Qt

Writing Nix expressions for Qt libraries and applications is largely similar as for other C++ software. This section assumes some knowledge of the latter.

The major caveat with Qt applications is that Qt uses a plugin system to load additional modules at runtime, from a list of well-known locations. In Nixpkgs, we patch QtCore to instead use an environment variable, and wrap Qt applications to set it to the right paths. This effectively makes the runtime dependencies pure and explicit at build-time, at the cost of introducing an extra indirection.

Nix expression for a Qt package (default.nix)

{ stdenv, lib, qtbase, wrapQtAppsHook }:

stdenv.mkDerivation {
  pname = "myapp";
  version = "1.0";

  buildInputs = [ qtbase ];
  nativeBuildInputs = [ wrapQtAppsHook ];
}

It is important to import Qt modules directly, that is: qtbase, qtdeclarative, etc. Do not import Qt package sets such as qt5 because the Qt versions of dependencies may not be coherent, causing build and runtime failures.

Additionally all Qt packages must include wrapQtAppsHook in nativeBuildInputs, or you must explicitly set dontWrapQtApps.

Locating runtime dependencies

Qt applications must be wrapped to find runtime dependencies. Include wrapQtAppsHook in nativeBuildInputs:

{ stdenv, wrapQtAppsHook }:

stdenv.mkDerivation {
  # ...
  nativeBuildInputs = [ wrapQtAppsHook ];
}

Add entries to qtWrapperArgs are to modify the wrappers created by wrapQtAppsHook:

{ stdenv, wrapQtAppsHook }:

stdenv.mkDerivation {
  # ...
  nativeBuildInputs = [ wrapQtAppsHook ];
  qtWrapperArgs = [ ''--prefix PATH : /path/to/bin'' ];
}

The entries are passed as arguments to wrapProgram.

Set dontWrapQtApps to stop applications from being wrapped automatically. Wrap programs manually with wrapQtApp, using the syntax of wrapProgram:

{ stdenv, lib, wrapQtAppsHook }:

stdenv.mkDerivation {
  # ...
  nativeBuildInputs = [ wrapQtAppsHook ];
  dontWrapQtApps = true;
  preFixup = ''
      wrapQtApp "$out/bin/myapp" --prefix PATH : /path/to/bin
  '';
}