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 <__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