Difference between revisions of "Goto"
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As an example, here an excerpt from FPC's code base [https://svn.freepascal.org/cgi-bin/viewvc.cgi/tags/release_3_0_4/rtl/inc/extres.inc?revision=37113&view=markup#l324 <tt>rtl/inc/extres.inc</tt>]: | As an example, here an excerpt from FPC's code base [https://svn.freepascal.org/cgi-bin/viewvc.cgi/tags/release_3_0_4/rtl/inc/extres.inc?revision=37113&view=markup#l324 <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: | ||
Revision as of 17:39, 11 April 2018
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goto is an unconditional jump to a previously declared label (either before or after the goto command).
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 is 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.
