instructions were chosen. | ||
| + | |||
| + | == See also == | ||
| + | * [[Exit|<syntaxhighlight lang="delphi" inline>exit</syntaxhighlight>]] | ||
| + | * [[sGoto|<syntaxhighlight lang="pascal" inline>{$goto}</syntaxhighlight> compiler directive]] | ||
| + | * {{Doc|package=RTL|unit=system|identifier=longjmp|text=<syntaxhighlight lang="pascal" inline>system.longJmp</syntaxhighlight>}} | ||
| + | |||
| + | [[Category: Code]] | ||
Latest revision as of 01:36, 25 January 2022
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goto is an unconditional jump to a previously declared label (either before or after the goto command).
It is a reserved word.
Usage of goto in high-level programming languages such as Pascal is highly discredited, since control structures of all sorts are available.
The last situation a goto is agreed with bad grace to be a significant system error, where a “graceful exit” is better than causing a system breakdown.
As an example, here an excerpt from FPC’s code base rtl/inc/extres.inc]:
324procedure InitResources;
328label ExitErrMem, ExitErrFile, ExitNoErr;
329begin
339 ResHeader:=GetMem(sizeof(TExtHeader));
340 if ResHeader=nil then goto ExitErrFile;
366 goto ExitNoErr;
367
368 ExitErrMem:
369 FreeMem(ResHeader);
370 ResHeader:=nil;
371 ExitErrFile:
372 {$I-}
373 Close(fd);
374 {$I+}
375 ExitNoErr:
376end;
According to the value of returnNilIfGrowHeapFails getMem possibly may return nil.
Instead of placing everything in a “success”-branch, a couple goto instructions were chosen.
