IPC using RegisterWindowMessage

I have implemented IPC mechanism using RegisterWindowMessage .
The sending app allocates memory using GlobalAlloc and gets the pointer using GlobalLock and casts it to ptr to struct and assigns value to it. Then it calls GlobalUnlock and sends the HGLOBAL as lparam of PostMessage. The receiving app converts the HGLOBAL into
a pointer(GlobalLock) and the pointer is not NULL. But when I try to read the contents of the pointer I get an access violation. I tried various ways of GlobalLock etc but in vain.
mfc_speakAsked:
Who is Participating?

[Webinar] Streamline your web hosting managementRegister Today

x
 
jhanceConnect With a Mentor Commented:
I'm not really interested in HOW YOU FEEL about this.  

Under WIN32 you CANNOT SHARE POINTERS BETWEEN PROCESSES.  Each process runs in its OWN 32-BIT MEMORY SPACE!

The clipboard is a different animal and is supported via the Windows API.  You don't operate on the clipboard via a pointer you get a handle to the clipboard from Windows.

The reason that WM_COPYDATA must use SendMessage is that the receiving application must accept the data while it is guaranteed to be valid.  If PostMessage were used the data might not still be available whenever the receiving process got around to accepting the message.

If you need some other scheme that doesn't rely on SendMessage you could use a memory mapped file, a disk file, a network socket connection, a pipe, a mailslot, or a shared memory area.

In none of the above cases, however, will you be sharing a memory pointer.  You will reference a handle to the object that Windows will manage for you.
0
 
jhanceCommented:
You CAN use RegisterWindowMessage to send messages between processes but you cannot send a memory pointer from one process to another.  Each process runs in its own virtual memory space and the addresses don't mix, at all.

There are a number of ways of doing this but the one that may lend itself to your method here is to use the WM_COPYDATA window message.
0
 
mfc_speakAuthor Commented:
I feel that pointers can be shared across processes. I did a similar thing for clipboard copy and it worked fine. Since the global handles are allocated in the shared addr space common to all processes, I feel this should be possible.

Regarding WM_COPYDATA message, I read thru documentation. It says that an app must use SendMessage and not PostMessage to send it. But I need to send the data by PostMessage( which wont block my thread).
0
Free Tool: SSL Checker

Scans your site and returns information about your SSL implementation and certificate. Helpful for debugging and validating your SSL configuration.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 
mfc_speakAuthor Commented:
WM_COPYDATA did not work with PostMessage. It worked only with SendMessage!!!
0
 
jhanceCommented:
Jerk!  

Next time keep your stupid points.
0
 
mfc_speakAuthor Commented:
Hey!!! dont get hot. I given you the grade you deserved (and points of course). You said that WM_COPYDATA works with PostMessage (but it didn't)
Any way :-)
0
 
jhanceCommented:
I said:

"The reason that WM_COPYDATA must use SendMessage..."

I did NOT say that WM_COPYDATA works with PostMessage!

I'm HOT because you graded me with a "D".  That's why you're a jerk!  If this answer was so damned bad you should have just rejected it.

Next time, keep your stinking points!!

0
All Courses

From novice to tech pro — start learning today.