Re: Libraries using longjmp for error handling

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From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Libraries using longjmp for error handling
Date	Fri, 29 Sep 2023 21:56:02 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>>
>> [..how should errors be handled..]
>>
>>> How would you prefer these sorts of thing to be signalled?
>>
>> Let me give just one example.
>>
>> I wrote some code not long ago to add a value to a set of values,
>> where the set is represented using a recursive binary tree structure
>> (like red-black trees, but a little different).  Usually we may
>> expect that a request to add a value would offer a value not already
>> included in the set, but certainly it can happen that there is a
>> call to add a value that is already included, in which case the top
>> level return value should be the original tree structure.
>>
>> I thought it would be easier to write a simple recursive routine
>> that assumes the to-be-added value is not yet included in the set,
>> and if that assumption is violated raise an exception that is caught
>> at the outermost level and simply returns the original argument tree
>> value.  Certainly the code could have been written to handle the
>> already-present situation at each level of the recursion, but it was
>> much easier and much cleaner to handle it by raising an exception.
>
> This is one of those cases where I just have to take your word for it,
> (and I am happy to do that).

Responding here to just this one part.

Here is a simpler version of the code, for an ordinary
binary tree, and without any rebalancing:

  module TreeSet = struct
    type 'a treeset = Empty | Node of 'a treeset * 'a * 'a treeset
    exception Present

    let add tree element =
      let rec add t e =
        match t with
        | Empty -> Node( Empty, e, Empty )
        | Node( a, v, b ) ->
            if  e < v  then  Node( add a e, v, b       )  else
            if  e > v  then  Node( a,       v, add b e )  else
            raise Present
      in
      try  add tree element  with  Present -> tree

  end

Transliterating that into C, the central functions might look like
this (I have left out the type definition and the constructor
function used to build a new node value):

  static Tree add( jmp_buf, Tree, double );
  static Tree raise_present( jmp_buf );
  static Tree new_node( Tree, double, Tree );

  Tree
  add_element( Tree tree, double new_element ){
    jmp_buf jb;
    if(  setjmp( jb )  )  return  tree;

    return  add( jb, tree, new_element );
  }

  Tree
  add( jmp_buf jb, Tree t, double d ){
    return
	! t	  ?  new_node( 0, d, 0 )
    :   d < t->v  ?  new_node( add( jb, t->a, d ), t->v,          t->b      )
    :   d > t->v  ?  new_node(          t->a,      t->v, add( jb, t->b, d ) )
    :	/*********/  raise_present( jb );
  }

  Tree
  raise_present( jmp_buf jb ){
    longjmp( jb, 1 );
  }

Of course it's unlikely that sets would be implemented this way
in C, but that's not the point;  the point is that exceptions
can result in code that is cleaner and easier to understand
than an alternate approach where return values would have to
be checked at every level.

(Incidentally, I want to acknowlege you comment about taking my
word for the result.  I still thought it would be good to give
a concrete example, even if it is a very simplified one.)

Thread

Libraries using longjmp for error handling (was: Re: More on NNTP testing) Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-09-27 22:52 +0000
  Re: Libraries using longjmp for error handling (was: Re: More on NNTP testing) Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2023-09-28 08:19 +0800
    Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-28 02:18 +0100
      Re: Libraries using longjmp for error handling Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-09-28 22:30 +0000
        Re: Libraries using longjmp for error handling scott@slp53.sl.home (Scott Lurndal) - 2023-09-28 22:33 +0000
          Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-09-29 01:41 +0300
            Re: Libraries using longjmp for error handling scott@slp53.sl.home (Scott Lurndal) - 2023-09-28 23:45 +0000
          Re: Libraries using longjmp for error handling Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-28 17:35 -0700
            Re: Libraries using longjmp for error handling David Brown <david.brown@hesbynett.no> - 2023-09-29 16:49 +0200
      Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-28 18:24 -0700
        Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-29 03:19 +0100
          Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-29 07:46 -0700
            Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-29 21:02 +0100
              Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-29 21:56 -0700
                Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-01 00:06 +0100
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-30 20:35 -0700
                Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-10-01 14:44 +0300
                Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-01 15:45 +0100
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-01 09:28 -0700
                Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-01 20:49 +0100
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-02 02:31 -0700
                Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-02 23:55 +0100
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-03 05:01 -0700
            Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-09-29 23:58 +0300
              Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-01 02:52 -0700
                Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-10-01 14:33 +0300
  Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-28 01:48 +0100
  Re: Libraries using longjmp for error handling (was: Re: More on NNTP testing) Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-28 05:11 +0000
    Re: Libraries using longjmp for error handling (was: Re: More on NNTP testing) Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-28 06:26 +0000
  Re: Libraries using longjmp for error handling (was: Re: More on NNTP testing) Anton Shepelev <anton.txt@gmail.moc> - 2023-09-28 16:02 +0300
    Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-28 14:44 +0100
      Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-09-28 18:31 +0300
        Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-28 16:43 +0000
          Re: Libraries using longjmp for error handling scott@slp53.sl.home (Scott Lurndal) - 2023-09-28 17:00 +0000
            Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-28 17:16 +0000
            Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-28 19:38 +0100
          Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-28 16:05 -0700
            Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-29 00:26 +0000
              Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-28 23:20 -0700
            Re: Libraries using longjmp for error handling Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-28 18:31 -0700
              Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-29 03:24 +0100
                Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-29 03:12 +0000
              Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-28 22:45 -0700
                Re: Libraries using longjmp for error handling Spiros Bousbouras <spibou@gmail.com> - 2023-09-29 12:02 +0000
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-29 08:10 -0700
                Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-29 17:03 +0100
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-29 10:15 -0700
                Re: Libraries using longjmp for error handling gazelle@shell.xmission.com (Kenny McCormack) - 2023-09-29 17:17 +0000
                Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-29 18:53 +0000
                Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-09-29 18:11 +0300
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-29 10:20 -0700
                Re: Libraries using longjmp for error handling Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-29 11:30 -0700
                Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-29 18:31 +0000
        Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-28 19:27 +0100
          Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-28 20:24 +0000
            Re: Libraries using longjmp for error handling Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-28 22:23 +0100
              Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-28 15:43 -0700
              Re: Libraries using longjmp for error handling Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-28 23:11 +0000
                Re: Libraries using longjmp for error handling Anton Shepelev <anton.txt@gmail.moc> - 2023-09-29 17:23 +0300
                Re: Libraries using longjmp for error handling Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-29 08:19 -0700
        Re: Libraries using longjmp for error handling David Brown <david.brown@hesbynett.no> - 2023-09-28 21:57 +0200
  Re: Libraries using longjmp for error handling Richard Kettlewell <invalid@invalid.invalid> - 2023-09-30 09:37 +0100

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