How to call self made C function in Qt?
Hi, I need to use C to make a function and then be called in Qt. And I tried to compile the C funtion to be a static or dynamic lib first, then call it in Qt. But I failed.
Can anyone help to analyse the reason why it always tell me ‘ ‘sum2’ was not declared in this scope’. If I change the source code to call sleep(1), it can succeed. But why my static lib fails, although I set the path to the g++ correctly already?
Thanks!
To generate the static lib:
- gcc -c sum.c
- ar -rc libsum.a sum.o
The make under my scratchbox output is below: As you can see, I put the static lib path there ‘-I. libsum.a’
> make
- g++ -c -pipe -O3 -g -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/targets/FREMANTLE_ARMEL/usr/share/qt4/mkspecs/linux-g++-gles2 -I. -I/targets/FREMANTLE_ARMEL/usr/include/qt4/QtCore -I/targets/FREMANTLE_ARMEL/usr/include/qt4/QtGui -I/targets/FREMANTLE_ARMEL/usr/include/qt4 -I. -I. -I. libsum.a -o mainwindow.o mainwindow.cpp
- mainwindow.cpp: In member function 'int MainWindow::setValue(int)':
- mainwindow.cpp:24: error: 'sum2' was not declared in this scope
- mainwindow.cpp: At global scope:
- mainwindow.cpp:20: warning: unused parameter 'duty_value'
- make: *** [mainwindow.o] Error 1
The C function for test is below. sum.c and sum.h
> cat sum.c
- sum2(int a, int b)
- {return a+b;}
> cat sum.h
- int sum2(int a, int b);
The Qt cpp and header files are below.
[sbox-FREMANTLE_ARMEL: ~/LED_Driving] > cat mainwindow.cpp
- #include "mainwindow.h"
- #include "ui_mainwindow.h"
- #include <QtCore>
- ui(new Ui::MainWindow)
- {
- ui->setupUi(this);
- connect(timer, SIGNAL(timeout()), this, SLOT(timerUpDate()));
- timer->start(20); //20ms one time
- }
- void MainWindow::timerUpDate()
- {
- connect(ui->horizontalSlider, SIGNAL(valueChanged(int)),
- this, SLOT(setValue(int)));
- }
- int MainWindow::setValue(int duty_value)
- {
- // gpio(duty_value);
- // sleep(1);
- return sum2(8,7);
- }
- MainWindow::~MainWindow()
- {
- delete ui;
- }
> cat mainwindow.h
- #ifndef MAINWINDOW_H
- #define MAINWINDOW_H
- #include <QMainWindow>
- namespace Ui {
- class MainWindow;
- }
- {
- Q_OBJECT
- public:
- ~MainWindow();
- public slots:
- int setValue(int);
- private:
- Ui::MainWindow *ui;
- void timerUpDate();
- };
- #endif // MAINWINDOW_H
8 replies
Are you including your sum.h header file so that your C++ code knows about your function?
Also, take a look here [parashift.com] for some more important information you will need.
Yes, I tried to include sum.h in mainwindow.h but still failed and got below output of ‘make’.
> make
- > make
- /usr/bin/uic mainwindow.ui -o ui_mainwindow.h
- g++ -c -pipe -O3 -g -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/targets/FREMANTLE_ARMEL/usr/share/qt4/mkspecs/linux-g++-gles2 -I. -I/targets/FREMANTLE_ARMEL/usr/include/qt4/QtCore -I/targets/FREMANTLE_ARMEL/usr/include/qt4/QtGui -I/targets/FREMANTLE_ARMEL/usr/include/qt4 -I. -I. -o main.o main.cpp
- g++ -c -pipe -O3 -g -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/targets/FREMANTLE_ARMEL/usr/share/qt4/mkspecs/linux-g++-gles2 -I. -I/targets/FREMANTLE_ARMEL/usr/include/qt4/QtCore -I/targets/FREMANTLE_ARMEL/usr/include/qt4/QtGui -I/targets/FREMANTLE_ARMEL/usr/include/qt4 -I. -I. -I. -L. libsum.a -o mainwindow.o mainwindow.cpp
- mainwindow.cpp: In member function 'int MainWindow::setValue(int)':
- mainwindow.cpp:26: error: 'sum2' was not declared in this scope
- mainwindow.cpp: At global scope:
- mainwindow.cpp:22: warning: unused parameter 'duty_value'
- make: *** [mainwindow.o] Error 1
And let’s see another trial. With below program and the libsum.a, it can run normally without the header (sum.h) included.
> cat testsum.c
- #include <stdio.h>
- int main()
- { printf("%d\n", sum2(8, 7));
- return 0;
- }
then compile
- > gcc testsum.c -I. libsum.a -o testsum
then run
- > ./testsum
- 15
So why this testsum.c C code can call the lib while the Qt code cannot call it?
The C function for test is below. sum.c and sum.h> cat sum.c
> cat sum.h
If this is all you have implemented in your files, it might be a little too minimalistic.
One should at least include the header file in the source file. So, this looks a little better:
- #include "sum.h"
- int sum2(int a, int b)
- {
- return a+b;
- }
Note, that I have declared also the return type. You should have at least seen a number of warnings through compilation.
No, obviously you have not included sum.h in your code where you are using function sum2.
You may add the header in mainwindow.cpp
- #include "mainwindow.h"
- #include "ui_mainwindow.h"
- #include <QtCore>
- #include "sum.h"
- ui(new Ui::MainWindow)
- {
- ui->setupUi(this);
- connect(timer, SIGNAL(timeout()), this, SLOT(timerUpDate()));
- timer->start(20); //20ms one time
- }
- void MainWindow::timerUpDate()
- {
- connect(ui->horizontalSlider, SIGNAL(valueChanged(int)),
- this, SLOT(setValue(int)));
- }
- int MainWindow::setValue(int duty_value)
- {
- // gpio(duty_value);
- // sleep(1);
- return sum2(8,7);
- }
- MainWindow::~MainWindow()
- {
- delete ui;
- }
I assume your code may be compilable now. However, I have not tried it.
The things, you are fighting with, are C/C++ basics. It would be probably more helpful for you to work through a C/C++ tutorial in more detail before you are digging into Qt.
Thank you for your reply!
Now the make output is below
- ...
- mainwindow.o: In function `MainWindow::setValue(int)':
- /home/maemo/LED_Driving/mainwindow.cpp:33: undefined reference to `sum2(int, int)'
- collect2: ld returned 1 exit status
- make: *** [LED_Driving] Error 1
In fact I know those points you mentioned, and I tried before this post. But the test result shows it is not as easy as it looks. I am still wondering how to solve it…
I think you should study the basics of C programming, namely how the compilation and linking processes work…
That undefined reference error means that the linker did not see a definition for that function (symbol). Are you sure you’re linking against the object files that contain the implementation of sum2?
Also, take a look here [parashift.com] for some more important information you will need.
Have you read and followed what’s written there? You usually cannot just link C and C++ code without any further preparations.
You need to use extern “C” when including C headers to C++. Usually you write your C headers (which you think might be used in C++ projects) like this:
- #ifdef __cplusplus
- extern "C" {
- #endif
- int mySum(int a, int b);
- ...
- #ifdef __cplusplus
- }
- #endif
In general, mixing C and C++ is very easy. Just remember (1) using extern “C”, (2) compiling C files with C (not C++) compilers. If you use C++ compiler, and want it to produce “C-style” libraries (which can be used by many different languages, e.g. C++ and Python), use extern “C” to use C-style naming convention. You find more information about this topic easily from Internet.
You must log in to post a reply. Not a member yet? Register here!
