How to stop ibwait quickly. (Can't set timeout to be too short)

[Robert Liang]

What I want to do:
Send a command to a GPIB instrument to trigger a time-consuming task, then I should be able to stop the time-consuming task anytime before it finishes normally.

What I have done:
Call these functions in one thread,

ibtmo(ud, 16)   #set timeout to 300s
ibwrta(ud, "...", ...)  #write the command asynchronously
ibwait(ud, TIMO | RQS)  #wait for RQS or timeout

When I try to cancel the time-consuming task, I call these functions in another thread,
ibstop(ud)
ibwrt(ud, "...", ...)  #write instrument specific cancel command

I've also tried to use ibonl,
ibonl(ud, 1)   #reset the interface
ibwrt(ud, "...", ...)  #write instrument specific cancel command

Problem:
both ibstop and ibonl take too long to return, they seems to wait the time-consuming task to finish and then return, which isn't what I want to do. Did I do the right thing? Where is the problem? Any help is greatly appreciated.

PS:
I wrote the program in C

GPIB driver version is 2.4

[GPIB_Guru]

Why don't you use either:
1) ibwait (0) in a loop and check the response, or
2) ibnotify to notify on the condition. This can be cancelled if necessary.