1 // mutex -*- C++ -*-
2 
3 // Copyright (C) 2008-2021 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library.  This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 #include <mutex>
26 
27 #ifdef _GLIBCXX_HAS_GTHREADS
28 
29 namespace std _GLIBCXX_VISIBILITY(default)
30 {
31 _GLIBCXX_BEGIN_NAMESPACE_VERSION
32 
33 #ifdef _GLIBCXX_HAVE_TLS
34   __thread void* __once_callable;
35   __thread void (*__once_call)();
36 
__once_proxy()37   extern "C" void __once_proxy()
38   {
39     // The caller stored a function pointer in __once_call. If it requires
40     // any state, it gets it from __once_callable.
41     __once_call();
42   }
43 
44 #else // ! TLS
45 
46   // Explicit instantiation due to -fno-implicit-instantiation.
47   template class function<void()>;
48 
49   function<void()> __once_functor;
50 
51   mutex&
52   __get_once_mutex()
53   {
54     static mutex once_mutex;
55     return once_mutex;
56   }
57 
58 namespace
59 {
60   // Store ptr in a global variable and return the previous value.
61   inline unique_lock<mutex>*
62   set_lock_ptr(unique_lock<mutex>* ptr)
63   {
64     static unique_lock<mutex>* __once_functor_lock_ptr = nullptr;
65     return std::__exchange(__once_functor_lock_ptr, ptr);
66   }
67 }
68 
69   // code linked against ABI 3.4.12 and later uses this
70   void
71   __set_once_functor_lock_ptr(unique_lock<mutex>* __ptr)
72   {
73     (void) set_lock_ptr(__ptr);
74   }
75 
76   // unsafe - retained for compatibility with ABI 3.4.11
77   unique_lock<mutex>&
78   __get_once_functor_lock()
79   {
80     static unique_lock<mutex> once_functor_lock(__get_once_mutex(), defer_lock);
81     return once_functor_lock;
82   }
83 
84   // This is called via pthread_once while __get_once_mutex() is locked.
85   extern "C" void
86   __once_proxy()
87   {
88     // Get the callable out of the global functor.
89     function<void()> callable = std::move(__once_functor);
90 
91     // Then unlock the global mutex
92     if (unique_lock<mutex>* lock = set_lock_ptr(nullptr))
93     {
94       // Caller is using the new ABI and provided a pointer to its lock.
95       lock->unlock();
96     }
97     else
98       __get_once_functor_lock().unlock();  // global lock
99 
100     // Finally, invoke the callable.
101     callable();
102   }
103 #endif // ! TLS
104 
105 _GLIBCXX_END_NAMESPACE_VERSION
106 } // namespace std
107 
108 #endif // _GLIBCXX_HAS_GTHREADS
109