How do I convert a synchronous file handle into an asynchronous one?

Raymond Chen

Raymond

Say you opened a file in synchronous mode,
and then you realize that you want to issue asynchronous I/O on it, too.
One way to do this is to call
Create­File a second time with the
FILE_FLAG_OVERLAPPED,
but this requires you to know the file name,
and the file name
may not be readily available to the function that
wants to do the conversion,
or it may not even be valid any longer if the file has
been renamed in the meantime.

Enter Re­Open­File.
This basically lets you do a Create­File
based on another handle rather than a file name.
It differs from Duplicate­Handle because
it actually goes and opens the file again
(as opposed to merely creating another reference to the same
file object in the kernel).
This means that you have the opportunity to choose new
handle attributes, like whether you want the handle
to be synchronous or asynchronous.

#include <windows.h>
#include <stdio.h>
int __cdecl main(int, char **)
{
 HANDLE h = CreateFile("test", GENERIC_WRITE, FILE_SHARE_READ, NULL,
                       CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
 HANDLE h2 = ReOpenFile(h, GENERIC_READ, FILE_SHARE_READ |
                        FILE_SHARE_WRITE, FILE_FLAG_OVERLAPPED);
 DWORD cbResult;
 WriteFile(h, "!", 1, &cbResult, NULL);
 OVERLAPPED o = { 0 };
 o.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
 char ch = 0;
 BOOL fRc = ReadFile(h2, &ch, 1, &cbResult, &o);
 if (fRc) {
  printf("read completed synchronously\n");
 } else if (GetLastError() == ERROR_IO_PENDING) {
  printf("read proceeding asynchronously\n");
 } else {
  printf("read failed\n");
 }
 GetOverlappedResult(h2, &o, &cbResult, TRUE);
 printf("Result was %c\n", ch);
 CloseHandle(o.hEvent);
 CloseHandle(h2);
 CloseHandle(h);
 return 0;
}

The program opens a test file for writing,
and then uses the
Re­Open­File function to open the same file
for reading.
(Since you are opening the file twice,
be careful to choose compatible sharing modes.)
We synchronously write an exclamation point to the file via the first handle,
and then we asynchronously read it back with the second handle.

It’s really not that exciting.

Raymond Chen
Raymond Chen

Follow Raymond   

0 comments

Comments are closed.