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.
