Asynchronous Events Using Timers

Recently I was had an issue with Timer Call back function not working on Windows. I registered a simple call back function after 1 second, and it wasn't getting called, I was wondering how can it happen like that.

Then I did some study and found that, the windows will only send messages to your application that the timer is expired, and that message contains all the details such as the registered call back function etc, its the duty of the application to process these events and the call the windows API, DispatchMessage() which does the job of calling the call back function.

Here is the sample code, which works this way,

#include <windows.h>
#include <stdio.h>

int flag = 1;

void CALLBACK TimerProc(HWND, UINT, UINT, DWORD);

int main()
{
    UINT_PTR id;
    MSG msg;

    id = SetTimer(NULL, 
                  0, 
                  3000, 
                  (TIMERPROC) TimerProc);

    while(flag)
    {
        GetMessage(&msg, NULL, 0, 0);
        DispatchMessage(&msg);
    }

    KillTimer(NULL, id);

    return 0;
}

void CALLBACK TimerProc(HWND hwnd, 
                        UINT uMsg, 
                        UINT idEvent, 
                        DWORD dwTime)
{
    flag = 0;
    printf("I am in Timer Call Back Function \n");
}

So, if You are having the same problem as mine, please do follow this, and hope your problem is resolved, and do get back to me if you need any assistance.

The reason I think windows has been designed like this, is to make sure the call is made from the application context, hope that make sense, and I could not think of any other reason for doing it this way.

Inter-working with C/ C++ files

There are some times in the product development, especially in porting projects, and where we need to use the existing source codes, we may have to co-work with both C and C++ files, then we need to make them inter-work. Here I will show you how you can achieve that,

Accessing the C functions and data in C++ files.

supposed assume there is function func_c in a C file, my_file.c

void func_c(void)
{
    printf("I am a C function!");
}

to access this inside a C++ file, we need to declare it as shown below.

extern "C" void func_c(void);

The same methods can be followed for accessing all kind of data structures and variables.

Accessing C++ functions and Data Structures in C files.

Here things are other way round, just because C files does not take the key word extern "C", so all things need to be inside the C++ files, for C files its just another extern function or data.

So if I have a function func_cpp(void) as shown below, in my_file.cpp, it needs to be decalred as shown below

extern "C" void func_cpp(void)
{
    cout << "I am in C++ function";
}

To access this function inside the C file, just declare it as extern as you would normally do.

extern void func_cpp(void)

Hope it helps!