IOCP and ReadFileEx usage

Question

i'm playing with IOCP. I'm trying to write simple application that async reads data from the file in the main thread. However i'm getting error(ERROR_INVALID_PARAMETER) in ReadFileEx function, but seems i'm doing it ok. What am i doing wrong? Here is my sample:

#include "stdafx.h"
#include <windows.h>
#include <assert.h>
#include <stdint.h>

VOID CALLBACK ReadCb(DWORD dwErrorCode,DWORD dwNumberOfBytesTransfered,LPOVERLAPPED lpOverlapped)
{
    fprintf(stderr,"i was here\n");
}


int _tmain(int argc, _TCHAR* argv[])
{
    HANDLE main_io,file_i;
    HANDLE file_handle;
    DWORD bytes_recvd;
    ULONG_PTR data = 0;
    OVERLAPPED overlapped;
    LPOVERLAPPED poverlapped = &overlapped;
    uint8_t read_data[1024];
    DWORD err;

    main_io = CreateIoCompletionPort(INVALID_HANDLE_VALUE,NULL,0,0);
    assert(main_io != NULL);

    file_handle = CreateFile(L"test.txt",GENERIC_READ,0,NULL,OPEN_EXISTING,FILE_FLAG_OVERLAPPED, NULL);
    assert(file_handle != INVALID_HANDLE_VALUE);

    file_i = CreateIoCompletionPort(file_handle,main_io,data,0);
    assert(file_i != NULL);

    memset(&overlapped,0,sizeof(OVERLAPPED));

    err = ReadFileEx(file_handle,(LPVOID)read_data,1024,&overlapped,ReadCb);
    fprintf(stderr,"err %d\n",GetLastError());
    assert(err != 0);

    assert(GetQueuedCompletionStatus(file_i,&bytes_recvd,&data,&poverlapped,INFINITE));

    CloseHandle(main_io);
    return 0;
}

Answer 1

'If the function succeeds, the return value is nonzero' - don't call GetLastError() for a non-zero return! Move the GetLastError line after the assert check so that, if an assertion violation exception occurs, the GetLastError is not reached.

Answer 2

As Martin says, first fix your error handling

const BOOL result = ReadFileEx(...);

const DWORD lastError = GetLastError();

if (lastError != ERROR_SUCCESS)
{
   fprintf(stderr, "err %d\n", lastError);
}

Note that Martin is incorrect about his assumption that you shouldn't call GetLastError() for success returns from ReadFileEx() , though this is a bit of a special case... From the MSDN docs :

When using ReadFileEx you should check GetLastError even when the function returns "success" to check for conditions that are "successes" but have some outcome you might want to know about. For example, a buffer overflow when calling ReadFileEx will return TRUE, but GetLastError will report the overflow with ERROR_MORE_DATA. If the function call is successful and there are no warning conditions, GetLastError will return ERROR_SUCCESS.

But that doesn't solve your problem, it just makes your example code more correct...

The problem is that ReadFileEx() does asynchronous file reading using a completion routine and you're trying to do asynchronous file reading using an I/O Completion Port. For that you should use ReadFile() . See this answer for why I feel that IOCP is the better route to take and why completion routines are a bit nasty...

So, simply change your ReadFileEx() call to a ReadFile() call and your problem will go away and the code will post a completion to the IOCP when the read completes...

ReadFile(file_handle,(LPVOID)read_data,1024,&overlapped);

and you're done.