[fpc-pascal]Program is busy

MichaƂ Kamburelis michalis at camelot.homedns.org
Sat Jul 19 09:52:40 CEST 2003

If you're doing anything time-consumig (like reading data from internet 
socket) and you want your program to respond to user actions (to Windows 
/ XWindows / any-other-event-driven-environment events), to repaint its 
windows etc. you have to merge event loop with your data-reading code.

Something like that:

procedure Read_Data_And_Process_Some_Events;
   while (not end of data stream) do
     read some data
     process some events

If you're reading from a file, "read some data" can be something like 
"read next 1000 bytes from a stream". BUT if you're reading from an 
internet socket you don't know how long it will take to get "next 1000 
bytes" so you should rather limit your reading code by time. Something like
   while (Now-time < 5 seconds) do
     peek if there are some data in a stream, if there are - read them

However, that kind of loop is called busy-waiting because if the stream 
has no more data yet, your program will use a lot of processor time just 
for peeking. It can be solved by doing Sleep(<for some small amount of 
time>) after every unsuccesful peek (that's an easy but poor solution) 
or by doing time-limited waiting for data using OS functions, like 
select() on sockets (and that's a corect solution).

"process some events" under WinAPI should be something like
   if PeekMessage(msg, 0, 0, 0, PM_REMOVE) then
     if msg.message=WM_QUIT then break;

If you want only to repaint your windows you can ignore all events 
except WM_PAINT. In general, you should disable some menus/buttons
etc. before calling Read_Data_And_Process_Some_Events. This should make 
  things easier, and it will prevent Read_Data_And_Process_Some_Events 
from being invoked recursively.

Hope this helps,
Michalis Kamburelis
michalis at camelot.homedns.org
http://www.camelot.homedns.org/~michalis/ (in Polish)

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