Submitter: Owen Shepherd
Submission Date: 2012-08-12
Source: WG14
Reference Document:
N1627
Version: 1.3
Date: April 2014
Subject: tss_t destruction unspecified
The standard does not specify if or when destructors for
thread specific data keys (created with the tss_create
function) are invoked.
This proposal suggests to align the behavior with that
specified by POSIX for pthread_key_t.
This behavior is hoped to both match the intention of
the standard, and be possible to implement on other systems (it is
noted that a pthreads implementation exists for Windows, for example,
while the behavior of POSIX and Windows thread local storage
implementations differ greatly)
After 7.26.5.1p2, add
Returning from func
shall have the same behaviour as invoking thrd_exit
with the returned value
Change 7.26.5.5 parts 2 and 3 from
The thrd_exit function
terminates execution of the calling thread and sets its
result code to res.
The program shall terminate normally after the last thread has been terminated. The behavior shall be as if the program called the exit function with the status EXIT_SUCCESS at thread termination time.
to
For every thread specific storage key
which was created with a non-NULL destructor and for which the value is
non-NULL, thrd_exit shall set the value associated with
the key to NULL and then invoke the destructor with its previous value.
The order in which destructors are invoked is unspecified.
If after this process there remain keys
with both non-NULL destructors and values, the implementation shall
repeat this process up to TSS_DTOR_ITERATIONS times.
If thrd_exit is not
being invoked in the last thread in the process, then the
implementation shall terminate execution of the calling thread and
set its result code to res. Otherwise, the
implementation shall behave as if exit(EXIT_SUCCESS)
was invoked.
(This change provides application writers guarantees about the
identity of the thread executing functions registered with atexit)
After 7.26.6.1p2, add
The value NULL shall be associated with the newly created key in all existing threads. Upon thread creation, the value associated with all keys shall be initialized to NULL
Note that destructors associated thread specific storage are not invoked at process exit.
To 7.26.6.2p2, append
If tss_delete is called
while another thread is executing destructors, whether this will affect
the number of invocations of the destructor associated with key
on that thread is unspecified. If the thread from which tss_delete
is invoked is executing destructors, then no further invocations of the
destructor associated with key will occur on said
thread.
Calling tss_delete
will not result in the invocation of any destructors.
After 7.26.6.4p2, add
This action will not invoke the destructor associated with the key on the value being replaced.
Committee Discussion
Apr 2013 meeting
- This paper was discussed in conjunction with DR424 which covers one additional related issue.
- The key observation is that these papers are about the deliberate underspecification of threads, since that allows the greatest opportunity for implementation on a variety of operating systems.
- Pete Becker, an implementor of the C11 threads library, was asked about these papers and replied in SC22/WG14 message 12813.
- Based on that response the committee is concerned that there could be subtleties in adopting the Proposed Technical Corrigendum and that, as such, these changes are substantial enough to warrant a proposal and to not be considered a defect.
Committee Discussion
Oct 2013 meeting
- A revised paper N1687 was submitted, although not in the recommended html format.
- The suggested technical corrigendum would specify new instances of undefined behavior as well as new requirements on implementations.
- The committee expressed concern that some of the new requirements may be onerous or impossible to provide above the native platform implementation.
Committee Discussion
After several papers N1750, N1751, revisions, and much discussion, the committee has agreed on the following as the resolution.
Proposed Technical Corrigendum
After 7.26.5.1 paragraph 2, add
Returning from func shall have the same behavior as invoking thrd_exit with the value returned from func.Change 7.26.5.5, replace paragraph 2 with:
For every thread-specific storage key which was created with a non-null destructor and for which the value is non-null, thrd_exit shall set the value associated with the key to NULL and then invoke the destructor with its previous value. The order in which destructors are invoked is unspecified.After 7.26.6.1 paragraph 2, add the following new paragraphs:If after this process there remain keys with both non-null destructors and values, the implementation shall repeat this process up to TSS_DTOR_ITERATIONS times.
Following this, the thrd_exit function terminates execution of the calling thread and sets its result code to res.
The value NULL shall be associated with the newly created key in all existing threads. Upon thread creation, the value associated with all keys shall be initialized to NULL.In 7.26.6.2 paragraph 2, add the following new second sentence:Destructors associated with thread-specific storage are not invoked at program termination.
A call to tss_create from within a destructor results in undefined behavior.
A call to tss_delete function results in undefined behavior if the call to tss_create which set key completed after the thread commenced executing destructors.After 7.26.6.2 paragraph 2, add the following new paragraphs:
If tss_delete is called while another thread is executing destructors, whether this will affect the number of invocations of the destructor associated with key on that thread is unspecified.In 7.26.6.3 paragraph 2, add the following new second sentence:Calling tss_delete will not result in the invocation of any destructors.
A call to tss_get function results in undefined behavior if the call to tss_create which set key completed after the thread commenced executing destructors.
In 7.26.6.4 paragraph 2, add the following new second sentence:
This action will not invoke the destructor associated with the key on the value being replaced.*)*) This clarifies whether or not a destructor will be invoked for storage created after a thread has already begun executing destructors: because tss_set is an undefined operation, a value may never be associated with the storage and therefore the destructor may never be invoked.