why getting first chace exception in c++

Question

I made a sample to check what happen when Ctrl+C is pressed in windows console application:

bool    TerminationFlag=true;

int main()
{
    g_hTerminateEvent = ::CreateEvent(NULL, FALSE, FALSE, NULL);
    ::SetConsoleCtrlHandler(ConsoleCtrlHandler, TRUE);
    while(1)
    {
        if(TerminationFlag == false)
        {
            break;
        }
    }
    return 0;
}

BOOL WINAPI ConsoleCtrlHandler(DWORD dwCtrlType)
{

    if (dwCtrlType == CTRL_C_EVENT ||
        dwCtrlType == CTRL_BREAK_EVENT ||
        dwCtrlType == CTRL_CLOSE_EVENT)
    {
        TerminationFlag=false;
        ::SetEvent(g_hTerminateEvent);
        return TRUE;
    }
    return FALSE;
}

I tested the code by running it using start debugging option in visual studio when I press ctrl+c I get the following message

First-chance exception at 0x7c87647d

when I press on continue option my code comes to the line TerminationFlag=false; even though I have handled Ctrl+C in control handler. Can you please tell me whats the problem?

Answer 1

I assume you're using Microsoft Visual Studio from your description of the problem. The first chance exception being raised is the CTRL-C event which is trapped by the debugging environment. This is expected behaviour.

You can choose to ignore this: go to Debug menu/Exceptions/Win32 Exceptions and take out the CONTROL-C check from the "Thrown" column menu. This will ensure that the debugger only breaks on CONTROL-C when it is user-unhandled. See picture below:

Incidentally, you should be waiting for the termination event not polling for a flag. You may want something like:

#include "windows.h"
#include <iostream>
HANDLE g_hTerminateEvent;

BOOL WINAPI ConsoleCtrlHandler(DWORD dwCtrlType)
{

    if (dwCtrlType == CTRL_C_EVENT ||
        dwCtrlType == CTRL_BREAK_EVENT ||
        dwCtrlType == CTRL_CLOSE_EVENT)
    {
        std::cout << "Terminating" << std::endl;
        SetEvent(g_hTerminateEvent);

        return TRUE;
    }
    return FALSE;
}
int main()
{
    g_hTerminateEvent = ::CreateEvent(NULL, FALSE, FALSE, NULL);
    SetConsoleCtrlHandler(ConsoleCtrlHandler, TRUE);

    DWORD result = WaitForSingleObject(g_hTerminateEvent, INFINITE);
    return 0;
}