[fpc-devel] Different handling of try..except depending on OS?

Michael Van Canneyt michael at freepascal.org
Thu Dec 10 09:38:25 CET 2020

On Thu, 10 Dec 2020, Bart via fpc-devel wrote:

> On Thu, Dec 10, 2020 at 7:34 AM Sven Barth via fpc-devel
> <fpc-devel at lists.freepascal.org> wrote:
>> That is correct, because without the SysUtils unit (which declared the
>> Exception type) the RTL can't convert the triggered runtime error to an
>> exception type that can be caught inside the try ... except ... end block.
> I would have expected that in that scenario (no sysutils) try..except
> or try..finally should raise a compilation error then.
> Since you teel the compiler to do things it cannot do?


Sysutils is only needed to convert run-time errors to exceptions.

You can perfectly use exceptions without sysutils, but they can not descend
from Exception, but from TObject. That exception classes descend from Exception
is a convention, not a compiler-enforced rule.

If you change your code to this
     On O : TObject do
      Writeln('except block : ',O.ClassName)

You'll see there is no output, because there is no exception object.

The reason is as Sven surmised: the setjmp mechanism used on non-windows

> It would be Delphi incompatible of course ...
> B.t.w. (cannot test now, am at work): does it catc exceptions if any
> other used unit in the main program uses sysutils?
> I.o.w.: is it enough if sysutils gets linked in, or must it be in the
> unit/prgram that actually has the try..except/try..finally code in it?

It is enough that sysutils gets linked in.

I checked that, the program you sent does not have sysutils linked in.


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