mirror of
https://git.FreeBSD.org/src.git
synced 2024-12-24 11:29:10 +00:00
3c87aa1d3d
load of _l suffixed versions of various standard library functions that use the global locale, making them take an explicit locale parameter. Also adds support for per-thread locales. This work was funded by the FreeBSD Foundation. Please test any code you have that uses the C standard locale functions! Reviewed by: das (gdtoa changes) Approved by: dim (mentor)
160 lines
7.3 KiB
Plaintext
160 lines
7.3 KiB
Plaintext
$FreeBSD$
|
|
|
|
Design of xlocale
|
|
=================
|
|
|
|
The xlocale APIs come from Darwin, although a subset is now part of POSIX 2008.
|
|
They fall into two broad categories:
|
|
|
|
- Manipulation of per-thread locales (POSIX)
|
|
- Locale-aware functions taking an explicit locale argument (Darwin)
|
|
|
|
This document describes the implementation of these APIs for FreeBSD.
|
|
|
|
Goals
|
|
-----
|
|
|
|
The overall goal of this implementation is to be compatible with the Darwin
|
|
version. Additionally, it should include minimal changes to the existing
|
|
locale code. A lot of the existing locale code originates with 4BSD or earlier
|
|
and has had over a decade of testing. Replacing this code, unless absolutely
|
|
necessary, gives us the potential for more bugs without much benefit.
|
|
|
|
With this in mind, various libc-private functions have been modified to take a
|
|
locale_t parameter. This causes a compiler error if they are accidentally
|
|
called without a locale. This approach was taken, rather than adding _l
|
|
variants of these functions, to make it harder for accidental uses of the
|
|
global-locale versions to slip in.
|
|
|
|
Locale Objects
|
|
--------------
|
|
|
|
A locale is encapsulated in a `locale_t`, which is an opaque type: a pointer to
|
|
a `struct _xlocale`. The name `_xlocale` is unfortunate, as it does not fit
|
|
well with existing conventions, but is used because this is the name the Darwin
|
|
implementation gives to this structure and so may be used by existing (bad) code.
|
|
|
|
This structure should include all of the information corresponding to a locale.
|
|
A locale_t is almost immutable after creation. There are no functions that modify it,
|
|
and it can therefore be used without locking. It is the responsibility of the
|
|
caller to ensure that a locale is not deallocated during a call that uses it.
|
|
|
|
Each locale contains a number of components, one for each of the categories
|
|
supported by `setlocale()`. These are likewise immutable after creation. This
|
|
differs from the Darwin implementation, which includes a deprecated
|
|
`setinvalidrune()` function that can modify the rune locale.
|
|
|
|
The exception to these mutability rules is a set of `mbstate_t` flags stored
|
|
with each locale. These are used by various functions that previously had a
|
|
static local `mbstate_t` variable.
|
|
|
|
The components are reference counted, and so can be aliased between locale
|
|
objects. This makes copying locales very cheap.
|
|
|
|
The Global Locale
|
|
-----------------
|
|
|
|
All locales and locale components are reference counted. The global locale,
|
|
however, is special. It, and all of its components, are static and so no
|
|
malloc() memory is required when using a single locale.
|
|
|
|
This means that threads using the global locale are subject to the same
|
|
constraints as with the pre-xlocale libc. Calls to any locale-aware functions
|
|
in threads using the global locale, while modifying the global locale, have
|
|
undefined behaviour.
|
|
|
|
Because of this, we have to ensure that we always copy the components of the
|
|
global locale, rather than alias them.
|
|
|
|
It would be cleaner to simply remove the special treatment of the global locale
|
|
and have a locale_t lazily allocated for the global context. This would cost a
|
|
little more `malloc()` memory, so is not done in the initial version.
|
|
|
|
Caching
|
|
-------
|
|
|
|
The existing locale implementation included several ad-hoc caching layers.
|
|
None of these were thread safe. Caching is only really of use for supporting
|
|
the pattern where the locale is briefly changed to something and then changed
|
|
back.
