IOCP readfile example

main.cpp

#include <Windows.h>
#include <stdio.h>

typedef struct _PER_SOCKET_CONTEXT {
    OVERLAPPED ol;
    char buf[100];
    HANDLE hFile;
} PER_IO_CONTEXT, * PPER_IO_CONTEXT;

DWORD WINAPI Worker(LPVOID param);

int wmain()
{
    HANDLE iocp = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
    if (iocp == NULL) {
        puts("****** CreateIoCompletionPort");
        return 1;
    }
    HANDLE hFile = CreateFileW(L"main.cpp",
        GENERIC_READ,
        0,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
        NULL);
    if (hFile == NULL) {
        puts("****** CreateFileW");
        return 1;
    }PER_IO_CONTEXT ctx{.hFile=hFile};

    iocp = CreateIoCompletionPort(hFile, iocp, (ULONG_PTR)&ctx, 0);

    if (iocp==NULL){
        puts("****** CreateIoCompletionPort twice failed!");
    }
    HANDLE hThread = CreateThread(NULL, 0, Worker, iocp, 0, NULL);
    if (hThread == NULL) {
        puts("****** CreateThread");
        return 1;
    }
    if (ReadFile(hFile, ctx.buf, sizeof(ctx.buf), NULL, &ctx.ol)){ 
        puts("****** ReadFile failed");
    }
    else {
        WaitForSingleObject(hThread, INFINITE);
    }
    CloseHandle(iocp);
}

DWORD WINAPI Worker(LPVOID param) {
    DWORD dwIoSize = 0;
    LPOVERLAPPED ol;
    PPER_IO_CONTEXT ctx;
    for(;;){
        BOOL bSuccess = GetQueuedCompletionStatus((HANDLE)param, &dwIoSize,
            (PDWORD_PTR)&ctx,
            &ol,
            INFINITE);
        if (ctx == NULL) {
            puts("****** ctx is NULL, maybe you call PostQueuedCompletionStatus?");
            break;
        }
        if (bSuccess==FALSE || dwIoSize == 0) {
            if (GetLastError()==ERROR_HANDLE_EOF){
                puts("\n****** EOF reached, stop");
            }else{
                printf("****** Unexpect Error GetLastError=%d(0x%x): Worker is exiting...\n", GetLastError(), GetLastError());
            }
            CloseHandle(ctx->hFile);
            break;
        }
        fwrite(ctx->buf, dwIoSize, 1, stdout);
        ctx->ol.Offset += dwIoSize;
        if (ReadFile(ctx->hFile, ctx->buf, sizeof(ctx->buf), NULL, &ctx->ol)){
            puts("****** ReadFile failed");
        }
    }
    return 0;
}

ReadFile returns FALSE and ERROR_IO_PENDING=GetLastError() if asynchronously