QT Creator：无法在QT Creator c项目中包含来自其他文件的功能

Question

I have never tried QT Creator before and thought I'd give it a spin for some test code I am using.

Long story short, I was unable to access functions defined in my other .c files. So I made a very simple, stupid program in QT Creator to try and see if the problem persisted, and it does.

The program is a single window with a button. However, my error is probably totally irrelevant to the button.

The error I am getting is (I know, this is a common one that is usually very simple): main.o: In function main': /home/user/Documents/dev/st2/st2/main.cpp:9: undefined reference to doubler(int)'

Basically I have a C file, myfuncts.c that looks like this:

#include "myfuncts.h"

int doubler(int a){
return a*2;
}

And the header file myfuncts.h:

#ifndef MYFUNCTS_H
#define MYFUNCTS_H

int doubler(int a);


#endif // MYFUNCTS_H

(so far I am keeping it as simple as I can until I get it working).

Now in main.cpp I have referenced this function only once, and this is indeed where the error comes up while I am compiling. Here is main.cpp :

#include <QtGui/QApplication>
#include "widget.h"

#include "myfuncts.h"

int main(int argc, char *argv[])
{
    int num = 42;
    num = doubler(num);
    QApplication a(argc, argv);
    Widget w;
    w.show();
    return a.exec();
}

When I try and build, here is my error:

make: Entering directory /home/user/Documents/dev/st2/st2' g++ -o st2 main.o widget.o myfuncts.o moc_widget.o -L/usr/lib -lQtGui -lQtCore -lpthread main.o: In function main': /home/user/Documents/dev/st2/st2/main.cpp:9: undefined reference to doubler(int)' collect2: ld returned 1 exit status make: *** [st2] Error 1 make: Leaving directory /home/user/Documents/dev/st2/st2' Exited with code 2. Error while building project st2 When executing build step 'Make'

Here is the project file, st2.pro:

# -------------------------------------------------
# Project created by QtCreator 2013-11-22T17:06:59
# -------------------------------------------------
TARGET = st2
TEMPLATE = app
SOURCES += main.cpp \
    widget.cpp \
    myfuncts.c
HEADERS += widget.h \
    myfuncts.h
FORMS += widget.ui

I am trying to keep with convention here and have allowed QT Creator to build the project file for me. I have also used the "Add new..." menu to add the myfunct.h and .c files, both of which exist in the same directory.

Dropping into the terminal and trying to make manually results in the same error, I am posting it so that the steps prior to the error can be seen:

user@singularity:st2$ make /usr/bin/uic-qt4 widget.ui -o ui_widget.h g++ -c -pipe -g -Wall -W -D_REENTRANT -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++ -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4 -I. -I. -o main.o main.cpp g++ -c -pipe -g -Wall -W -D_REENTRANT -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++ -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4 -I. -I. -o widget.o widget.cpp gcc -c -pipe -g -Wall -W -D_REENTRANT -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++ -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4 -I. -I. -o myfuncts.o myfuncts.c /usr/bin/moc-qt4 -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++ -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4 -I. -I. widget.h -o moc_widget.cpp g++ -c -pipe -g -Wall -W -D_REENTRANT -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++ -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4 -I. -I. -o moc_widget.o moc_widget.cpp g++ -o st2 main.o widget.o myfuncts.o moc_widget.o -L/usr/lib -lQtGui -lQtCore -lpthread main.o: In function main': /home/user/Documents/dev/st2/st2/main.cpp:9: undefined reference to doubler(int)' collect2: ld returned 1 exit status make: *** [st2] Error 1

And nm does show the symbols exist in myfunct.o:

user@singularity:st2$ nm myfuncts.o 0000000000000000 T doubler user@singularity:st2$

I have coded huge projects in the past with multiple libraries and c files without trouble, and I don't understand what is happening in QT Creator that is so different. It seems like everything is in the right place and accounted for, but, this is my first time with QT so there's got to be something!

Any help would be greatly appreciated! Platform is Debian Linux 6.0.2 on AMD64.

Answer 1

Add to your .pro file the following:

INCLUDEPATH += dir/where/is/myfuncts.h

EDIT: And try adding extern "C":

#ifndef MYFUNCTS_H
#define MYFUNCTS_H

/* Extern "C" */
#ifdef __cplusplus
extern "C" {
#endif


int doubler(int a);


/* Extern "C" */
#ifdef __cplusplus
}
#endif


#endif // MYFUNCTS_H

Answer 2

This is a problem about "name mangled". If you want to call a C function in the C++, you shall declare the C function with extern "C" in the C++ file.

/* file: main.cpp */
...
extern "C" int doubler(int a);
...

Usually, we modify the C header file so it's easier to #include in C++ code. So, you can do it like this,

/ * file: myfuncts.h */
#ifndef MYFUNCTS_H
#define MYFUNCTS_H

#ifdef __cplusplus
extern "C" {
#endif

int doubler(int a);

#ifdef __cplusplus
}
#endif

#endif // MYFUNCTS_H

Answer 3

I had a very similar problem: I solved it by putting the extern "C" around the include in the .cpp file, since the other methods posted here didn't seem to work.

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QTimer>

extern "C" {
#include "file.h"
#include "other_file.h"
}