1 //===----------------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #ifndef _LIBCPP_REFSTRING_H
10 #define _LIBCPP_REFSTRING_H
11 
12 #include "atomic_support.h"
13 #include <__config>
14 #include <cstddef>
15 #include <cstring>
16 #include <stdexcept>
17 
18 // MacOS and iOS used to ship with libstdc++, and still support old applications
19 // linking against libstdc++. The libc++ and libstdc++ exceptions are supposed
20 // to be ABI compatible, such that they can be thrown from one library and caught
21 // in the other.
22 //
23 // For that reason, we must look for libstdc++ in the same process and if found,
24 // check the string stored in the exception object to see if it is the GCC empty
25 // string singleton before manipulating the reference count. This is done so that
26 // if an exception is created with a zero-length string in libstdc++, libc++abi
27 // won't try to delete the memory.
28 #if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
29 #  define _LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE
30 #  include <dlfcn.h>
31 #  include <mach-o/dyld.h>
32 #endif
33 
34 _LIBCPP_BEGIN_NAMESPACE_STD
35 
36 namespace __refstring_imp {
37 namespace {
38 typedef int count_t;
39 
40 struct _Rep_base {
41   std::size_t len;
42   std::size_t cap;
43   count_t count;
44 };
45 
46 inline _Rep_base* rep_from_data(const char* data_) noexcept {
47   char* data = const_cast<char*>(data_);
48   return reinterpret_cast<_Rep_base*>(data - sizeof(_Rep_base));
49 }
50 
51 inline char* data_from_rep(_Rep_base* rep) noexcept {
52   char* data = reinterpret_cast<char*>(rep);
53   return data + sizeof(*rep);
54 }
55 
56 #if defined(_LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE)
57 inline const char* compute_gcc_empty_string_storage() noexcept {
58   void* handle = dlopen("/usr/lib/libstdc++.6.dylib", RTLD_NOLOAD);
59   if (handle == nullptr)
60     return nullptr;
61   void* sym = dlsym(handle, "_ZNSs4_Rep20_S_empty_rep_storageE");
62   if (sym == nullptr)
63     return nullptr;
64   return data_from_rep(reinterpret_cast<_Rep_base*>(sym));
65 }
66 
67 inline const char* get_gcc_empty_string_storage() noexcept {
68   static const char* p = compute_gcc_empty_string_storage();
69   return p;
70 }
71 #endif
72 
73 } // namespace
74 } // namespace __refstring_imp
75 
76 using namespace __refstring_imp;
77 
78 inline __libcpp_refstring::__libcpp_refstring(const char* msg) {
79   std::size_t len = strlen(msg);
80   _Rep_base* rep  = static_cast<_Rep_base*>(::operator new(sizeof(*rep) + len + 1));
81   rep->len        = len;
82   rep->cap        = len;
83   rep->count      = 0;
84   char* data      = data_from_rep(rep);
85   std::memcpy(data, msg, len + 1);
86   __imp_ = data;
87 }
88 
89 inline __libcpp_refstring::__libcpp_refstring(const __libcpp_refstring& s) noexcept : __imp_(s.__imp_) {
90   if (__uses_refcount())
91     __libcpp_atomic_add(&rep_from_data(__imp_)->count, 1);
92 }
93 
94 inline __libcpp_refstring& __libcpp_refstring::operator=(__libcpp_refstring const& s) noexcept {
95   bool adjust_old_count     = __uses_refcount();
96   struct _Rep_base* old_rep = rep_from_data(__imp_);
97   __imp_                    = s.__imp_;
98   if (__uses_refcount())
99     __libcpp_atomic_add(&rep_from_data(__imp_)->count, 1);
100   if (adjust_old_count) {
101     if (__libcpp_atomic_add(&old_rep->count, count_t(-1)) < 0) {
102       ::operator delete(old_rep);
103     }
104   }
105   return *this;
106 }
107 
108 inline __libcpp_refstring::~__libcpp_refstring() {
109   if (__uses_refcount()) {
110     _Rep_base* rep = rep_from_data(__imp_);
111     if (__libcpp_atomic_add(&rep->count, count_t(-1)) < 0) {
112       ::operator delete(rep);
113     }
114   }
115 }
116 
117 inline bool __libcpp_refstring::__uses_refcount() const {
118 #if defined(_LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE)
119   return __imp_ != get_gcc_empty_string_storage();
120 #else
121   return true;
122 #endif
123 }
124 
125 _LIBCPP_END_NAMESPACE_STD
126 
127 #endif //_LIBCPP_REFSTRING_H
128