Why the api functions whait for a response?

i have a program that send some api functions to an external app
but some times my app when it send an api function, it stop, waiting something from the other app.
is there a way to avoiding that?
telling the api function to not wait or something like that?

What Exactly are you doing, Are you using SendMessage, if so there should be no reason for it to stop waitng, if you're using postmessage or sendmeassage timeout then use sendmessage instead.

i'm usind Call SetWindowPos and
a = SendMessage(lHwnd, WM_LBUTTONDOWN......
a = SendMessage(lHwnd, WM_LBUTTONUP.....

Tried SendMessageCallback?
It returns immediately, and when the other app is done it calls your callback-function. (Look at my Q about SendMessageCallback.)

SendMessage waits for a responce.
PostMessage doesn't.
So if you don't want it to wait, then use PostMessage.