Difference between revisions of "Goto"
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(→See also: insert Exit) |
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{{goto}} | {{goto}} | ||
− | <syntaxhighlight | + | <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 | + | 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. |
− | |||
− | As an example, here an excerpt from | + | 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="324">procedure InitResources;</syntaxhighlight> | ||
− | <syntaxhighlight lang="pascal" line start="328" highlight=" | + | <syntaxhighlight lang="pascal" line start="328" highlight="1">label ExitErrMem, ExitErrFile, ExitNoErr; |
begin</syntaxhighlight> | begin</syntaxhighlight> | ||
− | <syntaxhighlight lang="pascal" line start="339" highlight=" | + | <syntaxhighlight lang="pascal" line start="339" highlight="2"> ResHeader:=GetMem(sizeof(TExtHeader)); |
if ResHeader=nil then goto ExitErrFile;</syntaxhighlight> | if ResHeader=nil then goto ExitErrFile;</syntaxhighlight> | ||
− | <syntaxhighlight lang="pascal" line start="366" highlight=" | + | <syntaxhighlight lang="pascal" line start="366" highlight="6"> goto ExitNoErr; |
ExitErrMem: | ExitErrMem: | ||
Line 23: | Line 25: | ||
ExitNoErr: | ExitNoErr: | ||
end;</syntaxhighlight> | end;</syntaxhighlight> | ||
− | According to the value of {{Doc|package=RTL|unit=system|identifier=returnnilifgrowheapfails|text=<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 | + | 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>{$goto}</syntaxhighlight> compiler directive]] | ||
+ | * {{Doc|package=RTL|unit=system|identifier=longjmp|text=<syntaxhighlight lang="pascal" inline>system.longJmp</syntaxhighlight>}} | ||
− | [[Category: | + | [[Category: Code]] |
− |
Latest revision as of 01:36, 25 January 2022
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goto
is an unconditional jump to a previously declared label
(either before or after the goto
command).
It is a reserved word.
Usage of goto
in high-level programming languages such as Pascal is highly discredited, since control structures of all sorts are available.
The last situation a goto
is agreed with bad grace to be a significant system error, where a “graceful exit” is better than causing a system breakdown.
As an example, here an excerpt from FPC’s code base rtl/inc/extres.inc]:
324procedure InitResources;
328label ExitErrMem, ExitErrFile, ExitNoErr;
329begin
339 ResHeader:=GetMem(sizeof(TExtHeader));
340 if ResHeader=nil then goto ExitErrFile;
366 goto ExitNoErr;
367
368 ExitErrMem:
369 FreeMem(ResHeader);
370 ResHeader:=nil;
371 ExitErrFile:
372 {$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.