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PEP 539 -- A New C-API for Thread-Local Storage in CPython

Title:A New C-API for Thread-Local Storage in CPython
Author:Erik M. Bray, Masayuki Yamamoto
BDFL-Delegate:Nick Coghlan
Type:Standards Track
Post-History:16-Dec-2016, 31-Aug-2017, 08-Sep-2017


The proposal is to add a new Thread Local Storage (TLS) API to CPython which would supersede use of the existing TLS API within the CPython interpreter, while deprecating the existing API. The new API is named the "Thread Specific Storage (TSS) API" (see Rationale for Proposed Solution for the origin of the name).

Because the existing TLS API is only used internally (it is not mentioned in the documentation, and the header that defines it, pythread.h, is not included in Python.h either directly or indirectly), this proposal probably only affects CPython, but might also affect other interpreter implementations (PyPy?) that implement parts of the CPython API.

This is motivated primarily by the fact that the old API uses int to represent TLS keys across all platforms, which is neither POSIX-compliant, nor portable in any practical sense [1].


Throughout this document the acronym "TLS" refers to Thread Local Storage and should not be confused with "Transportation Layer Security" protocols.


The current API for TLS used inside the CPython interpreter consists of 6 functions:

PyAPI_FUNC(int) PyThread_create_key(void)
PyAPI_FUNC(void) PyThread_delete_key(int key)
PyAPI_FUNC(int) PyThread_set_key_value(int key, void *value)
PyAPI_FUNC(void *) PyThread_get_key_value(int key)
PyAPI_FUNC(void) PyThread_delete_key_value(int key)
PyAPI_FUNC(void) PyThread_ReInitTLS(void)

These would be superseded by a new set of analogous functions:

PyAPI_FUNC(int) PyThread_tss_create(Py_tss_t *key)
PyAPI_FUNC(void) PyThread_tss_delete(Py_tss_t *key)
PyAPI_FUNC(int) PyThread_tss_set(Py_tss_t *key, void *value)
PyAPI_FUNC(void *) PyThread_tss_get(Py_tss_t *key)

The specification also adds a few new features:

  • A new type Py_tss_t--an opaque type the definition of which may depend on the underlying TLS implementation. It is defined:

    typedef struct {
        int _is_initialized;
        NATIVE_TSS_KEY_T _key;
    } Py_tss_t;

    where NATIVE_TSS_KEY_T is a macro whose value depends on the underlying native TLS implementation (e.g. pthread_key_t).

  • A constant default value for Py_tss_t variables, Py_tss_NEEDS_INIT.

  • Three new functions:

    PyAPI_FUNC(Py_tss_t *) PyThread_tss_alloc(void)
    PyAPI_FUNC(void) PyThread_tss_free(Py_tss_t *key)
    PyAPI_FUNC(int) PyThread_tss_is_created(Py_tss_t *key)

    The first two are needed for dynamic (de-)allocation of a Py_tss_t, particularly in extension modules built with Py_LIMITED_API, where static allocation of this type is not possible due to its implementation being opaque at build time. A value returned by PyThread_tss_alloc is in the same state as a value initialized with Py_tss_NEEDS_INIT, or NULL in the case of dynamic allocation failure. The behavior of PyThread_tss_free involves calling PyThread_tss_delete preventively, or is a no-op if the value pointed to by the key argument is NULL. PyThread_tss_is_created returns non-zero if the given Py_tss_t has been initialized (i.e. by PyThread_tss_create).

The new TSS API does not provide functions which correspond to PyThread_delete_key_value and PyThread_ReInitTLS, because these functions were needed only for CPython's now defunct built-in TLS implementation; that is the existing behavior of these functions is treated as follows: PyThread_delete_key_value(key) is equalivalent to PyThread_set_key_value(key, NULL), and PyThread_ReInitTLS() is a no-op [8].

The new PyThread_tss_ functions are almost exactly analogous to their original counterparts with a few minor differences: Whereas PyThread_create_key takes no arguments and returns a TLS key as an int, PyThread_tss_create takes a Py_tss_t* as an argument and returns an int status code. The behavior of PyThread_tss_create is undefined if the value pointed to by the key argument is not initialized by Py_tss_NEEDS_INIT. The returned status code is zero on success and non-zero on failure. The meanings of non-zero status codes are not otherwise defined by this specification.

Similarly the other PyThread_tss_ functions are passed a Py_tss_t* whereas previously the key was passed by value. This change is necessary, as being an opaque type, the Py_tss_t type could hypothetically be almost any size. This is especially necessary for extension modules built with Py_LIMITED_API, where the size of the type is not known. Except for PyThread_tss_free, the behaviors of PyThread_tss_ are undefined if the value pointed to by the key argument is NULL.

