I couldn't think a good title for this.Simple question

[ZekDe]

Hello friends,
I am not able to understand this,could you help me.I am using Qt creature

//This is in .h
 class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    void S7API CliCompletion(void *usrPtr, int opCode, int opResult);
    ~MainWindow();
.
.
.

//When I say right click and "add definition in mainwindow" I have got this in .cpp

void MainWindow::CliCompletion(void *usrPtr, int opCode, int opResult)
{
 
}
// Now, My problem what is S7API  here,I have not encountered like this in C language.What does it do here.what is it.Where do I have to look in Dietel book. 

[mbozzi]

It's certainly a name from some third-party library. You will not find it used in any textbook.

A google search suggests that S7API is a macro for ABI compatibility from libsnap7, which appears to be some sort of Ethernet communications library.

From this file:
https://github.com/SCADACS/snap7/blob/b0260d8eb8fcebd955a769e6043a301b6c1e63a6/examples/cpp/snap7.h

It is either __stdcall (which is a MSVC-specific extension specifying the calling convention) or nothing, depending on whether or not the macro OS_WINDOWS is defined.

[ZekDe]

Yes I know this is related with third-party library, as actual that I didn't understand,why was it used .What is the aim?Because this function does not return a value.When is this use?

[mbozzi]

What is the aim [of using the macro S7ABI]?

To ensure ABI compatibility between the caller and callee.

It tells the compiler how to generate code to call this function in the way that the called function expects.

[ZekDe]

I am new communication engineer ,this is why I don't know perfectly C++.Could you explain more ,if it is possible with little example