Waitting on hEvent of Mailslot

ForNow

Hi I am using Mailslots for Interprocess communication. Process A does a WriteFile with NULL for the overlapped parameter. Process B has a overlapped parameter on the ReadFile. My question is Can process A do WaitForSingleObject on the hEvent of process B overlapped to know when the read has completed Thsnks

CPallini

I think you have to create a named event, see Interprocess Synchronization (Windows)[^].

Victor Nijegorodov

There is an excellent article about using mailslots: Using Mailslots for Interprocess Communication[^] Did you read it?

ForNow

I did in a ReadFile for a mail slot when the i/o is completed shouldn't hOverLap.hEvent be signaled ?

ForNow

thanks

ForNow

I read the article Which basically Said if I CreateFile on the Client Side with FILE_FLAG_OVERALAPPED File then when doing an I/O which I would assume would include ReadFile on the Server side An Overlapped structure would be used (A member of which is m_hEvent I did a OpenEvent on the Client side as The CreateProcess let me inherit objects tried WaitForSingleObject (m_hEvent of the overlapped of the ReadFile) the First wait worked the second didn't as I did three writes

Richard Andrew x64

ForNow wrote:

First wait worked the second didn't as I did three writes

"Didn't work" is hardly a technical description of the problem. Are you forgetting to reset a manual-reset event?

The difficult we do right away... ...the impossible takes slightly longer.

ForNow

Don't think so the flag is set to False it should auto-reset

      sysblk.mail = CreateEvent(&sa,
                                FALSE,           // let Windows Re-set
                                FALSE,           // Intially set off
                               (LPCTSTR)"MyEvent");

Richard Andrew x64

Well maybe the writes are occurring so quickly that the system only has time to signal the event one time. Whenever the event is signaled, you should check for more messages before looping back to the wait function. Don't expect the event to be signaled exactly once for each message you write.

The difficult we do right away... ...the impossible takes slightly longer.

ForNow

You are probably right the WriteFile has a NULL for the overlapped While the ReadFile has a overlapped paramater

leon de boer

The WriteFile doesn't need or require an overlapped access and I would be intrigued what security attributes you are feeding in via sa. Naming the event is fine but why are you needing to provide specific security attributes are you doing something special? The worry I gleaned was you hold the event handle in sysblk.mail are you remembering to copy the handle reference into your overlapped structure? It's sort of odd what you are doing there but not wrong so long as you remember to transfer the handle. The usual read thread code direct from MSDN looks like

char buffer[100];
OVERLAPPED ovlp = {0};

// Create event in overlapped structure
// This directly places Event handle in empty Overlapped structure
ovlp.hEvent = CreateEvent(NULL, false, false, NULL);

if (ovlp.hEvent == NULL) {
    // You have some error creating the event run some error code
    
}

DWORD read;

do {
    ReadFile(mailslot, buffer, sizeof(buffer), &read, &ovlp);  // ovlp.handle in your case must be sysblk.mail
    buffer\[read\] = 0;
    WaitForSingleObject(ovlp.hEvent, INFINITE);                // you are then waiting on your event sysblk.mail 
    process\_message(buffer);
} while (strcmp(buffer, "exit"));   // <= Your exit thread condition

The write thread is create function is non overlapped

HANDLE mailslot = CreateFile("\\\\.\\mailslot\\myslot",
GENERIC_WRITE,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL,
OPEN_ALWAYS,
0,
NULL);

They are the by the book MSDN examples and your code should be some extension of that process inside your sysblk code. Also remember this sort of thing is for relatively low speed, small packet exchanges if you want to go beyond that you need to change to memory mapped files.

In vino veritas