GetLastError

The GetLastError function returns the calling thread's last-error code value. The last-error code is maintained on a per-thread basis. Multiple threads do not overwrite each other's last-error code.

DWORD GetLastError(VOID)

Parameters

This function has no parameters.

Return Values

The return value is the calling thread's last-error code value. Functions set this value by calling the SetLastError function. The Return Value section of each reference page notes the conditions under which the function sets the last-error code.

Remarks

You should call the GetLastError function immediately when a function's return value indicates that such a call will return useful data. That is because some functions call SetLastError(0) when they succeed, wiping out the error code set by the most recently failed function.

Most functions in the Win32 API that set the thread's last error code value set it when they fail; a few functions set it when they succeed. Function failure is typically indicated by a return value error code such as FALSE, NULL, 0xFFFFFFFF, or 1. Some functions call SetLastError under conditions of success; those cases are noted in each function's reference page.

Error codes are 32-bit values (bit 31 is the most significant bit). Bit 29 is reserved for application-defined error codes; no system error code has this bit set. If you are defining an error code for your application, set this bit to one. That indicates that the error code has been defined by an application, and ensures that your error code does not conflict with any error codes defined by the operating system.

To obtain an error string for operating system error codes, use the FormatMessage function. For a complete list of error codes, see the WINNT.H header file in the Win32 SDK.

See Also

FormatMessage, SetLastError, SetLastErrorEx

Comment from Greatis Sofware

GetLastError function is unique. There are no other WinAPI functions that are mentioned as often as GetLastError. You can always find phrase "If the function fails, the return value is blah-blah-blah. To get extended error information, call GetlastError" in every topic of win32.hlp file, where subject is the function, which can return an error. Function GetLastError returns integer code, that is not very useful, if we are talkin about error message to be displayed to user, because it's just error number in DWORD. Of course, error codes can be found in WINNT.H, but end user of our application will hardly do this. It would be great to show some clear text message, which he could send to us with angry reports about program instability... It turned out, that is quite possible. We just need to take FormatMessage, and use it with our error code to get text message in user's language on it. Very friendly, isnt't it? By the way there's C example in FormatMessage topic. And Delphi example is here:

function GetLastErrorText: string;
var
  C: array[Byte] of Char;
begin
  FormatMessage(
    FORMAT_MESSAGE_FROM_SYSTEM,
    nil,
    GetLastError,
    LOCALE_USER_DEFAULT,
    C,
    SizeOf(C),
    nil);
  Result:=C;
end;

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