Moreover, because of the use of Py_tss_t instead of int, there are behaviors in the new API which differ from the existing API with regard to key creation and deletion. PyThread_tss_create can be called repeatedly on the same key--calling it on an already initialized key is a no-op and immediately returns success. Similarly for calling PyThread_tss_delete with an uninitialized key.

The behavior of PyThread_tss_delete is defined to change the key's initialization state to "uninitialized"--this allows, for example, statically allocated keys to be reset to a sensible state when restarting the CPython interpreter without terminating the process (e.g. embedding Python in an application) [12].

The old PyThread_*_key* functions will be marked as deprecated in the documentation, but will not generate runtime deprecation warnings.

Additionally, on platforms where sizeof(pthread_key_t) != sizeof(int), PyThread_create_key will return immediately with a failure status, and the other TLS functions will all be no-ops on such platforms.

Comparison of API Specification

API Thread Local Storage (TLS) Thread Specific Storage (TSS)
Version Existing New
Key Type int Py_tss_t (opaque type)
Handle Native Key cast to int conceal into internal field
Function Argument int Py_tss_t *
  • create key
  • delete key
  • set value
  • get value
  • delete value
  • reinitialize keys (after fork)
  • create key
  • delete key
  • set value
  • get value
  • (set NULL instead) [8]
  • (unnecessary) [8]
  • dynamically (de-)allocate key
  • check key's initialization state
Default Value (-1 as key creation failure) Py_tss_NEEDS_INIT
Requirement native threads (since CPython 3.7 [9]) native threads
Restriction No support for platforms where native TLS key is defined in a way that cannot be safely cast to int. Unable to statically allocate keys when Py_LIMITED_API is defined.


With the proposed changes, a TSS key is initialized like:

static Py_tss_t tss_key = Py_tss_NEEDS_INIT;
if (PyThread_tss_create(&tss_key)) {
    /* ... handle key creation failure ... */

The initialization state of the key can then be checked like:


The rest of the API is used analogously to the old API:

int the_value = 1;
if (PyThread_tss_get(&tss_key) == NULL) {
    PyThread_tss_set(&tss_key, (void *)&the_value);
    assert(PyThread_tss_get(&tss_key) != NULL);
/* ... once done with the key ... */

When Py_LIMITED_API is defined, a TSS key must be dynamically allocated:

static Py_tss_t *ptr_key = PyThread_tss_alloc();
if (ptr_key == NULL) {
    /* ... handle key allocation failure ... */
/* ... once done with the key ... */
ptr_key = NULL;

Platform Support Changes

A new "Native Thread Implementation" section will be added to PEP 11 that states:

  • As of CPython 3.7, all platforms are required to provide a native thread implementation (such as pthreads or Windows) to implement the TSS API. Any TSS API problems that occur in an implementation without native threads will be closed as "won't fix".


The primary problem at issue here is the type of the keys (int) used for TLS values, as defined by the original PyThread TLS API.

The original TLS API was added to Python by GvR back in 1997, and at the time the key used to represent a TLS value was an int, and so it has been to the time of writing. This used CPython's own TLS implementation which long remained unused, largely unchanged, in Python/thread.c. Support for implementation of the API on top of native thread implementations (pthreads and Windows) was added much later, and the built-in implementation has been deemed no longer necessary and has since been removed [9].

The problem with the choice of int to represent a TLS key, is that while it was fine for CPython's own TLS implementation, and happens to be compatible with Windows (which uses DWORD for the analogous data), it is not compatible with the POSIX standard for the pthreads API, which defines pthread_key_t as an opaque type not further defined by the standard (as with Py_tss_t described above) [14]. This leaves it up to the underlying implementation how a pthread_key_t value is used to look up thread-specific data.

This has not generally been a problem for Python's API, as it just happens that on Linux pthread_key_t is defined as an unsigned int, and so is fully compatible with Python's TLS API--pthread_key_t's created by pthread_create_key can be freely cast to int and back (well, not exactly, even this has some limitations as pointed out by issue #22206).

However, as issue #25658 points out, there are at least some platforms (namely Cygwin, CloudABI, but likely others as well) which have otherwise modern and POSIX-compliant pthreads implementations, but are not compatible with Python's API because their pthread_key_t is defined in a way that cannot be safely cast to int. In fact, the possibility of running into this problem was raised by MvL at the time pthreads TLS was added [2].

