Subj : Re: pthread_cond to create sema?
To   : comp.programming.threads
From : Paolo
Date : Thu Jan 20 2005 08:00 am

Giancarlo Niccolai <searchforme@ingoogle.com> wrote:
> Paolo wrote:

>> 
>> Are these P() and V() correct if i want to implement
>> semaphores p() and v() functions?
>> 
>> messagecounter is a global int variable,
>> semaforo is a mutex used only here
>> sema is a cond variable
>> 
>> 
>> 
>> #define P() {\
>> pthread_mutex_lock(&semaforo);\
>> while ( TW_messagecounter <= 0) {\
>>  pthread_cond_wait(&sema, &semaforo);\
>> }\
>> messagecounter--;\
>> pthread_mutex_unlock(&semaforo);\
>> }
>> 
>> #define V() {\
>> pthread_mutex_lock(&semaforo);\
>> messagecounter++;\
>> pthread_cond_signal(&sema);\
>> pthread_mutex_unlock(&semaforo);\
>> }
>> 
>> Thank you!
>> Paolo

> Yes, but you can make it better by checking if there are waiting agents
> before signaling and pushing the cleanup function before waiting (and
> popping it after waiting). 

Do you think i could get a speed boost avoiding a signal for each V() with
a new variable and a check on it?

>Minimal cleanup is the pthread_unlock_mutex, so
> that if your wait is killed by another thread, the mutex gets unlocked
> before the agent is definitely stopped.

How can i unlock the mutex if i'm killed, before i have to exit?
with a signal handler?

> Bests,
> Giancarlo.

Thank you!
Paolo

.