Difference between revisions of "Goto"

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The last situation a <syntaxhighlight enclose="none" lang="pascal">goto</syntaxhighlight> is agreed with bad grace is a significant system error, where a “graceful exit” is better than causing a system breakdown.
 
The last situation a <syntaxhighlight enclose="none" lang="pascal">goto</syntaxhighlight> 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 [https://svn.freepascal.org/cgi-bin/viewvc.cgi/tags/release_3_0_4/rtl/inc/extres.inc?revision=37113&view=markup <tt>rtl/inc/extres.inc</tt>]:
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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>]:
 
<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="328">label ExitErrMem, ExitErrFile, ExitNoErr;
 
<syntaxhighlight lang="pascal" line start="328" highlight="328">label ExitErrMem, ExitErrFile, ExitNoErr;

Revision as of 18:51, 7 March 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.

see also