[vlc-devel] How to share a variable between qt4 ui (c++) and playlist thread (c)

fayyaz lodhi fayyazkl at gmail.com
Thu Jan 10 05:15:04 CET 2013

To elaborate further, i created a global variable in
It is extern in playlist_internal.h

This variable is read from in the LoopRequest function when a playlist is
signaled to start playing.

Before the start of a play list some gui dialogs appear which are located
in modules/gui/qt4/dialogs (some what similar to firstrun.cpp).

Now if firstrun.cpp has extern the same variable i.e. (extern type name),
and it is filled with in this dialog (actually it is a c string i.e. char
array filled from a QString using  strcpy(c_string,
q_string.toLocal8Bit().constData()). This compiles successfully, but i get
following run time errors with no GUI loaded.

no suitable dialogs provider found (hint: compile the qt4 plugin, and make
sure it is loaded properly)
[0x1ea18b8] skins2 interface error: cannot instantiate qt4 dialogs provider
[0x1ea18b8] main interface error: no suitable interface module
[0x1e72388] main libvlc error: interface "default" initialization failed

I have tried several other combinations like instead of using extern,
declaring the variable in playlist_internal.h or say vlc_common.h and
including the header file on the gui side (if not included already) or vice
versa. But usually i start getting multiple inclusion errors though the
variable is only created once.

All errors go away as soon as i comment out the variable from any one side
or even if i make it static on one side. GUI starts loading normally.

Can some one help?


On Wed, Jan 9, 2013 at 5:16 PM, Fayyaz Khan <fayyazkl at gmail.com> wrote:

> Thank you Remi.
> I understand that well and probably that is not a problem in my case.
> The shared variable (char array) that i wish to create will be updated by
> a dialog in the beginning of the program (though it can be chosen from a
> list of dynamic options), before playlist components picks it up. So there
> will be no sync issue.
> Moreover, i am using PL Lock / unlock in both scenarios to ensure
> synchronized access.
> Could you please elaborate a little more on a good location in source code
> to prevent other include file issues? Right now i am a victim of those.
> Thank you
> Fayyaz
> On 1/9/2013 5:11 PM, Rémi Denis-Courmont wrote:
>> On Wed, 9 Jan 2013 14:28:28 +0500, fayyaz lodhi <fayyazkl at gmail.com>
>> wrote:
>>> What is the correct way of sharing data between the two?
>> I don't need to remind you that the multi-threaded memory model (either
>> POSIX or C11) forbids sharing data directly unless the data is constant
>> read-only.
>> You need to add functions instead, including appropriate synchronization
>> depending on the actual case.
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