LIBS vs PRE_TARGETDEPS in .pro files of Qt

Question

I am a newbie with in Qt & started appreciating the framework that qmake provides in .pro files. Primary objective of my question is to understand in detail the difference between qmake variables "LIBS" & "PRE_TARGETDEPS" with static linking of libraries.

My Qt App uses a bunch of C++ static libraries that it depends on. Again, the static libraries have interdependencies between themselves. Each library has a .pro file included in it to support qmake way of building. And of course the app also has a .pro file.

Now in the static libraries, if libStaticA is dependent on libStaticB where both are C++ libraries. And both of them have a .pro file each. Is it enough to mention the dependency in libStaticA.pro with +LIBS & -l like below ? +LIBS += -L/path_To_libStaticB/ -llibStaticB

Or is it enough to mention the dependency with PRE_TARGETDEPS like below +PRE_TARGETDEPS += /path_To_libStaticB/libStaticB.a

Or should I mention both ? +PRE_TARGETDEPS += /path_To_libStaticB/libStaticB.a +LIBS += /path_To_libStaticB/libStaticB.a

What is the relevance of LIBS & PRE_TARGETDEPS ?

PS: My development machine is osx. Thanks in advance for any explanations to clarify my understanding here

Answer 1

LIBS：

  

指定要链接到项目的库列表。如果使用Unix -l（库）和-L（库路径）标志，qmake会在Windows上正确处理库（即，将库的完整路径传递给链接器）。 qmake必须存在库才能找到-l lib所在的目录。

PRE_TARGETDEPS：

  

列出目标所依赖的库。某些后端（如Visual Studio和Xcode项目文件的生成器）不支持此变量。通常，这些构建工具在内部支持此变量，它对于显式列出依赖的静态库非常有用。

Qt使用PRE_TARGETDEPS变量来存储静态链接库的依赖项。每次构建应用程序时，它都会强制您的库重新链接。
如果您没有指定此变量并且更新并重建库，则您的程序仍将使用旧库。

对于您的问题，如果您使用静态库，您应该（几乎）始终同时使用LIB和PRE_TARGETDEPS。

引用：Qmake variable reference
同样有趣：Adding libraries to Qt Projects