Pthreads & Signals

[gerald.feldman at hp.com (Jerry Feldman)]

On Tuesday 16 November 2004 9:35 pm, Anthony Gabrielson wrote:
> Hello all,
> 	I'm trying to figure out howto resend a signal after I do
> something.  I have reduced my problem to the attached example.  If look
> though it the main thread gets a signal and then just tries to resend it
> back to the thr thread.  It just holds at the wait command...
I took a look at your program. The problem is synchronization. 
First, the mutex should be locked before a condition wait. Main fails to 
lock and unloack the mutex. Remember that pthread_cond_wait() unlocks the 
mutex, then reacquires it when it wakes up. 

So, in your code, the thread acquires the mutex, waits 1 usec, then signals 
main who is waiting. Main wakes up, tries to reacquire the mutex. Or, the 
thread sends the signal before main issues the condition wait. 
I removed the usleep(3), locked the mutex before the pthread create. This 
will block the child thread. Main then creates the child. The child blocks. 
Then main waits, and the cond_wait unlocks the mutex. The child wakes up 
and sends the signal. Main then wakes up and blocks on the mutex from the 
cond_wait. The child then issues a cond_wait, which unlocks the mutex. Main 
wakes up, signals the child, releases the mutex. The child wakes up, 
releases the mutex, and exits. Main then completes the jon and exits. 

gaf at toulouse:~/src> ./pthrsignal
main: wait cond
thr: Got Mutex
thr: Sent Signal
main: got cond
main: send cond
thr: Got Signal
thr: Release Mutex




-- 
Jerry Feldman <gerald.feldman at hp.com>
Partner Technology Access Center (contractor) (PTAC-MA)
Hewlett-Packard Co.
550 King Street LKG2a-X2
Littleton, Ma. 01460
(978)506-5243
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