CXXFLAGS modification of Qt pro file?

Question

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Configuring the GCC compiler switches in Qt, QtCreator, and QMake

I would like to use -O1 instead of -O2 in my makefile (CFLAGS and CXXFLAGS) for my Linux build. My understanding of how these makefiles are generated based on the .pro file is somewhat lacking. This is because the version of Qt combined with the version of G++ I am using has instabilities when -O2 is present.

Presently, I am running a replacement script, after I run qmake, which does this:

sed -i 's/\-O2/\-O1/g' AllProjects/Makefile.Release

This is a ghetto solution. A much better solution would be to modify the .pro file somehow to pass along these directives. I am not sure how CFLAGS and CXXFLAGS are being generated though.

I have tried passing a

linux-g++-{ 
      CFLAGS += -O1
      CXXFLAGS += -O1
      CONFIG += -O1
}

which did not work.

linux-g++-{ QMAKE_CXXFLAGS += -O1 } doesn't work, linux-g++-{ QMAKE_CXXFLAGS_RELEASE += -O1 } also doesn't work, I should have mentioned that I tried this in my solution. — Brian Stinar, Apr 29 '11 at 20:52
I'm thinking my check is a bit too tight too, since it seems like QMAKE_CXXFLAGS SHOULD work. — Brian Stinar, Apr 29 '11 at 20:53

score 29 · Accepted Answer · edited May 19 '13 at 15:45

29

You were very close. What you want is:

QMAKE_CXXFLAGS += -O1

If you would like to apply flags to just the release build, then you can use this:

QMAKE_CXXFLAGS_RELEASE += -O1

You also probably want to change your condition to be a little more flexible. In summary, something like this:

*-g++* {
    QMAKE_CXXFLAGS += -O1
}

More in the documentation here: http://qt-project.org/doc/qt-5.0/qtdoc/qmake-variable-reference.html#qmake-cxxflags

edited May 19 '13 at 15:45

Nobody moving away from SE

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answered Apr 29 '11 at 20:38

Evan Teran

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Thanks, my flag was ALSO too tight, exactly like you suggested. – Brian Stinar Apr 29 '11 at 20:55
1

@Brian: yea, you had a trailing dash which killed it. – Evan Teran Apr 29 '11 at 21:06
1

If the flag you are setting also relates to linking, you will also need to set eg. QMAKE_LFLAGS += -m32 – iforce2d Sep 30 '12 at 16:37
1

This doesn't work for me. You solution changed `-O2` to `-O1 -O2`, which is hardly helpful. – Joseph Quinsey Oct 31 '12 at 04:51
qmake docs moved to https://doc.qt.io/qt-6/qmake-variable-reference.html – milahu Jun 12 '22 at 17:54

CXXFLAGS modification of Qt pro file?

1 Answers1