QMake -> MSVC project: is there a way to disable the /FC flag?

Violet Giraffe

I'm having the same problem that CMake users had until it was fixed, described and explained here: https://gitlab.kitware.com/cmake/cmake/-/issues/18261

In short, the /FC flag (use full paths for __FILE__ and similar macros) is the default behavior in MSBuild, so if /FC is NOT set, a line <UseFullPaths>false</UseFullPaths MUST be emitted in the generated project. I assume that because of this inverse logic there is no qmake-native solution (e. g. tricks like QMAKE_CXXFLAGS = $$replace(QMAKE_CXXFLAGS, "/FC ", "") won't work). But there is a potential workaround:

"Create a ProjectName.vcxproj.user file that turns explicitly sets <UseFullPaths> to false. Because MSBuild merges .user project files with project files of the same name, this lets CMake users disable the setting anyway."

Can this be done in qmake? More specifically, can this be done once in a .pri file that's included into every .pro for a SUBDIRS project with many libraries?

SGaist

Hi,

Wouldn't:

QMAKE_CXXFLAGS -= /FC

Do the job ?

Violet Giraffe

@SGaist, no, because qmake is not trying to add this flag - it's not present in QMAKE_CXXFLAGS . But it also doesn't include the following line into the resulting vcxproj, and by default it's true in MSBuild:

<ClCompile>
      <UseFullPaths>false</UseFullPaths>
</ClCompile>

So, /FC is on by default and there seems to be no way to turn it off with qmake.

Since posting the question I have implemented the suggested workaround (creating a .vcxproj.user file), it can be easily done in qmake, but I still can't get it to work - it would appear that MSBuild ignores any <ClCompile> options in a .user file.

SGaist

Based on the flag documentation I am wondering whether you have also ZI defined which implies FC.

Violet Giraffe

@SGaist, I didn't know about this connection, thanks! No, I'm not specifying /Zi in the .pro file, but it is added to the generated project nonetheless. Not sure where it's coming from; QMAKE_CXXFLAGS -= /Zi doesn't affect it. I've also printed QMAKE_CXXFLAGS to make sure it's not there. It must be hard-coded in the vcxproj generator, or in some template file that's part of qmake?

Violet Giraffe

I have somehow managed to remove /Zi from the generated project, but /FC is still effectively set by virtue of UseFullPaths not being specified in the vcxproj.

Violet Giraffe

Not related to the initial question, but here are more examples of thing you can't get right with qmake in a vcxproj:

QMAKE_LFLAGS += /LTCG:INCREMENTAL results in /LTCG being set (the non-incremental option).
QMAKE_LFLAGS += /DEBUG:FULL results in /DEBUG being set (without :FULL). It's probably the same thing, but still annoying.

What's the status of qmake, is it end of life? I guess these problems won't be fixed if I report them?

SGaist

Did you try to quote these expressions ?

As for qmake, it shall be supported for the lifetime of Qt 6 so I would say it's worth opening tickets since there are still quite a lot of projects using it.

Violet Giraffe

@SGaist, like so? This, unfortunately, changed nothing. And note that things like /OPT:REF and /DEBUG:FASTLINK do work correctly, while /DEBUG:FULL does not.

Release:QMAKE_LFLAGS += "/DEBUG:FULL" /OPT:REF /OPT:ICF "/LTCG:INCREMENTAL"

In case with DEBUG it's just a nag, but LTCG is a functionality issue of not being able to get the right behavior with qmake.
P. S. Thanks for your help and advice!

SGaist

Then it is likely an issue with the backend.

Don't forget to post a link to your ticket(s) !

Violet Giraffe

Posted an issue: https://bugreports.qt.io/browse/QTBUG-104450