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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.

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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 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);. This function is used for other std::print overloads that don't take an ostream& argument.

[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] 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] 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.

[debugging.utility] is_debugger_present

Libc++ implements std::is_debugger_present() differently depending on the host platform:

  • Linux: /proc/self/status is read for the TracerPid entry and if it is equal to 0, returns false. Otherwise, the command name of the tracer PID is looked up, and a best effort is made to identify whether the tracer process' name is a recognized debugger: “gdb”, “gdbserver” or “lldb-server”. False is returned if it is not in this list.

  • Darwin & FreeBSD: sysctl is called to retrieve the current process information and check if the P_TRACED flag is set in the returned info. On Darwin, kinfo_proc::kp_proc.p_flag is checked. On FreeBSD, kinfo_proc::ki_flag is checked.

  • AIX: Reads the file /proc/{current_pid}/status (subtituting current_pid for the process' PID) into a pstatus_t structure, then checks if the STRC flag is set in pstatus_t::pr_flag.

  • Windows: Calls IsDebuggerPresent().

  • Default: Returns false.

It is also defined as a weak symbol in the library to allow it to be replaceable, as per the standard, so a user may redefine it with their own implementation.

Listed in the index of implementation-defined behavior

The order of the entries matches the entries in the draft of the Standard.