On Sat, 26 Nov 2005 18:57:03 -0500, Harry Vennik <[EMAIL PROTECTED]>
wrote:
Op zaterdag 26 november 2005 20:03, schreef Youness Alaoui:
Hi,
that's nice!
the thing is "why does it only happen with pwc" ?
I think I understand. The list contains the attributes we can set. And
there
are some attributes that cannot be set with the pwc driver, so those
attributes won't appear in the list. All other drivers we know support
these
attributes. So only with the pwc driver it happens that we have to loop
through all the list, to know that the attribute is not available, so
only
with the pwc driver we get far enough to reach the bad pointer!
humm.. probably, now I get it... I don't know what causes this to be like
that, but what if we initialize (memset(0, list)) the list to NULL and
each time we add something, we set to NULL the next pointer... I guess
that's what it should be... I'm too tired to look at the code now, but
MAYBE tomorrow, I"ll have a peek
anyways, what you found confirms what someone else told me a few weeks
ago, I just never had time to look into it... here's more info about the
linked list problem and maybe a fix for it :
original post (see the comments) :
https://sourceforge.net/tracker/?group_id=54091&atid=472655&func=detail&aid
=1281369
and here's a transcipt of the discussion with him on the pwc forum
thread :
By: Willem Monsuwe - monsuwe
RE: segfault when trying to start webcam
2005-10-02 15:20
To be more exact, the driver doesn't support changing all the properties
that the program wants to.
For example, on a LT Quickcam 4000, you can set brightness and contrast
just fine, it just doesn't happen to have a hue setting.
IMHO, the webcam plugin should check what properties the driver supports
and be able to change those, but that's a lot of work to do, so that's
for
later.
No, let's just have a set of attributes to set, and check if those are
available, and disable them if not. (That's what we are trying to do
now, but
it fails because of the dangling pointer bug. Please also note that
doing it
the way proposed here - detect features and use those - will always fail,
with every driver (!), because of the same bug.)
I didn't really get it but do you mean initialize the list with all
possible value and add a boolean enable == TRUE | FALSE ? so if we find an
attribute, instead of adding it to the list, we just set the enabled to
TRUE? I'm not sure that's what you meant, but in any case, I'd prefer
libng stays clean and portable...
maybe someone could go check xawtv's cvs and see if libng was in any way
updated..
I remind those who don't know that libng is the library used by xawtv, it
is not available anywhere besides in xawtv's source code (no separate
individual download for it), it's internal to xawtv and may have been
updated...
In any case, the reason for the crash is a fault in the way linked lists
are handled in libng(*). If that is remedied, the webcam works better,
although not okay, because then the function returns an error, which the
tcl code doesn't handle.
Hmm, have to look at it better to know if this is true. But even if it
is, it
probably won't hurt anyway.
*) Linked lists in libng are built by linking to a root pointer, that is
allocated by the caller. This means that if you have a linked list as
member of a struct, the first item in the list will actually be that
member. Furthermore, it's a doubly linked, circular list. This is all
fine, as long as you use the original root pointer. As soon as you copy
it, for example by assigning the struct, the root pointer will be at a
different address. However, inside the linked list there are two
pointers
pointing to the old address, so as soon as a function traverses the
entire
list, you will have a dangling pointer bug.
This is how things may go, but I think the dangling pointer problem has a
different cause in this case. I think so because of the unlikely value
of the
pointer in an otherwise correctly filled structure for the 'Gain Level'
attribute. I think there is just some buggy code in libng that happens to
overwrite the pointer with something else.
Harry
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