Re: [XForms] infrequent address exception in XCreatePixmap, FYI

[Jens Thoms Toerring]

Hi Peter,

On Tue, Mar 03, 2020 at 11:09:07AM -0800, Peter Rowat via xforms-development 
wrote:
> My program runs successfully 90% of the time, but about every 10th run there
> is an address exception appearing in XCreatePixmap.

Sorry to be stupid, but what exactly is an "address exception"? I'm
aware of SEGFAULT (dereferencing a non-existing address) and SIGBUS
(an unaligned access), but the term "address exception" doesn't ring
a bells. A quick search for the term on Google comes up with some
Java-ish stuff.

> Here is the lldb output:
> 
> Process 4581 launched: '/Users/prowat/BioPrep/N2Prep/Izh2p2/mod' (x86_64)
> ......
> ......
> Process 4581 stopped
> * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS 
> (code=1, address=0x968)
>     frame #0: 0x000000010012b52b libX11.6.dylib`XCreatePixmap + 29
> libX11.6.dylib`XCreatePixmap:
> ->  0x10012b52b <+29>: movq   0x968(%rbx), %rax
>     0x10012b532 <+36>: testq  %rax, %rax
>     0x10012b535 <+39>: je     0x10012b53c               ; <+46>
>     0x10012b537 <+41>: movq   %rbx, %rdi
> Target 0: (mod) stopped.
> (lldb) bt
> * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS 
> (code=1, address=0x968)
>   * frame #0: 0x000000010012b52b libX11.6.dylib`XCreatePixmap + 29
>     frame #1: 0x000000010012a589 libX11.6.dylib`XCreateBitmapFromData + 53
>     frame #2: 0x000000010029f593 libforms.2.dylib`fli_init_stipples + 67
>     frame #3: 0x0000000100250046 libforms.2.dylib`fli_init_colormap + 198
>     frame #4: 0x0000000100258546 libforms.2.dylib`fl_initialize + 5334
>     frame #5: 0x0000000100001577 mod`main(argc=1, argv=0x00007ffeefbff888) at 
> runit1.c:315:2
>     frame #6: 0x00007fff674a07fd libdyld.dylib`start + 1
>     frame #7: 0x00007fff674a07fd libdyld.dylib`start + 1
> (end of lldb print)===========

Had a look at the last potential culprit in XForms, fli_init_stipples().
But there's nothing that looks like it could result in a segmentation
fault or unaligned access. And I'd guess that this would be something
that would happen consistently and not just every tenth time the pro-
gram is started as everything used there is global variables which
should be the same each time round.

> Frame #5: is at statement: `fl_initialize( &argc, argv, "BioPrep", 0, 0 );’
> It was not the first executable statement but was before any other XForms
> statement.

> I’ve moved the fl_initialize statement to be the first executable, in case
> this stops the address exceptions. None so far after 2 runs. If I get no
> more address exceptions after another 20-30 runs, I’ll write again.

To be honest I'm not a big fan of movings stuff around in the code
to get rid of annoying behaviour as it, in my experience, tends to
at best move a bug to a place where it's less likely to be triggered,
but not to fix the underlying issue. My preference would be to figure
out what is really going on there;-) But then I don't really know yet
what this "address exeption" thingy actually signifies...

                          Best regards, Jens
-- 
  \   Jens Thoms Toerring  ________      address@hidden
   \_______________________________      http://toerring.de