Subj : Re: Did pthread_once init_routine succeed?
To   : comp.programming.threads
From : David Schwartz
Date : Wed Feb 23 2005 04:25 pm


"Ian Pilcher" <i.pilcher@comcast.net> wrote in message 
news:ksidnY-GyNgDkoDfRVn-2w@comcast.com...

> Is it safe to use a file-scope variable to report the result of an
> init_routine called via pthread_once?  I believe that it is, if the
> following conditions are met:

    Yes. What would be the use of pthread_once if you couldn't modify memory 
in the initialization routine and access the results after pthread_once 
returns? (Think about it, that's about all pthread_once is good for.)

>   * The variable is only set by the init_routine.
>
>   * The variable is only read immediately following a call to
>     pthread_once (with the appropriate parameters).
>
> It seems to me that one of three things can happen when a thread hits a
> call to pthread_once:
>
>   1. The thread is the "first" to call pthread_once (with these
>      parameters).  It will execute the init_routine, which sets the
>      value of the variable, and then return from pthread_once.  The
>      value of the variable is determinate when the thread reads it.
>
>   2. The thread is not the "first" to call pthread_once, but the
>      init_routine has not yet returned (in whatever thread it is
>      executing).  The thread will block until the other thread
>      completes the init_routine (setting the variable).  The value
>      of the variable is determinate when the thread reads it.
>
>   3. The thread is not the "first" to call pthread_once, and
>      init_routine has already completed (in this or another thread).
>      The variable was set by init_routine, so its value is determinate
>      when the thread reads it.

    Correct, this is the semantics of pthread_once and the reason it is 
useful at all.

    DS

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