blob: f54ff2a60b735ac4172744d9ced6274b3bf4c3db [file] [log] [blame]
// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#pragma once
#include <stddef.h>
#if __cplusplus >= 201103L && __has_include(<type_traits>)
#include <type_traits>
// This represents a UTF-8 string constant provided by the vDSO itself.
// This pointer remains valid and the string doesn't change for the
// life of the process (if not the system).
// This type exists to be the return value type for vDSO functions.
// In current machine ABIs, it's returned "for free" in two registers.
// To a C caller, these functions have ABIs indistinguishable from if
// they simply returned `const char*` so there is no overhead to
// supporting the explicit-length API as well as the traditional C
// string API, though it does require writing out `.c_str` in the
// source. C++ 17 callers can take advantage of direct coercion to
// the standard std::string_view and std::u8string_view types, which
// also allows e.g. direct construction of std::string.
typedef struct {
const char* c_str; // UTF-8, guaranteed to be '\0'-terminated.
size_t length; // Length, not including the '\0' terminator.
#ifdef __cplusplus
// This is ABI-identical to the usual implementation of std::string_view,
// when applied to NUL-terminated C strings. But this API doesn't presume
// that std::string_view has a particular implementation or exists at all.
// For convenience of use without directly using the C++ standard library
// API, a templatized implicit coercion is defined to types that have the
// API of std::string_view or std::u8string_view. With the most common
// implementations, this coercion will be compiled away to nothing.
template <
typename _T
#if __cplusplus >= 201103L && __has_include(<type_traits>)
typename = typename std::enable_if<sizeof(typename _T::value_type) == sizeof(char)>::type
operator _T() {
// It's preferable to exclude incompatible types via SFINAE so that
// the user's diagnostic experience is exactly as if no coercion
// operator existed. SFINAE should exclude this definition when a
// C++11 <type_traits> is available to define std::enable_if. If
// no standard C++ library header is available, this will provide
// a specific diagnostic.
static_assert(sizeof(typename _T::value_type) == sizeof(char),
"zx_string_view_t can be coerced to C++ 17 std::string_view"
" or std::u8string_view or types with equivalent API");
return {reinterpret_cast<typename _T::const_pointer>(c_str), length};
// Preferably zx_string_view_t values should just be coerced to
// std::string_view. But it provides the most minimal aspects of
// the equivalent API in case a return value expression is used
// directly as `zx_foo_string().data()`, for example.
using value_type = char;
using const_pointer = const char*;
using size_type = size_t;
const_pointer data() const { return c_str; }
size_type size() const { return length; }
} zx_string_view_t;