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 <__config>
13 #include <stdexcept>
14 #include <cstddef>
15 #include <cstring>
16 #include "atomic_support.h"
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__) || \
29     defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
30 #   define _LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE
31 #   include <dlfcn.h>
32 #   include <mach-o/dyld.h>
33 #endif
34 
35 _LIBCPP_BEGIN_NAMESPACE_STD
36 
37 namespace __refstring_imp { 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
58 const char* compute_gcc_empty_string_storage() noexcept
59 {
60     void* handle = dlopen("/usr/lib/libstdc++.6.dylib", RTLD_NOLOAD);
61     if (handle == nullptr)
62         return nullptr;
63     void* sym = dlsym(handle, "_ZNSs4_Rep20_S_empty_rep_storageE");
64     if (sym == nullptr)
65         return nullptr;
66     return data_from_rep(reinterpret_cast<_Rep_base *>(sym));
67 }
68 
69 inline
70 const char*
71 get_gcc_empty_string_storage() noexcept
72 {
73     static const char* p = compute_gcc_empty_string_storage();
74     return p;
75 }
76 #endif
77 
78 }} // namespace __refstring_imp
79 
80 using namespace __refstring_imp;
81 
82 inline
83 __libcpp_refstring::__libcpp_refstring(const char* msg) {
84     std::size_t len = strlen(msg);
85     _Rep_base* rep = static_cast<_Rep_base *>(::operator new(sizeof(*rep) + len + 1));
86     rep->len = len;
87     rep->cap = len;
88     rep->count = 0;
89     char *data = data_from_rep(rep);
90     std::memcpy(data, msg, len + 1);
91     __imp_ = data;
92 }
93 
94 inline
95 __libcpp_refstring::__libcpp_refstring(const __libcpp_refstring &s) noexcept
96     : __imp_(s.__imp_)
97 {
98     if (__uses_refcount())
99         __libcpp_atomic_add(&rep_from_data(__imp_)->count, 1);
100 }
101 
102 inline
103 __libcpp_refstring& __libcpp_refstring::operator=(__libcpp_refstring const& s) noexcept {
104     bool adjust_old_count = __uses_refcount();
105     struct _Rep_base *old_rep = rep_from_data(__imp_);
106     __imp_ = s.__imp_;
107     if (__uses_refcount())
108         __libcpp_atomic_add(&rep_from_data(__imp_)->count, 1);
109     if (adjust_old_count)
110     {
111         if (__libcpp_atomic_add(&old_rep->count, count_t(-1)) < 0)
112         {
113             ::operator delete(old_rep);
114         }
115     }
116     return *this;
117 }
118 
119 inline
120 __libcpp_refstring::~__libcpp_refstring() {
121     if (__uses_refcount()) {
122         _Rep_base* rep = rep_from_data(__imp_);
123         if (__libcpp_atomic_add(&rep->count, count_t(-1)) < 0) {
124             ::operator delete(rep);
125         }
126     }
127 }
128 
129 inline
130 bool __libcpp_refstring::__uses_refcount() const {
131 #if defined(_LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE)
132     return __imp_ != get_gcc_empty_string_storage();
133 #else
134     return true;
135 #endif
136 }
137 
138 _LIBCPP_END_NAMESPACE_STD
139 
140 #endif //_LIBCPP_REFSTRING_H
141