ReadFile doesn't work asynchronously on Win7 and Win2k8

Question

According to MSDN, ReadFile can read data 2 different ways: synchronously and asynchronously. I need the second one. The folowing code demonstrates usage with OVERLAPPED struct:

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

void Read()
{
    HANDLE hFile = CreateFileA("c:\\1.avi", GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
    if ( hFile == INVALID_HANDLE_VALUE )
    {
        printf("Failed to open the file\n");
        return;
    }

    int dataSize = 256 * 1024 * 1024;
    char* data = (char*)malloc(dataSize);
    memset(data, 0xFF, dataSize);

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

    printf("reading: %d\n", time(NULL));
    BOOL result = ReadFile(hFile, data, dataSize, NULL, &overlapped);
    printf("sent: %d\n", time(NULL));


    DWORD bytesRead;
    result = GetOverlappedResult(hFile, &overlapped, &bytesRead, TRUE); // wait until completion - returns immediately
    printf("done: %d\n", time(NULL));

    CloseHandle(hFile);
}



int main()
{
        Read();
}

On Windows XP output is: reading: 1296651896 sent: 1296651896 done: 1296651899

It means that ReadFile didn't block and returned imediatly at the same second, whereas reading process continued for 3 seconds. It is normal async reading.

But on windows 7 and windows 2008 I get following results: reading: 1296661205 sent: 1296661209 done: 1296661209. It is a behavior of sync reading.

MSDN says that async ReadFile sometimes can behave as sync (when the file is compressed or encrypted for example). But the return value in this situation should be TRUE and GetLastError() == NO_ERROR. On Windows 7 I get FALSE and GetLastError() == ERROR_IO_PENDING. So WinApi tells me that it is an async call, but when I look at the test I see that it is not!

I'm not the only one who found this "bug": read the comment on ReadFile MSDN page.

So what's the solution? Does anybody know? It is been 14 months after Denis found this strange behavior.

Answer 1

I don't know the size of the "c:\1.avi" file but the size of the buffer you give to Windows (256M!) is probably big enough to hold the file. So windows decides to read the whole file and put it in the buffer the way it likes. You don't say to windows "I want async", you say "I know how to handle async".

Just change the buffer size say 1024, and your program will behave exactly the same, but read only 1024 bytes (and return ERROR_IO_PENDING as well).

In general, you do asynchronous because you want to do something else during the operation. Look at the sample here: Testing for the End of a File, as it demonstrate an async ReadFile. If you change the sample's buffer and set it to a big value, it should behave exactly like yours.

PS: I suggest you don't rely on time samples to check things, use return codes and events