Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.programming.threads > #4060

Re: pthread_once_t in dynamically allocated (and freed) memory?

Show all headers | View raw

On 2018-02-25, dave@boostpro.com <dave@boostpro.com> wrote:
> Hi,
>
> I want to dynamically allocate memory, then once, on demand, thread-safely initialize part of that memory, and potentially free the memory.  Then later I want to repeat the process with what is potentially the same memory block (and thus the same address of pthread_once_t) that has been returned from the allocator.  Does this 
>
> a. "just work" as long as it is set up with PTHREAD_ONCE_INIT, or 
> b. do I need to do insert some kind of fence to ensure that all threads see the re-initialized memory, or
> c. never work; don't do that!
> d. something else I haven't thought of?

It must work.

Firstly, POSIX says that PTHREAD_ONCE_INIT is a constant.
Thus it may be used in assignment. This is important because, by
contrast, the PTHREAD_MUTEX_INITIALIZER is described as a macro
that can be used for initialization, and not a constant.

Secondly, there is a storage restriction on the phread_once_t
control variable: namely, it may not be defined in automatic storage
(i.e. non-static local variable).  There is no restriction against
pthread_once_t being in dynamic storage.

> I ask because all the examples I can find store the pthread_once_t in
> static storage and never re-initialize it.

However, if that is in a shared library, it's dynamic anyway.  The
dynamic case is effectively exercised any time a shared library is
dlopen'ed that has a file scope pthread_once_t.

Back to comp.programming.threads | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

pthread_once_t in dynamically allocated (and freed) memory? dave@boostpro.com - 2018-02-25 11:09 -0800
  Re: pthread_once_t in dynamically allocated (and freed) memory? Kaz Kylheku <217-679-0842@kylheku.com> - 2018-02-26 02:29 +0000
    Re: pthread_once_t in dynamically allocated (and freed) memory? dave@boostpro.com - 2018-02-25 22:10 -0800
      Re: pthread_once_t in dynamically allocated (and freed) memory? Kaz Kylheku <217-679-0842@kylheku.com> - 2018-02-26 19:14 +0000

csiph-web