Win32 Programming Lesson 5: Error Codes

Before We Begin  Much of the time in this class we’ll be calling Win32 Functions  However, sometimes they’re not going to work as you expect  How do you deal with the unexpected?

First, a question  What’s the difference between these lines, and how is that difference realized: void main() { node *foo; foo = new node(); node bar; … }

Return Codes  Windows functions (and C/C++ calls in general) can only directly return one value… why?

Common Win32 Returns  VOID: This function cannot possibly fail (Ha!)  BOOL: Success is 0. Non-zero is a failure. Check for 0 or not 0… don’t check for TRUE  HANDLE: NULL (failure) or an OBJECT which can be manipulated  PVOID: NULL (failure) or a Pointer to an area of memory  LONG/DWORD: 0 or -1 for failure – sometimes. Varies from function to function

Tracking Errors  The actual error code is stored in thread local storage Remember that term, we’ll be looking at it a lot later For now, that means every thread has its own error codes  MSDN: DWORD GetLastErrorGetLastError

The Actual Values  Stored in WinError.h // // MessageId: ERROR_INVALID_FUNCTION // // MessageText: // // Incorrect function. // #define ERROR_INVALID_FUNCTION 1L // dderror // // MessageId: ERROR_FILE_NOT_FOUND // // MessageText: // // The system cannot find the file specified. // #define ERROR_FILE_NOT_FOUND 2L

Not Very Helpful…  People don’t do well with numbers  Need to convert into a Text String  Fortunately, there is a way!

FormatMessage  DWORD FormatMessage( DWORD dwFlags, LPCVOID pSource, DWORD dwMessageId, DWORD dwLanguageId, PTSTR pszBuffer, DWORD nSize, va_list *Arguments);FormatMessage

MSDN  It’s an invaluable resource but you have to learn how to use it  I’ll do this one for you… some of the rest you will have to do on your own

Parm by Parm  dwFlags: (in) Something we supply. FORMAT_MESSAGE_FROM_SYSTEM gives us the result from GetLastError  lpSource: (in) Where the definitions are – for us, NULL.  dwMessageId: (in) The message we want to translate

Parm by Parm (ctd)  dwLanguageID: (in) What language to format the message in  lpBuffer: (out) The buffer for the message Can be auto-alloced  nSize: (in) Either: the size of the output, or the minimum number of TCHARS the message will contain  Arguments: (in) Pointer to an array of values to insert into the formatted message

FormatString in Action  BOOL fOk = FormatMessage( FORMAT_MESSAGE_FROM_SYSTEM | FORM AT_MESSAGE_ALLOCATE_BUFFER, NULL, dwError, MAKELANGID(LANG_ENGLISH, SUBLANG_E NGLISH_US), (LPTSTR) &hlocal, 0, //dwSize NULL);

A GUI Version  Turns out to be very tricky, due to one parameter which is hard to marshal: va_list  Why is this hard? .NET implements its own object to get around this: Win32ExceptionWin32Exception Note here how we’re getting into some of the “gnarliness” of Win32