Why are the rules for GetWindowText so weird?
Joel Spolsky rightly points out that the
rules for GetWindowText exhibit abstraction leakage. Why are the
rules for GetWindowText so weird?
Set the wayback machine to 1983. Your typical PC had an 8086 processor running at
a whopping 4.7MHz, two 360K 5½-inch floppy drives (or if you were really loaded, one
floppy drive and a 10MB hard drive), and 256KB of memory. [Original entry said
256MB - oops. Thanks to
Joe Beda for pointing this out.]
This was the world of Windows 1.0.
Windows 1.0 was a coöperatively-multitasked system. No preëmptive multitasking here.
When your program got control, it had control for as long as it wanted it. Only when
you called a function like PeekMessage or GetMessage did
you release control to other applications.
This was important, because in the absence of a hardware memory manager, you really
had to make sure that your memory didn't get ripped out from under you.
One important consequence of coöperative multitasking is that if your program is running,
not only do you know that no other program is running, but you also know that every
window is responding to messages. Why? Because if they were hung, they wouldn't
have released control to you!
This means that it is always safe to send a message. You never had
to worry about the possibility of sending a message to a hung window, since you knew
that no windows were hung.
In this simpler world, GetWindowText was a straightforward function:
int WINAPI
GetWindowText(HWND hwnd, LPSTR pchBuf, int cch)
{
// ah for the simpler days
return SendMessage(hwnd, WM_GETTEXT, (WPARAM)cch, (LPARAM)pchBuf);
}
This worked for all windows, all the time. No special handling of windows in a different
process.
It was the transition to Win32 and preëemptive multitasking that forced the change
in the rules, because for the first time, there was the possibility that (gasp) the
window you were trying to communicate with was not responding to messages.
Now you have the backwards compatibility problem. As I described in
my original article, many parts of the system and many programs rely on the ability
to retrieve window text without hanging. So how do you make it possible to retrieve
window text without hanging, while still giving controls like the edit control the
ability to do their own window text management?
The Win32 rules on GetWindowText are the result of this attempt to reconcile
conflicting goals.
(This same story, with slight changes, also works as a discussion of why DDE works
the way it does. But fewer people use DDE nowadays, so the effect is not as dramatic.)
