Table of Contents

Introduction

The final release of the C11 international standard leaves many aspects regarding thread local storage unspecified. Specifically, the following aspects are unspecified:

• If or when destructors for thread specific storage ("tss") objects are invoked
• The ordering (or lack thereof) of destructor invocation
• The identity of the thread invoking the TSS destructors
• The number of times TSS destructors may be invoked (and the meaning of the TSS_DTOR_ITERATIONS constant)
• The behavior of TSS destructor invocation in the face of parallel modifications

This ambiguity leaves the utility of the TSS feature in a fully conforming application (i.e. one not relying on additional assertions by the implementation) greatly reduced. In particular, it prevents the usage of the thread specific storage feature for reliable resource cleanup

This proposal is submitted in relation to specification defect reports DR 416 (by the same author as this proposal) and DR 424. This proposal will look at existing implementations of thread specific storage and their behavior, and will then propose alterations to the C11 standard .

Existing implementations and specifications

This proposal will look at the specifications of thread specific storage and related mechanisms under two common platforms (POSIX.1 2008 and Microsoft Windows) and additionally at the defined behaviour of the C++11 international standard.

POSIX.1 2008

POSIX.1 implements thread specific storage under the POSIX Threads ("pthreads") API. It is implemented in terms of the type pthread_key_t, which mirrors tss_t, and four functions, which exactly mirror those provided by the C11 standard:

In addition, POSIX.1 defines the constant PTHREAD_DESTRUCTOR_ITERATIONS which has a description which presumably matches the intent of the C11 specification's TSS_DTOR_ITERATIONS constant.

POSIX.1 defines that, at thread exit time

• For each key which was created with a destructor, the value associated with the key will be set to NULL and the key's destructor will be invoked with the value that the key had immediately prior to being set to NULL
• The ordering of destructor calls for distinct keys is undefined
• If after invoking the destructor for each key created with one there remain keys with destructors which have values which are non-NULL, the process will be repeated up to PTHREAD_DESTRUCTOR_ITERATIONS times.
• POSIX.1 leaves undefined the behaviour of invoking pthread_exit (the function which exits a thread, analogously to C11's thrd_exit) from within a destructor

POSIX.1 defines that pthread_key_delete and exit do not cause destructor invocations.

POSIX.1 leaves undefined whether destructors for keys created or destroyed concurrently with a thread running destructors (in another thread, or from a destructor running on the thread executing destructors) will alter the set of destructors run by said thread. 

Microsoft Windows

Thread Specific Storage on Microsoft Windows is implemented in terms of four functions:

// Common Win32 API types, defined for those unfamiliar:
typedef uint32_t DWORD;
typedef void *LPVOID;
 
// Win32 TLS functions
DWORD TlsAlloc(void);
BOOL TlsFree(DWORD dwTlsIndex);
BOOL TlsSetValue(DWORD dwTlsIndex, LPVOID lpTlsValue);
LPVOID TlsGetValue(DWORD dwTlsIndex);

The function TlsAlloc is used in order to allocate a new thread specific storage "index.", Note that the Windows API does not directly have any concept of a thread local storage destructor. The TlsFree function is used to deallocate a thread specific storage index. The TlsSetValue and TlsGetValue functions, aside from the differences in the types involved and method of indicating errors, behave in the same manner as the tss_set and tss_get functions in the C11 standard.

Windows does not directly provide destructor support for thread specific storage objects, but does provide mechanisms by which they may be implemented:

• A dynamic link library ("DLL") may provide an entry point, conventionally called DllMain, which receives notifications on various events, including thread startup and termination
• On recent versions of Windows (5.1/XP, released 2001) and above, an executable may request to have similar notifications delivered to one or more functions by placing a structure pointing to them in a table placed in a specifically located executable segment

Either method may be used to implement thread local storage destructors (the former is used by, for example, the pthreads-win32 library to implement the POSIX.1 threading library on top of Windows).

A caveat which must be noted with either of the above methods of implementing thread specific storage destructors is that in both cases the notifications are delivered while the system holds a lock on an internal mutex (The "Loader lock", which is also taken internally during calls to certain functions exposed by the system)

C++2011

C++2011 introduces the language keyword thread_local, aligned to the thread_local macro introduced by <threads.h> and _Thread_local keyword in C11, which introduces an object of thread local storage duration. C++ objects have both constructors and destructors, and therefore must be allocated and constructed before first use in a thread, and destroyed and deallocated at thread exit.

C++ defines that thread local storage destructors are called 

• As a result of calling exit for objects associated with the thread that invoked exit, prior to commencing invocation of functions registered with atexit
• Upon return from main, for the objects associated with the process' initial thread (following the rule that an application which returns from main shall behave as if exit was invoked with the value returned)
• Upon thread exit (in unspecified order)

It is noted that the requirement that destructors be called from exit differs from that of POSIX.1. It is also noted that C++ does not need to invoke destructors multiple times because of the nature of C++ objects.

Proposed Behavior

The proposed behavior is to align the C specification behavior with POSIX.1

Implementation Considerations

The behavior of the thread specific storage primitives defined in the POSIX.1 specification are thought to be possible to implement on all platforms which support threads. On platforms which do not implement thread specific storage destructors natively, or which do so in a manner incompatible with the mechanisms defined by POSIX.1 can be handled either by

• Implementation of the invocation of thread specific storage destructors using a platform dependent mechanism, as done by pthreads-win32, referenced above
• Implementation of the invocation of thread specific storage destructors entirely within the C library

An example implementation would be for the C library to invoke destructors manually from within the thrd_exit function. This would be sufficient to support fully conforming C programs, though may not be useful for applications where some components may not use the C library threading primitives.

Proposed Technical Corrigendum

This proposed corrigendum is a lightly edited version of that originally proposed in DR 416:

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 part 2 from

The thrd_exit function terminates execution of the calling thread and sets its result code to res.

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.

Following this, the thrd_exit function terminates execution of the calling thread and sets its result code to res.

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 with thread specific storage are not invoked at process exit.

It is undefined to call tss_create from within a destructor invocation.

It is undefined if calls to tss_set, tss_get or tss_delete for a storage are valid on a thread if the tss_create call which allocated it completed after the thread commenced executing destructors.

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.

(This additionally clarifies whether or not a destructor will be invoked for a 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)

Changes since DR 416:

• Revert change to 2.26.5.5 part 3 to language in the existing international standard
• State that invoking tss_create from within a destructor invocation is undefined
• State that invoking tss_set, tss_get or tss_delete on a storage created after a thread has begun executing destructors from within that thread is undefined

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