libcxx/src/include/refstring.h

   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