Dear Dave, Yes, the perl interpreters are not the same: 
# perl thread
(gdb) p my_perl
$1 = (PerlInterpreter *) 0xb
(...)
 dSP;
(gdb) p my_perl
$7 = (PerlInterpreter *) 0x555555953260
(...)
# mpv thread:
(gdb) p my_perl
$2 = (PerlInterpreter *) 0x1
(...)
dSP;
(gdb) p my_perl
$3 = (PerlInterpreter *) 0x0

[download]
Regarding the creation of the extra threads: To be honest, I don't know how they are created and what with the main perl thread happens. This is managed by libmpv and I don't know and understand the code very well.. The error seems to occur in mpv_initialize from mpv-player/client.c, which calls mp_wakeup_core from mpv-player/playloop.c that triggers an event from the saved dispatch queue. mpv/misc/dispatch.c describes it as follows: "A dispatch queue lets other threads run callbacks in a target thread. The target thread is the thread which calls mp_dispatch_queue_process()." This is here mp_dispatch_interrupt which was called in mp_wakeup_core. So I think the target thread is created in mpv_create with the following lines: 
pthread_t thread;
    if (pthread_create(&thread, NULL, core_thread, mpctx) != 0) {
        ctx->clients->have_terminator = true; // avoid blocking
        mpv_terminate_destroy(ctx);
        mp_destroy(mpctx);
        return NULL;
}

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core_thread() consists of the following lines: 
static void *core_thread(void *p)
{
    struct MPContext *mpctx = p;

    mpthread_set_name("mpv core");

    while (!mpctx->initialized && mpctx->stop_play != PT_QUIT)
        mp_idle(mpctx);

    if (mpctx->initialized)
        mp_play_files(mpctx);

    // This actually waits until all clients are gone before actually
    // destroying mpctx. Actual destruction is done by whatever destro
+ys
    // the last mpv_handle.
    mp_shutdown_clients(mpctx);

    return NULL;
}

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I don't know whether this is helpful. Perhaps the task is a number too big for me because I have no clue regarding threads? Nevertheless thank you very much for all your help and patience..

Max

PS.: I also don't understand why one time the callback is called in the perl thread? Perhaps because of the following line in mpv_set_wakeup_callback? 
if (ctx->wakeup_cb)
ctx->wakeup_cb(ctx->wakeup_cb_ctx);

[download]

In reply to Re^3: XS callback to mpv_set_wakeup_callback by MaxPerl
in thread XS callback to mpv_set_wakeup_callback by MaxPerl

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