GetLastError function

The GetLastError function retrieves the calling thread's last-error code value. This function is often used to obtain extended error information after a Windows API call fails.

Header

#include <windows.h>

Library

Kernel32.lib

Syntax

DWORD WINAPI GetLastError(void);

Return Value

Returns the calling thread's last-error code. The value is of type DWORD. If the function succeeds, the return value is zero.

Remarks

The last-error code is maintained on a per‑thread basis.
Many Windows API functions set the last-error value when they fail. Call GetLastError immediately after the function that failed.
Do not call SetLastError unless you are explicitly resetting the error code.
When writing multithreaded code, ensure you call GetLastError from the same thread that called the failing API.

Example

Note: This example demonstrates retrieving the error code after a failed DeleteObject call.

#include <windows.h>
#include <stdio.h>

int main(void)
{
    HBITMAP hBmp = CreateBitmap(0, 0, 1, 1, NULL);
    if (!hBmp)
    {
        printf("CreateBitmap failed. Error: %lu\\n", GetLastError());
        return 1;
    }

    // Intentional error: DeleteObject expects a GDI object, not a handle to a bitmap created with zero dimensions.
    if (!DeleteObject((HGDIOBJ)hBmp))
    {
        DWORD err = GetLastError();
        printf("DeleteObject failed. Error: %lu\\n", err);
    }

    return 0;
}

Microsoft Docs