[vlc-devel] Callbacks in modules written in C++ have wrong linkage

[Filip Roséen]

> Note
> --------
> Related to this problem is the fact that loading of modules stores
> pointer-to-functions inside pointer-to-void, which is
> undefined-behavior according to both the C and C++ Standard.
>
> Fixing the above would require major changes to the codebase, and as
> such it is currently ignored.

We currently have an issue with C++ modules not utilizing the proper
linkage for the callbacks set when declaring the plugin. Since the given
callbacks are invoked from code compiled as C, the callbacks shall have
C linkage.



Example
-------

The following is taken from `modules/access/live555.cpp`:

     70: static int  Open ( vlc_object_t * );
     71: static void Close( vlc_object_t * );
       ...
     95: vlc_module_begin ()
       ...
     99:     set_callbacks( Open, Close )
       ...
    142: vlc_module_end ()

`Open` and `Close`, implicitly having C++ linkage, are indirectly stored
in a variable of type `int(*)(vlc_object_t*)`, implicitly having C
linkage, at `src/modules/modules.c:146` and `src/modules/modules.c:154`,
respectivelly.



Proposed Resolution
-------------------

In order to fix the issue the following patch can be applied. It will
ensure that the type of `Open` and `Close` has C linkage, even though
the functions themselves have internal linkage (due to `static`).

    --- a/modules/access/live555.cpp
    +++ b/modules/access/live555.cpp
    @@ -67,8 +67,10 @@ extern "C" {
     /*****************************************************************************
      * Module descriptor
      *****************************************************************************/
    -static int  Open ( vlc_object_t * );
    -static void Close( vlc_object_t * );
    +extern "C" {
    +  static int  Open ( vlc_object_t * );
    +  static void Close( vlc_object_t * );
    +}
     
     #define KASENNA_TEXT N_( "Kasenna RTSP dialect")
     #define KASENNA_LONGTEXT N_( "Kasenna servers use an old and nonstandard " \


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Questions
---------

  - Any thoughts on the matter? 
  - Should I go ahead and patch this everywhere it is required?

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