774ea531ea
When closing a filebuf, we would previously call setbuf(0, 0). Not only does this get rid of any underlying buffer set by the user, but it also sets the unbuffered mode. This means that if the filebuf is then reopened to keep writing to a file, it would have lost track of the original user-provided buffer (or the default one which has a size of 4096), and instead it would use the unbuffered mode, which is terribly slow. This led to a bug report where users were complaining that closing and reopening the filebuf led to a significantly worse performance than using it without having closed and reopened. While this is a slightly unusual usage pattern, it should definitely work. rdar://161833214
85 lines
3.6 KiB
ReStructuredText
85 lines
3.6 KiB
ReStructuredText
.. _implementation-defined-behavior:
|
|
|
|
===============================
|
|
Implementation-defined behavior
|
|
===============================
|
|
|
|
This document contains the implementation details of the implementation-defined behavior in libc++.
|
|
The C++ standard mandates that implementation-defined behavior is documented.
|
|
|
|
.. note:
|
|
This page is far from complete.
|
|
|
|
|
|
Implementation-defined behavior
|
|
===============================
|
|
|
|
Updating the Time Zone Database
|
|
-------------------------------
|
|
|
|
The C++ standard allows implementations to automatically update the
|
|
*remote time zone database*. Libc++ opts not to do that. Instead calling
|
|
|
|
- ``std::chrono::remote_version()`` will update the version information of the
|
|
*remote time zone database*,
|
|
- ``std::chrono::reload_tzdb()``, if needed, will update the entire
|
|
*remote time zone database*.
|
|
|
|
This offers a way for users to update the *remote time zone database* and
|
|
give them full control over the process.
|
|
|
|
|
|
`[ostream.formatted.print]/3 <http://eel.is/c++draft/ostream.formatted.print#3>`_ A terminal capable of displaying Unicode
|
|
--------------------------------------------------------------------------------------------------------------------------
|
|
|
|
The C++ standard specifies that the manner in which a stream is determined to refer
|
|
to a terminal capable of displaying Unicode is implementation-defined. This is
|
|
used for ``std::print`` and similar functions taking an ``ostream&`` argument.
|
|
|
|
Libc++ determines that a stream is Unicode-capable terminal by:
|
|
|
|
* First it determines whether the stream's ``rdbuf()`` has an underlying
|
|
``FILE*``. This is ``true`` in the following cases:
|
|
|
|
* The stream is ``std::cout``, ``std::cerr``, or ``std::clog``.
|
|
|
|
* A ``std::basic_filebuf<CharT, Traits>`` derived from ``std::filebuf``.
|
|
|
|
* The way to determine whether this ``FILE*`` refers to a terminal capable of
|
|
displaying Unicode is the same as specified for `void vprint_unicode(FILE*
|
|
stream, string_view fmt, format_args args);
|
|
<http://eel.is/c++draft/print.fun#7>`_. This function is used for other
|
|
``std::print`` overloads that don't take an ``ostream&`` argument.
|
|
|
|
`[sf.cmath] <https://wg21.link/sf.cmath>`_ Mathematical Special Functions: Large indices
|
|
----------------------------------------------------------------------------------------
|
|
|
|
Most functions within the Mathematical Special Functions section contain integral indices.
|
|
The C++ standard specifies the result for larger indices as implementation-defined.
|
|
Libc++ pursuits reasonable results by choosing the same formulas as for indices below that threshold.
|
|
E.g.,
|
|
|
|
- ``std::hermite(unsigned n, T x)`` for ``n >= 128``
|
|
|
|
|
|
`[filebuf.virtuals] <https://eel.is/c++draft/filebuf.virtual>`_ Effect of calling ``basic_filebuf::setbuf`` with nonzero arguments
|
|
----------------------------------------------------------------------------------------------------------------------------------
|
|
|
|
Libc++ uses the provided buffer as the underlying buffer for input and output, and
|
|
does not discard that buffer even when the stream is closed.
|
|
|
|
|
|
`[stringbuf.cons] <http://eel.is/c++draft/stringbuf.cons>`_ Whether sequence pointers are initialized to null pointers
|
|
----------------------------------------------------------------------------------------------------------------------
|
|
|
|
Libc++ does not initialize the pointers to null pointers. It resizes the buffer
|
|
to its capacity and uses that size. This means the SSO buffer of
|
|
``std::string`` is used as initial output buffer.
|
|
|
|
|
|
Listed in the index of implementation-defined behavior
|
|
======================================================
|
|
|
|
The order of the entries matches the entries in the
|
|
`draft of the Standard <http://eel.is/c++draft/impldefindex>`_.
|