C11 defect report: tss_t destruction unspecified

Submitter:
Submission Date:
Source: Owen Shepherd
Reference Document: N/A
Version:
Date: 
Subject: tss_t destruction unspecified

Summary

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)

Suggested Technical Corrigendum

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.