It could be argued that PEP-11 makes specific requirements for supporting a new, not otherwise officially-support platform (such as CloudABI), and that the status of Cygwin support is currently dubious. However, this creates a very high barrier to supporting platforms that are otherwise Linux- and/or POSIX-compatible and where CPython might otherwise "just work" except for this one hurdle. CPython itself imposes this implementation barrier by way of an API that is not compatible with POSIX (and in fact makes invalid assumptions about pthreads).

Rationale for Proposed Solution

The use of an opaque type (Py_tss_t) to key TLS values allows the API to be compatible with all present (POSIX and Windows) and future (C11?) native TLS implementations supported by CPython, as it allows the definition of Py_tss_t to depend on the underlying implementation.

Since the existing TLS API has been available in the limited API [13] for some platforms (e.g. Linux), CPython makes an effort to provide the new TSS API at that level likewise. Note, however, that the Py_tss_t definition becomes to be an opaque struct when Py_LIMITED_API is defined, because exposing NATIVE_TSS_KEY_T as part of the limited API would prevent us from switching native thread implementation without rebuilding extension modules.

A new API must be introduced, rather than changing the function signatures of the current API, in order to maintain backwards compatibility. The new API also more clearly groups together these related functions under a single name prefix, PyThread_tss_. The "tss" in the name stands for "thread-specific storage", and was influenced by the naming and design of the "tss" API that is part of the C11 threads API [15]. However, this is in no way meant to imply compatibility with or support for the C11 threads API, or signal any future intention of supporting C11--it's just the influence for the naming and design.

The inclusion of the special default value Py_tss_NEEDS_INIT is required by the fact that not all native TLS implementations define a sentinel value for uninitialized TLS keys. For example, on Windows a TLS key is represented by a DWORD (unsigned int) and its value must be treated as opaque [3]. So there is no unsigned integer value that can be safely used to represent an uninitialized TLS key on Windows. Likewise, POSIX does not specify a sentinel for an uninitialized pthread_key_t, instead relying on the pthread_once interface to ensure that a given TLS key is initialized only once per-process. Therefore, the Py_tss_t type contains an explicit ._is_initialized that can indicate the key's initialization state independent of the underlying implementation.

Changing PyThread_create_key to immediately return a failure status on systems using pthreads where sizeof(int) != sizeof(pthread_key_t) is intended as a sanity check: Currently, PyThread_create_key may report initial success on such systems, but attempts to use the returned key are likely to fail. Although in practice this failure occurs earlier in the interpreter initialization, it's better to fail immediately at the source of problem (PyThread_create_key) rather than sometime later when use of an invalid key is attempted. In other words, this indicates clearly that the old API is not supported on platforms where it cannot be used reliably, and that no effort will be made to add such support.

Rejected Ideas

  • Do nothing: The status quo is fine because it works on Linux, and platforms wishing to be supported by CPython should follow the requirements of PEP-11. As explained above, while this would be a fair argument if CPython were being to asked to make changes to support particular quirks or features of a specific platform, in this case it is a quirk of CPython that prevents it from being used to its full potential on otherwise POSIX-compliant platforms. The fact that the current implementation happens to work on Linux is a happy accident, and there's no guarantee that this will never change.
  • Affected platforms should just configure Python --without-threads: this is no longer an option as the --without-threads option has been removed for Python 3.7 [16].
  • Affected platforms should use CPython's built-in TLS implementation instead of a native TLS implementation: This is a more acceptable alternative to the previous idea, and in fact there had been a patch to do just that [4]. However, the built-in implementation being "slower and clunkier" in general than native implementations still needlessly hobbles performance on affected platforms. At least one other module (tracemalloc) is also broken if Python is built without a native TLS implementation. This idea also cannot be adopted because the built-in implementation has since been removed.
  • Keep the existing API, but work around the issue by providing a mapping from pthread_key_t values to int values. A couple attempts were made at this ([5], [6]), but this injects needless complexity and overhead into performance-critical code on platforms that are not currently affected by this issue (such as Linux). Even if use of this workaround were made conditional on platform compatibility, it introduces platform-specific code to maintain, and still has the problem of the previous rejected ideas of needlessly hobbling performance on affected platforms.


An initial version of a patch [7] is available on the bug tracker for this issue. Since the migration to GitHub, its development has continued in the pep539-tss-api feature branch [10] in Masayuki Yamamoto's fork of the CPython repository on GitHub. A work-in-progress PR is available at [11].

This reference implementation covers not only the new API implementation features, but also the client code updates needed to replace the existing TLS API with the new TSS API.