GetOverlappedResult

Retrieves the result of an asynchronous (overlapped) operation on a specified file, named pipe, or communications device.

Syntax

#include <windows.h>

BOOL GetOverlappedResult(
    HANDLE       hFile,
    LPOVERLAPPED lpOverlapped,
    LPDWORD      lpNumberOfBytesTransferred,
    BOOL         bWait
);

Parameters

Name	Type	Description
hFile	HANDLE	Handle to the file or pipe. Must have been opened with FILE_FLAG_OVERLAPPED.
lpOverlapped	LPOVERLAPPED	Pointer to the OVERLAPPED structure used in the asynchronous operation.
lpNumberOfBytesTransferred	LPDWORD	Pointer to a variable that receives the number of bytes transferred.
bWait	BOOL	If TRUE, the function does not return until the operation has completed. If FALSE, the function returns immediately.

Return value

If the function succeeds, the return value is TRUE. If it fails, the return value is FALSE. Call GetLastError for extended error information.

Remarks

The OVERLAPPED structure must remain valid until the operation completes.
When bWait is FALSE and the operation has not completed, the function returns FALSE with ERROR_IO_INCOMPLETE.
For pipe handles, the function can be used after ReadFile or WriteFile with overlapped I/O.

Example

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

int main()
{
    HANDLE hPipe = CreateFileW(
        L"\\\\.\\pipe\\MyPipe",
        GENERIC_READ | GENERIC_WRITE,
        0,
        NULL,
        OPEN_EXISTING,
        FILE_FLAG_OVERLAPPED,
        NULL);

    if (hPipe == INVALID_HANDLE_VALUE) {
        printf("CreateFile failed: %lu\\n", GetLastError());
        return 1;
    }

    char buffer[128];
    DWORD bytesRead;
    OVERLAPPED ov = {0};
    ov.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);

    BOOL ok = ReadFile(hPipe, buffer, sizeof(buffer), NULL, &ov);
    if (!ok && GetLastError() != ERROR_IO_PENDING) {
        printf("ReadFile failed: %lu\\n", GetLastError());
        return 1;
    }

    // Wait for the read to complete
    ok = GetOverlappedResult(hPipe, &ov, &bytesRead, TRUE);
    if (!ok) {
        printf("GetOverlappedResult failed: %lu\\n", GetLastError());
    } else {
        printf("Read %lu bytes: %.*s\\n", bytesRead, (int)bytesRead, buffer);
    }

    CloseHandle(ov.hEvent);
    CloseHandle(hPipe);
    return 0;
}