|
|
|
|
The current xlocale implementation removes the caching entirely. This pattern
|
|
is not one that should be encouraged. If you need to make some calls with a
|
|
modified locale, then you should use the _l suffix versions of the calls,
|
|
rather than switch the global locale. If you do need to temporarily switch the
|
|
locale and then switch it back, `uselocale()` provides a way of doing this very
|
|
easily: It returns the old locale, which can then be passed to a subsequent
|
|
call to `uselocale()` to restore it, without the need to load any locale data
|
|
from the disk.
|
|
|
|
If, in the future, it is determined that caching is beneficial, it can be added
|
|
quite easily in xlocale.c. Given, however, that any locale-aware call is going
|
|
to be a preparation for presenting data to the user, and so is invariably going
|
|
to be part of an I/O operation, this seems like a case of premature
|
|
optimisation.
|
|
|
|
localeconv
|
|
----------
|
|
|
|
The `localeconv()` function is an exception to the immutable-after-creation
|
|
rule. In the classic implementation, this function returns a pointer to some
|
|
global storage, which is initialised with the data from the current locale.
|
|
This is not possible in a multithreaded environment, with multiple locales.
|
|
|
|
Instead, each locale contains a `struct lconv` that is lazily initialised on
|
|
calls to `localeconv()`. This is not protected by any locking, however this is
|
|
still safe on any machine where word-sized stores are atomic: two concurrent
|
|
calls will write the same data into the structure.
|
|
|
|
Explicit Locale Calls
|
|
---------------------
|
|
|
|
A large number of functions have been modified to take an explicit `locale_t`
|
|
parameter. The old APIs are then reimplemented with a call to `__get_locale()`
|
|
to supply the `locale_t` parameter. This is in line with the Darwin public
|
|
APIs, but also simplifies the modifications to these functions. The
|
|
`__get_locale()` function is now the only way to access the current locale
|
|
within libc. All of the old globals have gone, so there is now a linker error
|
|
if any functions attempt to use them.
|
|
|
|
The ctype.h functions are a little different. These are not implemented in
|
|
terms of their locale-aware versions, for performance reasons. Each of these
|
|
is implemented as a short inline function.
|
|
|
|
Differences to Darwin APIs
|
|
--------------------------
|
|
|
|
`strtoq_l()` and `strtouq_l() `are not provided. These are extensions to
|
|
deprecated functions - we should not be encouraging people to use deprecated
|
|
interfaces.
|
|
|
|
Locale Placeholders
|
|
-------------------
|
|
|
|
The pointer values 0 and -1 have special meanings as `locale_t` values. Any
|
|
public function that accepts a `locale_t` parameter must use the `FIX_LOCALE()`
|
|
macro on it before using it. For efficiency, this can be emitted in functions
|
|
which *only* use their locale parameter as an argument to another public
|
|
function, as the callee will do the `FIX_LOCALE()` itself.
|
|
|
|
Potential Improvements
|
|
----------------------
|
|
|
|
Currently, the current rune set is accessed via a function call. This makes it
|
|
fairly expensive to use any of the ctype.h functions. We could improve this
|
|
quite a lot by storing the rune locale data in a __thread-qualified variable.
|
|
|
|
Several of the existing FreeBSD locale-aware functions appear to be wrong. For
|
|
example, most of the `strto*()` family should probably use `digittoint_l()`,
|
|
but instead they assume ASCII. These will break if using a character encoding
|
|
that does not put numbers and the letters A-F in the same location as ASCII.
|
|
Some functions, like `strcoll()` only work on single-byte encodings. No
|
|
attempt has been made to fix existing limitations in the libc functions other
|
|
than to add support for xlocale.
|
|
|
|
Intuitively, setting a thread-local locale should ensure that all locale-aware
|
|
functions can be used safely from that thread. In fact, this is not the case
|
|
in either this implementation or the Darwin one. You must call `duplocale()`
|
|
or `newlocale()` before calling `uselocale()`. This is a bit ugly, and it
|
|
would be better if libc ensure that every thread had its own locale object.
|