Qt

This section describes the differences between Nix expressions for Qt libraries and applications and Nix expressions for other C++ software. Some knowledge of the latter is assumed.

There are primarily two problems which the Qt infrastructure is designed to address: ensuring consistent versioning of all dependencies and finding dependencies at runtime.

Nix expression for a Qt package (default.nix)

<programlisting>
{ mkDerivation, lib, qtbase }: <co xml:id='qt-default-nix-co-1' />

mkDerivation { <co xml:id='qt-default-nix-co-2' />
  pname = "myapp";
  version = "1.0";

  buildInputs = [ qtbase ]; <co xml:id='qt-default-nix-co-3' />
}
</programlisting>

 <calloutlist>
  <callout arearefs='qt-default-nix-co-1'>
   <para>
    Import <literal>mkDerivation</literal> and Qt (such as <literal>qtbase</literal> modules directly. <emphasis>Do not</emphasis> import Qt package sets; the Qt versions of dependencies may not be coherent, causing build and runtime failures.
   </para>
  </callout>
  <callout arearefs='qt-default-nix-co-2'>
   <para>
    Use <literal>mkDerivation</literal> instead of <literal>stdenv.mkDerivation</literal>. <literal>mkDerivation</literal> is a wrapper around <literal>stdenv.mkDerivation</literal> which applies some Qt-specific settings. This deriver accepts the same arguments as <literal>stdenv.mkDerivation</literal>; refer to <xref linkend='chap-stdenv' /> for details.
   </para>
   <para>
    To use another deriver instead of <literal>stdenv.mkDerivation</literal>, use <literal>mkDerivationWith</literal>:
<programlisting>
mkDerivationWith myDeriver {
  # ...
}
</programlisting>
    If you cannot use <literal>mkDerivationWith</literal>, please refer to <xref linkend='qt-runtime-dependencies' />.
   </para>
  </callout>
  <callout arearefs='qt-default-nix-co-3'>
   <para>
    <literal>mkDerivation</literal> accepts the same arguments as <literal>stdenv.mkDerivation</literal>, such as <literal>buildInputs</literal>.
   </para>
  </callout>
 </calloutlist>

Locating runtime dependencies

Qt applications need to be wrapped to find runtime dependencies. If you cannot use mkDerivation or mkDerivationWith above, include wrapQtAppsHook in nativeBuildInputs:

stdenv.mkDerivation {
  # ...

  nativeBuildInputs = [ wrapQtAppsHook ];
}

Entries added to qtWrapperArgs are used to modify the wrappers created by wrapQtAppsHook. The entries are passed as arguments to wrapProgram executable makeWrapperArgs.

mkDerivation {
  # ...

  qtWrapperArgs = [ ''--prefix PATH : /path/to/bin'' ];
}

Set dontWrapQtApps to stop applications from being wrapped automatically. It is required to wrap applications manually with wrapQtApp, using the syntax of wrapProgram executable makeWrapperArgs:

mkDerivation {
  # ...

  dontWrapQtApps = true;
  preFixup = ''
      wrapQtApp "$out/bin/myapp" --prefix PATH : /path/to/bin
  '';
}

Note: wrapQtAppsHook ignores files that are non-ELF executables. This means that scripts won't be automatically wrapped so you'll need to manually wrap them as previously mentioned. An example of when you'd always need to do this is with Python applications that use PyQT.

Libraries are built with every available version of Qt. Use the meta.broken attribute to disable the package for unsupported Qt versions:

mkDerivation {
  # ...

  # Disable this library with Qt &lt; 5.9.0
  meta.broken = builtins.compareVersions qtbase.version "5.9.0" &lt; 0;
}

Adding a library to Nixpkgs

Qt libraries are added to qt5-packages.nix and are made available for every Qt version supported.

Example adding a Qt library

The following represents the contents of qt5-packages.nix.

{
  # ...

  mylib = callPackage ../path/to/mylib {};

  # ...
}

Adding an application to Nixpkgs

Applications that use Qt are also added to qt5-packages.nix. An alias is added in the top-level all-packages.nix pointing to the package with the desired Qt5 version.

Example adding a Qt application