OpenEuphoria

Re: Structs Question

• Posted by euman at bellsouth.net Jul 14, 2001
• 387 views

Pretty good lesson I think....>

Euman
euman at bellsouth.net
> 
> > Never mind I figured it out.
> > if I free(iParent) before I send this message
> > hParent = c_func(SendMessage,{TreeView1,TVM_INSERTITEM, 0, tvstruct})
> > there wont be any text for the root item.
> 
> Yep, that's the problem alright. Same with Child1 and child2.
> 
> > Now how in hek is this?
> >
> > I poked the text into the structure then free'd the allocated_string
> before
> > I sent the message and it doesnt work and when I move the free()
> > after the message......hmmm Im at a loss. (beats me) but it works now.
> 
> Well, actually you didn't poke the TEXT into the structure. All you do is
> poke the ADDRESS of the text into the structure.
> 
> > iParent = allocate_string("Parent Item")
> 
> The allocate_string() reserves a place in RAM for you text, copies the text
> in the sequence to that spot, and returns to your program the location
> (address) reserved for you.
> 
> > poke4( tvstruct + TVINSERTSTRUCTITEM_pszText,iParent)
> 
> This poke4() copies the address returned to the struct. An address is
> 4-bytes long and that's what the poke4() function does - copies 4-byte to an
> RAM location.
> 
> > free(iParent)
> 
> This tells Windows that you no longer need the RAM location that was
> reserved for your text and gives it back to Windows. After this, your text
> may or may not be in the RAM still, it all depends on how Windows uses that
> RAM again after you gave it back.
> 
> You cannot use or expect to use RAM that you have given back to Windows.
> 
> > hParent = c_func(SendMessage,{TreeView1,TVM_INSERTITEM, 0, tvstruct})
> 
> However, you then send the message to Windows to insert an item. The Windows
> code that does this doesn't know you gave back the RAM and will attempt to
> use the address you poked into the structure. But by this time, the spot
> could have been used by anything else and destroyed your text.
> 
> If you notice, in win32lib, we always release_mem() after sending messages.
> 
> -----
> Cheers,
> Derek.