What would be a pratical example of sysroot and prefix options for Qt

Question

I'm looking at all the options that can be run for the configure script provided with Qt. (specifically qt-everywhere-opensource-src-5.2.0).

After considerable searching, I've determined this stuff is poorly documented at best so I was hoping I could get some help. When I look at the descriptions for prefix and sysroot configuration options:

~/qt-everywhere-opensource-src-5.2.0$ ./configure -help | grep "sysroot"
-extprefix <dir> ... When -sysroot is used, install everything to <dir>,
-sysroot <dir> ...... Sets <dir> as the target compiler's and qmake's sysroot and also sets pkg-config paths.
-no-gcc-sysroot ..... When using -sysroot, it disables the passing of --sysroot to the compiler

~/qt-everywhere-opensource-src-5.2.0$ ./configure -help | grep "prefix"
-prefix <dir> ...... This will install everything relative to <dir>
-extprefix <dir> ... When -sysroot is used, install everything to <dir>,
-hostprefix [dir] .. Tools and libraries needed when developing

So I've used -prefix before, and it did exactly as described. It placed everything at the provided <dir>, then when I built my application using <prefix_dir>/bin/qmake and installed that on my target platform it wanted to find all the shared object libraries at <prefix_dir>/lib.

I'm under the impressions that if I use -sysroot it will install everything at <sysroot_dir> then when I install my application on the target platform it will search in /lib. At least I hope that's true.

Now if my assumption is correct... then what's the point of -extprefix? Are they saying that if I can redirect where things good if I use both -sysroot and -extprefix?

And what would be a reason why I would want to use -no-gcc-sysroot? If I wanted my Qt libs to be installed at "sysroot" why wouldn't I want gcc to use/know the same sysroot?

An explanation of some of these would be great, even better if I can get some practical examples of how to correctly use these options.

Answer 1

These are options that are used when building embedded platforms. Yes they are a royal mess. So here's only a partial answer:

-prefix

• tried and proven way to say /usr/lib instead of /usr/local/lib or similar for the entire installation of Qt
• when Qt is built for the platform it is currently running on (typical for desktop)

-sysroot /path

• intends to build Qt for a system which isn't installed at /
• for example -sysroot ~/mysystem where ~/myssytem contains /lib /bin etc...
• will pass --sysroot to other tools, like gcc and pkg-config, so they will search for their dependencies in ~/mysystem/lib rather than /lib

-extprefix /b

• when using -sysroot /a, don't actually write to /a
• write qt to /b instead
• this is intended for cross-compiling against read-only sysroots

-no-gcc-sysroot

• very specific hack for compilers that can't find their own crt inside --sysroot
• passes sysroot to pkgconfig and others, but not to gcc
• so that gcc will be called with -L/sysroot/lib/ correctly, but doesn't try to find implicit paths (crt) here.

-hostprefix /path

• when compiling for a different target than we are currently running on
• qmake will be the host architecture (for example x86) and qt itself will be the target architecture (lets say arm)
• so put qmake in /path instead of the target system specified by -sysroot. it won't be any useful on the target system

To add to the confusion:

-R /path

• sets the linkers runpath - which is where QtGui finds QtCore for example - independant of all the other options

Which flags you want to use when compiling for a target not your host, depends on a boatload of hardcoded assumptions inside configure.

generally -sysroot plus -prefix should work for most use cases.

i.e. when you have:

 $ ls ~/mytarget
 lib bin share dev

you could just use -sysroot ~/mytarget -prefix /