[TUHS] GOTO etc

[TUHS] GOTO etc

[Steve]

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Dennis added setjmp() and longjmp() so the shell could handle errors in a reasonable way.
There are two places where setjmp was used in the original shell (7th edition) code as I recall.  
Both at the top level
in main.c.

The idea came from Algol68 but I do not know where it was originally invented.  longjmp() was used 
in the "exitsh"
function that got called on the exit command, default trap routine and a fault with no trap set.

It was also used when executing a subshell to avoid a fork and exec.  In this case the setjmp() was 
at top level
in the initial sh setup.

Hope this makes sense.  But these were two different uses.  One for error recovery and one to reset 
the execution environment
back to initial state.

Steve

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[TUHS] Re: GOTO etc

[Bakul Shah]

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Perhaps you’re talking about non-local GOTOs in Algol68, where you can jump from a nested procedure to a label in a lexically enclosing procedure. Pascal has this too. C has no nested procedures but its setjmp/longjmp is much more powerful (& dangerous). Though both can be used to the top level of a REPL or to jump to a known place after an error.

> On Mar 12, 2023, at 11:24 AM, Steve <srb@unixsh.com> wrote:
> 
>  Dennis added setjmp() and longjmp() so the shell could handle errors in a reasonable way.
> There are two places where setjmp was used in the original shell (7th edition) code as I recall.  Both at the top level
> in main.c.
> 
> The idea came from Algol68 but I do not know where it was originally invented.  longjmp() was used in the "exitsh"
> function that got called on the exit command, default trap routine and a fault with no trap set.
> 
> It was also used when executing a subshell to avoid a fork and exec.  In this case the setjmp() was at top level
> in the initial sh setup.
> 
> Hope this makes sense.  But these were two different uses.  One for error recovery and one to reset the execution environment
> back to initial state.
> 
> Steve

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[TUHS] Re: GOTO etc

[Paul Winalski]

On 3/12/23, Bakul Shah <bakul@iitbombay.org> wrote:
> Perhaps you’re talking about non-local GOTOs in Algol68, where you can jump
> from a nested procedure to a label in a lexically enclosing procedure.
> Pascal has this too.

So does PL/I.

-Paul W.

[TUHS] Re: GOTO etc

[Steve Bourne]

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Hi Bakul, yes non local GOTOs.  In the Algol68C compiler we wrote we used the names setmp and longjmp
for the code generation routines to implement non local goto.  So as you say they were not partof 
the Algol68 language.

Steve

On 3/12/23 5:10 PM, Paul Winalski wrote:
> On 3/12/23, Bakul Shah<bakul@iitbombay.org>  wrote:
>> Perhaps you’re talking about non-local GOTOs in Algol68, where you can jump
>> from a nested procedure to a label in a lexically enclosing procedure.
>> Pascal has this too.
> So does PL/I.
>
> -Paul W.


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[TUHS] GOTO etc.

[Steve]

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After some digging - in the Algol68C compiler we used the names setmp and longjmp for the code 
generation routines to implement non local goto.  So as you say they were not part of the Algol68 
language. Steve
From: Bakul Shah<bakul@iitbombay.org>
Subject: [TUHS] Re: GOTO etc
To:srb@acm.org

Perhaps you’re talking about non-local GOTOs in Algol68, where you can jump from a nested procedure to a label in a lexically enclosing procedure. Pascal has this too. C has no nested procedures but its setjmp/longjmp is much more powerful (& dangerous). Though both can be used to the top level of a REPL or to jump to a known place after an error.

> On Mar 12, 2023, at 11:24 AM, Steve<srb@unixsh.com>  wrote:
>
>  Dennis added setjmp() and longjmp() so the shell could handle errors in a reasonable way.
> There are two places where setjmp was used in the original shell (7th edition) code as I recall.  Both at the top level
> in main.c.
>
> The idea came from Algol68 but I do not know where it was originally invented.  longjmp() was used in the "exitsh"
> function that got called on the exit command, default trap routine and a fault with no trap set.
>
> It was also used when executing a subshell to avoid a fork and exec.  In this case the setjmp() was at top level
> in the initial sh setup.
>
> Hope this makes sense.  But these were two different uses.  One for error recovery and one to reset the execution environment
> back to initial state.
>
> Steve

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