QMediaPlayer does not work on NixOS

Volker Weissmann

I tried building a video player like this:
example.pro:

QT      += core gui widgets multimedia multimediawidgets

CONFIG  += c++17

SOURCES += main.cpp

main.cpp:

#include <QApplication>                                                                                                                                                                                             
#include <QWidget>                                                                                                                                                                                                  
#include <QPushButton>                                                                                                                                                                                              
#include <QVBoxLayout>                                                                                                                                                                                              
#include <QMediaPlayer>                                                                                                                                                                                             
#include <QVideoWidget>                                                                                                                                                                                             
#include <QUrl>                                                                                                                                                                                                     
                                                                                                                                                                                                                    
int main(int argc, char *argv[]) {                                                                                                                                                                                  
    QApplication app(argc, argv);                                                                                                                                                                                   
                                                                                                                                                                                                                    
    QWidget window;                                                                                                                                                                                                 
    window.resize(640, 480);                                                                                                                                                                                        
                                                                                                                                                                                                                    
    QVideoWidget *videoWidget = new QVideoWidget;                                                                                                                                                                   
    QMediaPlayer *player = new QMediaPlayer;                                                                                                                                                                        
                                                                                                                                                                                                                    
    player->setVideoOutput(videoWidget);                                                                                                                                                                            
    //player->setMedia(QUrl::fromLocalFile("/home/user/Downloads/sample-5s.mp4")); // also works                                                                                                                    
    player->setMedia(QUrl("https://file-examples.com/storage/feaa6a7f0468517af9bc02d/2017/04/file_example_MP4_480_1_5MG.mp4"));                                                                                     
                                                                                                                                                                                                                    
    QVBoxLayout *layout = new QVBoxLayout;                                                                                                                                                                          
    layout->addWidget(videoWidget);                                                                                                                                                                                 
                                                                                                                                                                                                                    
    window.setLayout(layout);                                                                                                                                                                                       
    window.show();                                                                                                                                                                                                  
    player->play();                                                                                                                                                                                                 
                                                                                                                                                                                                                    
    return app.exec();                                                                                                                                                                                              
}

If I build and run this under ArchLinux with the packages qt5-base and qt5-multimedia installed, it works fine (except for a segmentation fault on exit, but that is a different topic). If however, I build and run this in nixos with nix-shell -p qt5Full libsForQt5.qt5.qtmultimedia I get an empty window, with nothing in stdout or stderr. If I run it with QT_DEBUG_PLUGINS=1 I cannot spot anything suspicious in the output.

Any idea how to make progress here? Is that a bug in the nixos package of Qt?

SGaist

Hi and welcome to devnet,

Which exact version of Qt are you using ?
Which window manager are running with ?
Which desktop environment are you using ?

Volker Weissmann

Hello SGaist,

Im using QtVersion: 5.15.16 and sway (a wayland compositor).

SGaist

And for the desktop environment ?

Volker Weissmann

@SGaist said in QMediaPlayer does not work on NixOS:

And for the desktop environment ?

Sway. Nothing else.

I installed it like this:

programs.sway = {
    enable = true;
    wrapperFeatures.gtk = true;
  };

JoeCFD

run
strace your_app
you may be able to find some clues

Volker Weissmann

@JoeCFD said in QMediaPlayer does not work on NixOS:

run
strace your_app
you may be able to find some clues

Just did that.

Here is the result:
https://weissmann.pm/2025-qt5-nixos-bug-strace.txt

JoeCFD

openat(AT_FDCWD, "/home/unpriv/.Xdefaults-nichts", O_RDONLY) = -1 ENOENT (No such file or directory)

<====X11 Fallback Attempts. Are your env settings correct for Wayland? Not familiar with your OS.

JoeCFD

setenv( "DISPLAY",         ":0",      1 );
setenv( "GDK_BACKEND",     "wayland", 1 );
setenv( "QT_QPA_PLATFORM", "wayland", 1 );

setenv( "XDG_RUNTIME_DIR", "/run/user/1000",            1 );
setenv( "WAYLAND_DISPLAY", "wayland-1",                 1 );
setenv( "QT_QPA_FONTDIR",  "/usr/share/fonts/truetype", 1 );

I need these settings in my Qt code on Yocto wayland .

Volker Weissmann

Xeyes shows that the application is using X11.
Once I'll find time, I try to force it to use Wayland.