Last-Error Code

When an error occurs, most functions in the Win32 API return an error code, usually zero, NULL, or –1. Many functions also set an internal error code called the last-error code. When a function succeeds, the last-error code is not reset. The error code is maintained separately for each running thread; an error in one thread does not overwrite the last-error code in another thread. An application can retrieve the last-error code by using the GetLastError function; the error code may tell more about what actually occurred to make the function fail.

The SetLastError function sets the error code for the current thread. The SetLastErrorEx function also allows the caller to set an error type indicating the severity of the error. These functions are intended primarily for dynamic-link libraries (DLL), so they can emulate the behavior of the Win32 API.

The system defines a set of error codes that can be set as last-error codes or be returned by these functions. 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 define error codes for your application, set this bit to indicate that the error code has been defined by an application and to ensure that the error codes do not conflict with any system-defined error codes. For more information, see Error Codes.