Re: How does PeekMessage really work?
See Below.....
"Denis Adamchuk" <DenisAdamchuk@discussions.microsoft.com> wrote in message
news:179A5207-EB0E-4381-9127-5BE1D90A0DBD@microsoft.com...
Hi everyone,
I'm writing an application where Windows messages are used in a special
manner and want to clarify how flexible is access to the thread message
queue.
I've read the MSDN article about PeekMessage and many web sites where
different opinions are expressed. My questions are the following...
1) What really does PeekMessage()?
Does it process messages sent from other threads by means of
SendMessage-family routines or just picks a message from the queue (or
from
where?) and give me a chance to analyze it and process it by myself?
PeekMessage retrieves a message from the calling thread's message queue, it
doesn't care if was sent view SendMessage or PostMessage. PeekMessage does
not process the message, that's job of the winproc to process messages (this
happens during the call to DispatchMessage), PeekMessage simply returns the
message that is at the top of the queue.
2) Does PeekMessage process ALL sent messages that are available now or
only
one of them?
PeekMessage returns one message at a time. Again it does not process message
simply returns a message. Not that wRemoveMsg parameter let you tell it to
either remove the message if you are calling PeekMessage to process the
message after you receive it, or to leave it in the queue, if you are
calling PeekMessage just out of curiousity and won't really be processing
the message. See GetMessage if you always want to remove the message from
the queue.
3) According to the MSDN - PeekMessage() does both things I've mentioned
above.
But what do wMsgFilterMin and wMsgFilterMax arguments mean?
Does PeekMessage pick from the queue messages from identifiers from the
range or does it process only such messages sent to the current thread?
wMsgFilterMin and wMsgFilterMax is used to only receive back message in the
specified range. And simply won't return any messages outside of that
range.
4) The most confusing one :)
What is tuned by PM_QS_*** flags - behavior for implicit message
processing
or behavior for message picking?
If PM_QS*** define what messages I want to proceed why there are any flags
except the PM_QS_SENDMESSAGE (since PeekMessage() is devoted to process
only
SendMessage() messages)?
Otherwise if PM_QS*** define what messages I want to pick why there is the
PM_QS_SENDMESSAGE flag if the messages that were sent will be processed
implicitly and I never get them as a result?
I have never used these before, but the docs say that they filter message by
type. Kind of like FilterMin and FilterMax
5) PeekMessage() does not fill lpMsg output parameter with a message(s) it
has processed?
I don't understand questions 5, and 6
6) Is it possible to disable an implicit message processing inside
PeekMessage() and make it to return as lpMsg messages that were sent from
another threads via SendMessage-routines BUT only within a defined range?
7) What is PM_NOYIELD?
PM_NOYIELD will make user that no other threads are released during the call
to PeekMessage. Explaining this one will take a couple of pages. each
thread in your application gets a time slice of the processer. So a thread
will run for a while and when it becomes idle the processor is given to
another thread in your app, and so far and so forth. Callling PeekMessage
might cause another thread to take over momentarily, PM_NOYIELD will pervent
that from happening.
Thanks in advance!
It will be great if this post become a consolidated place of information
about PeekMessage() for those who search...
AliR.