How to: Write a Run-Time Error Reporting Function


The new home for Visual Studio documentation is Visual Studio 2017 Documentation on

The latest version of this topic can be found at How to: Write a Run-Time Error Reporting Function.

A custom reporting function for run-time errors must have the same declaration as _CrtDbgReportW. It should return a value of 1 to the debugger.

The following example shows how to define a custom reporting function.

#include <stdio.h>  
int errorhandler = 0;  
void configureMyErrorFunc(int i)  
    errorhandler = i;  
int MyErrorFunc(int errorType, const wchar_t *filename,  
                int linenumber, const wchar_t *moduleName,  
                const wchar_t *format, ...)  
    switch (errorhandler)  
    case 0:  
    case 1:  
        wprintf(L"Error type %d at %s line %d in %s",  
                errorType, filename, linenumber, moduleName);  
    case 2:  
    case 3:  
        fprintf(stderr, "Error type");  
    return 1;  

The following example shows a more complex custom reporting function. In this example, the switch statement handles various error types, as defined by the reportType parameter of _CrtDbgReportW. Because you are replacing _CrtDbgReportW, you cannot use _CrtSetReportMode. Your function must handle the output. The first variable argument in this function takes a run-time error number. For more information, see _RTC_SetErrorType.

#include <windows.h>  
#include <stdarg.h>  
#include <rtcapi.h>  
#include <malloc.h>  
#pragma runtime_checks("", off)  
int Catch_RTC_Failure(int errType, const wchar_t *file, int line,   
                   const wchar_t *module, const wchar_t *format, ...)  
    // Prevent re-entrance.  
    static long running = 0;  
    while (InterlockedExchange(&running, 1))  
    // Now, disable all RTC failures.  
    int numErrors = _RTC_NumErrors();  
    int *errors=(int*)_alloca(numErrors);  
    for (int i = 0; i < numErrors; i++)  
        errors[i] = _RTC_SetErrorType((_RTC_ErrorNumber)i, _RTC_ERRTYPE_IGNORE);  
   // First, get the rtc error number from the var-arg list.  
    va_list vl;  
    va_start(vl, format);  
    _RTC_ErrorNumber rtc_errnum = va_arg(vl, _RTC_ErrorNumber);  
    wchar_t buf[512];  
    const char *err = _RTC_GetErrDesc(rtc_errnum);  
    swprintf_s(buf, 512, L"%S\nLine #%d\nFile:%s\nModule:%s",  
        file ? file : L"Unknown",  
        module ? module : L"Unknown");  
    int res = (MessageBox(NULL, buf, L"RTC Failed...", MB_YESNO) == IDYES) ? 1 : 0;  
    // Now, restore the RTC errortypes.  
    for(int i = 0; i < numErrors; i++)  
        _RTC_SetErrorType((_RTC_ErrorNumber)i, errors[i]);  
    running = 0;  
    return res;  
#pragma runtime_checks("", restore)  

Use _RTC_SetErrorFuncW to install your custom function in place of _CrtDbgReportW. For more information, see _RTC_SetErrorFuncW. The _RTC_SetErrorFuncW return value is the previous reporting function, which you can save and restore if necessary.

#include <rtcapi.h>  
int main()  
    _RTC_error_fnW oldfunction, newfunc;  
    oldfunction = _RTC_SetErrorFuncW(&MyErrorFunc);  
    // Run some code.  
    newfunc = _RTC_SetErrorFuncW(oldfunction);  
    // newfunc == &MyErrorFunc;  
    // Run some more code.  

Native Run-Time Checks Customization