如果在Qt Creator中进行编译，库函数的行为将有所不同

Question

I have a c program, that if I build it in the shell with this command:

gcc -o simpledemo -fpic -fsigned-char -DPLATFORM_LINUX -Iinclude/ simpledemo.c ../AcapelaLibs/libbabile.a -lstdc++

it compiles, runs, and produces the expected output.

gcc info:

/usr/bin/gcc
gcc (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4

However, if I compile it with Qt Creator (Qt 5.5.1), it compiles and runs but a NULL value is returned from a function of the library I use.

Qt uses the following g++ :

/usr/bin/g++
g++ (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4

I think it might be something with the compiler flags. This is what I'm doing in Qt to match the shell command:

in .pro file:

QMAKE_CXXFLAGS  += -DPLATFORM_LINUX
INCLUDEPATH     += $$PWD/include/

I think the INCLUDEPATH is OK because I can access files there from within my code (they are recognizable).

And note that in the shell compilation I use a library ../AcapelaLibs/libbabile.a , so in Qt I click Add Library... and choose that same library, and Qt adds it to the .pro file. Also here I can access library functions from within my code.

The problem is that a function from libbabile.a returns NULL if I compile and run from Qt (or compile in Qt and run from shell).

What's the difference between the 2 methods that makes one of them succeed and the other fail?

Answer 1

Library function behaves differently if compiled in Qt Creator. I think it might be something with the compiler flags.

What compiler flags are different from used in command line?

You can examine Qt Creator compiler output and get all the compiler options specified from there. Then you can compare those with expected set of options and deliberately add or remove those options in your project .pro file:

# C++ flags
QMAKE_CXXFLAGS += -opt1 -opt2 # add
QMAKE_CXXFLAGS -= -opt1 -opt2 # remove

QMAKE_CXXFLAGS_RELEASE += -opt1 -opt2 # add
QMAKE_CXXFLAGS_RELEASE -= -opt1 -opt2 # remove

QMAKE_CXXFLAGS_DEBUG += -opt1 -opt2 # add
QMAKE_CXXFLAGS_DEBUG -= -opt1 -opt2 # remove

# C flags, slightly different macro
QMAKE_CFLAGS += -opt1 -opt2 # mind add/remove/debug/release

For both debug and release modes and separately.