SDL startup segfault

[M374LX]

Sometimes, LMMS (master branch) segfaults at startup.

Update: after building SDL from source, the backtrace gave more useful information.

SDLAudioC3 suggests it is related to SDL. The "C" means it is a capture thread: https://github.com/SDL-mirror/SDL/blob/81117f4052b8ae50a1e49fe5c221e05433ac77ac/src/audio/SDL_audio.c#L1449

Thread 17 "SDLAudioC3" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffbd69d700 (LWP 26426)]
0x0000000000000000 in ?? ()
(gdb) bt full
#0  0x0000000000000000 in ?? ()
No symbol table info available.
#1  0x00007ffff7741cfa in SDL_DBus_Init () at /home/ax/src/SDL2-2.0.12/src/core/linux/SDL_dbus.c:129
        err = {name = 0x0, message = 0x0, dummy1 = 1, dummy2 = 0, dummy3 = 0, dummy4 = 0, dummy5 = 1, padding1 = 0xa8087600}
        is_dbus_available = SDL_FALSE
#2  0x00007ffff77422e1 in SDL_DBus_Init () at /home/ax/src/SDL2-2.0.12/src/core/linux/SDL_dbus.c:168
        is_dbus_available = SDL_FALSE
        err = {name = <optimized out>, message = <optimized out>, dummy1 = <optimized out>, dummy2 = <optimized out>, dummy3 = <optimized out>, dummy4 = <optimized out>, dummy5 = <optimized out>, padding1 = <optimized out>}
#3  SDL_DBus_GetContext () at /home/ax/src/SDL2-2.0.12/src/core/linux/SDL_dbus.c:170
No locals.
#4  0x00007ffff774c069 in rtkit_setpriority (nice_level=-10, thread=26426) at /home/ax/src/SDL2-2.0.12/src/core/linux/SDL_threadprio.c:62
        ui64 = 26426
        si32 = -10
        dbus = <optimized out>
#5  SDL_LinuxSetThreadPriority_REAL (threadID=<optimized out>, priority=-10) at /home/ax/src/SDL2-2.0.12/src/core/linux/SDL_threadprio.c:105
No locals.
#6  0x00007ffff7742e9b in SDL_SYS_SetThreadPriority (priority=priority@entry=SDL_THREAD_PRIORITY_HIGH) at /home/ax/src/SDL2-2.0.12/src/thread/pthread/SDL_systhread.c:207
        value = <optimized out>
        thread = <optimized out>
#7  0x00007ffff76b0235 in SDL_SetThreadPriority_REAL (priority=priority@entry=SDL_THREAD_PRIORITY_HIGH) at /home/ax/src/SDL2-2.0.12/src/thread/SDL_thread.c:470
No locals.
#8  0x00007ffff763d0c3 in SDL_CaptureAudio (devicep=devicep@entry=0x555555f35310) at /home/ax/src/SDL2-2.0.12/src/audio/SDL_audio.c:805
        device = 0x555555f35310
        silence = 0
        delay = 11
        data_len = 4096
        data = <optimized out>
        udata = 0x555555cd33b0
        callback = 0x5555557d4bee <AudioSdl::sdlInputAudioCallback(void*, unsigned char*, int)>
#9  0x00007ffff76afebc in SDL_RunThread (data=0x555555f429f0) at /home/ax/src/SDL2-2.0.12/src/thread/SDL_thread.c:289
        args = 0x555555f429f0
        userfunc = 0x7ffff763d070 <SDL_CaptureAudio>
        userdata = 0x555555f35310
        thread = 0x555555f42690
        statusloc = 0x555555f426a0
#10 0x00007ffff7742cb9 in RunThread (data=<optimized out>) at /home/ax/src/SDL2-2.0.12/src/thread/pthread/SDL_systhread.c:79
No locals.
#11 0x00007ffff7fa1f27 in start_thread (arg=<optimized out>) at pthread_create.c:479
        ret = <optimized out>
        pd = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {140736371218176, -968516196008548055, 140737488346942, 140737488346943, 140736371216192, 140736371218176, 968372279680608553, 968498574795516201}, mask_was_saved = 0}}, priv = {pad = {
              0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = 0
#12 0x00007ffff5fcbe0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
No locals.

[PhysSong]

I guess there might be some race condition in LoadDBUSLibrary() in SDL2's src/core/linux/SDL_dbus.c, between the audio playing thread and the audio capturing thread.
Can you try downgrading SDL2 to 2.0.10 and check the crash still happens?

[M374LX]

The problem seems to stop when using the latest development version of SDL.

It looks like the bug is in SDL, rather than LMMS.

[M374LX]

Adding a delay before opening the audio input device seems to solve the problem.

[PhysSong]

Closed in #5815.