Eric EricWF

Introduction

This document provides a detailed exploration of how C++ standard library headers are used when building, testing, and using the LLVM libc++ library. It covers the different types of headers involved, the main header layouts used, and the importance of include paths and how they are constructed. It also dives into the specific include paths used in key scenarios on Linux and Apple platforms. The goal is to explain the key concepts and complexities around libc++ headers to support informed decision making about the library's header layout.

The Headers

There are several types of headers involved in the libc++ library:

Unify the behavior of value-initialization and default-initialization.

Introduction

Currently default-initialization and value-initialization act bizzarely compared to each other. Specifically default-initialization is required to call trivial constructors when value-initialization is not.

For example:

struct T { int x; };
// default_init is initialized by a call to the trivial default ctor.

	v4l2-compliance SHA: not available, 64 bits

	Compliance test for uvcvideo device /dev/video0:

	Driver Info:
	Driver name : uvcvideo
	Card type : Live Streaming USB Device: Live
	Bus info : usb-0000:00:14.0-6
	Driver version : 5.2.17
	Capabilities : 0x84a00001

	// RUN: $CXX -c %s -o /tmp/first.o -std=c++17 -g
	// RUN: $CXX -c %s -o /tmp/second.o -std=c++17 -g -DWITH_MAIN
	// RUN: $CXX -o /tmp/test.out /tmp/first.o /tmp/second.o
	// RUN: /tmp/test.out

	#include <cassert>

	#define ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x) #x
	#define ABSL_INTERNAL_UNIQUE_SMALL_NAME1(x) ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x)
	#define ABSL_INTERNAL_UNIQUE_SMALL_NAME() \

	// RUN: $CXX -c %s -o /tmp/first.o -std=c++17 -g
	// RUN: $CXX -c %s -o /tmp/second.o -std=c++17 -g -DWITH_MAIN
	// RUN: $CXX -o /tmp/test.out /tmp/first.o /tmp/second.o
	// RUN: /tmp/test.out

	#include <cassert>

	namespace {
	__attribute__((noinline))
	int bar() asm(".my-name");

	#ifdef SYS_HEADER
	#pragma GCC system_header
	#endif

	template <class Ret, class Fn, class ...Args>
	constexpr Ret foo(Fn fn, Args... args) { return fn(args...); }

	32000 : __front_spare() == 384 : __back_spare() == 383 : __capacity() == 32767 : bytes allocated == 32840
	31000 : __front_spare() == 884 : __back_spare() == 883 : __capacity() == 32767 : bytes allocated == 32840
	30000 : __front_spare() == 1384 : __back_spare() == 1383 : __capacity() == 32767 : bytes allocated == 32840
	29000 : __front_spare() == 1884 : __back_spare() == 1883 : __capacity() == 32767 : bytes allocated == 32840
	28000 : __front_spare() == 2384 : __back_spare() == 2383 : __capacity() == 32767 : bytes allocated == 32840
	27000 : __front_spare() == 2884 : __back_spare() == 2883 : __capacity() == 32767 : bytes allocated == 32840
	26000 : __front_spare() == 3384 : __back_spare() == 3383 : __capacity() == 32767 : bytes allocated == 32840
	25000 : __front_spare() == 3884 : __back_spare() == 3883 : __capacity() == 32767 : bytes allocated == 32840
	24000 : __front_spare() == 288 : __back_spare() == 287 : __capacity() == 24575 : bytes allocated == 24648
	23000 : __front_spare() == 788 : __back_spare() == 787 : _


	Passing Tests (481):
	libc++ :: std/algorithms/alg.c.library/tested_elsewhere.pass.cpp
	libc++ :: std/atomics/atomics.fences/atomic_signal_fence.pass.cpp
	libc++ :: std/atomics/atomics.fences/atomic_thread_fence.pass.cpp
	libc++ :: std/atomics/atomics.flag/atomic_flag_clear.pass.cpp
	libc++ :: std/atomics/atomics.flag/atomic_flag_clear_explicit.pass.cpp
	libc++ :: std/atomics/atomics.flag/atomic_flag_test_and_set.pass.cpp
	libc++ :: std/atomics/atomics.flag/atomic_flag_test_and_set_explicit.pass.cpp
	libc++ :: std/atomics/atomics.flag/clear.pass.cpp

	// RUN: %clang_cc1 -triple x86_64-apple-darwin10.0.0 -fresumable-functions -emit-llvm -o - %s -fexceptions -std=c++17 \| FileCheck %s

	resumable int foo() {
	return 42;
	}

	void bar() {
	resumable auto r = foo();
	bool b = r.ready();
	int x = r.result();

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