Passing a HANDLE to a DLL - c

Question

I'm new to Win32 programming.
I'm trying to pass a HANDLE obtained using CreateFile() to a function in a DLL.
But upon trying to read bytes, dwBytesRead says 0.
Am I allowed to pass HANDLEs to DLL entries? I read here [Writing DLLs] that resources of the caller do not belong to callee, and hence I should not call CloseHandle() or things like free() for malloc() in caller.
Is my understanding correct? Kindly point me in the right direction. Here's the code:
main.c
#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>
#define BUFFERSIZE 5
int __declspec( dllimport ) hello( HANDLE );
void __cdecl _tmain(int argc, TCHAR *argv[])
{
HANDLE hFile;
printf("\n");
if( argc != 2 )
{
printf("Usage Error: Incorrect number of arguments\n\n");
_tprintf(TEXT("Usage:\n\t%s <text_file_name>\n"), argv[0]);
return;
}
hFile = CreateFile(argv[1], // file to open
GENERIC_READ, // open for reading
FILE_SHARE_READ, // share for reading
NULL, // default security
OPEN_EXISTING, // existing file only
FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, // normal file
NULL); // no attr. template
if (hFile == INVALID_HANDLE_VALUE)
{
_tprintf(TEXT("Terminal failure: unable to open file \"%s\" for read.\n"), argv[1]);
return;
}
printf( "Entered main, calling DLL.\n" );
hello(hFile);
printf( "Back in main, exiting.\n" );
CloseHandle(hFile);
}
hello.c
#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>
#define BUFFERSIZE 5
DWORD g_BytesTransferred = 0;
VOID CALLBACK FileIOCompletionRoutine(
__in DWORD dwErrorCode,
__in DWORD dwNumberOfBytesTransfered,
__in LPOVERLAPPED lpOverlapped )
{
_tprintf(TEXT("Error code:\t%x\n"), dwErrorCode);
_tprintf(TEXT("Number of bytes:\t%x\n"), dwNumberOfBytesTransfered);
g_BytesTransferred = dwNumberOfBytesTransfered;
}
int __declspec( dllexport ) hello( HANDLE hFile )
{
DWORD dwBytesRead = 0;
char ReadBuffer[BUFFERSIZE] = {0};
OVERLAPPED ol = {0};
if( FALSE == ReadFileEx(hFile, ReadBuffer, BUFFERSIZE-1, &ol, FileIOCompletionRoutine) )
{
DWORD lastError = GetLastError();
printf("Terminal failure: Unable to read from file.\n GetLastError=%08x\n", lastError);
return lastError;
}
dwBytesRead = g_BytesTransferred;
if (dwBytesRead > 0 && dwBytesRead <= BUFFERSIZE-1)
{
ReadBuffer[dwBytesRead]='\0';
printf("Data read from file (%d bytes): \n", dwBytesRead);
printf("%s\n", ReadBuffer);
}
else if (dwBytesRead == 0)
{
printf("No data read from file \n");
}
else
{
printf("\n ** Unexpected value for dwBytesRead ** \n");
}
printf( "Hello from a DLL!\n" );
return( 0 );
}

Answer 1

You are missing the SleepEx(5000, TRUE) call from the example.
You are using async-io, in which case you will receive a callback when the read occurs. If you don't wait for the callback you may get 0 bytes read depending on when the callback is triggered.