docs/latex/wx/mutex.tex

   1 \section{\class{wxMutex}}\label{wxmutex}
   2
   3 A mutex object is a synchronization object whose state is set to signaled when
   4 it is not owned by any thread, and nonsignaled when it is owned. Its name comes
   5 from its usefulness in coordinating mutually-exclusive access to a shared
   6 resource. Only one thread at a time can own a mutex object but the mutexes are
   7 recursive in the sense that a thread can lock a mutex which it had already
   8 locked before (instead of dead locking the entire process in this situation by
   9 starting to wait on a mutex which will never be released while the thread is
  10 waiting).
  11
  12 For example, when several thread use the data stored in the linked list,
  13 modifications to the list should be only allowed to one thread at a time
  14 because during a new node addition the list integrity is temporarily broken
  15 (this is also called {\it program invariant}).
  16
  17 \wxheading{Example}
  18
  19 {\small%
  20 \begin{verbatim}
  21     // this variable has an "s_" prefix because it is static: seeing an "s_" in
  22     // a multithreaded program is in general a good sign that you should use a
  23     // mutex (or a critical section)
  24     static wxMutex *s_mutexProtectingTheGlobalData;
  25
  26     // we store some numbers in this global array which is presumably used by
  27     // several threads simultaneously
  28     wxArrayInt s_data;
  29
  30     void MyThread::AddNewNode(int num)
  31     {
  32         // ensure that no other thread accesses the list
  33         s_mutexProtectingTheGlobalList->Lock();
  34
  35         s_data.Add(num);
  36
  37         s_mutexProtectingTheGlobalList->Unlock();
  38     }
  39
  40     // return true the given number is greater than all array elements
  41     bool MyThread::IsGreater(int num)
  42     {
  43         // before using the list we must acquire the mutex
  44         wxMutexLocker lock(s_mutexProtectingTheGlobalData);
  45
  46         size_t count = s_data.Count();
  47         for ( size_t n = 0; n < count; n++ )
  48         {
  49             if ( s_data[n] > num )
  50                 return false;
  51         }
  52
  53         return true;
  54     }
  55 \end{verbatim}
  56 }
  57
  58 Notice how wxMutexLocker was used in the second function to ensure that the
  59 mutex is unlocked in any case: whether the function returns true or false
  60 (because the destructor of the local object {\it lock} is always called). Using
  61 this class instead of directly using wxMutex is, in general safer and is even
  62 more so if your program uses C++ exceptions.
  63
  64 \wxheading{Derived from}
  65
  66 None.
  67
  68 \wxheading{Include files}
  69
  70 <wx/thread.h>
  71
  72 \wxheading{See also}
  73
  74 \helpref{wxThread}{wxthread}, \helpref{wxCondition}{wxcondition},
  75 \helpref{wxMutexLocker}{wxmutexlocker}, \helpref{wxCriticalSection}{wxcriticalsection}
  76
  77 \latexignore{\rtfignore{\wxheading{Members}}}
  78
  79 \membersection{wxMutex::wxMutex}\label{wxmutexconstr}
  80
  81 \func{}{wxMutex}{\void}
  82
  83 Default constructor.
  84
  85 \membersection{wxMutex::\destruct{wxMutex}}
  86
  87 \func{}{\destruct{wxMutex}}{\void}
  88
  89 Destroys the wxMutex object.
  90
  91 \membersection{wxMutex::Lock}\label{wxmutexlock}
  92
  93 \func{wxMutexError}{Lock}{\void}
  94
  95 Locks the mutex object.
  96
  97 \wxheading{Return value}
  98
  99 One of:
 100
 101 \twocolwidtha{7cm}
 102 \begin{twocollist}\itemsep=0pt
 103 \twocolitem{{\bf wxMUTEX\_NO\_ERROR}}{There was no error.}
 104 \twocolitem{{\bf wxMUTEX\_DEAD\_LOCK}}{A deadlock situation was detected.}
 105 \twocolitem{{\bf wxMUTEX\_BUSY}}{The mutex is already locked by another thread.}
 106 \end{twocollist}
 107
 108 \membersection{wxMutex::TryLock}\label{wxmutextrylock}
 109
 110 \func{wxMutexError}{TryLock}{\void}
 111
 112 Tries to lock the mutex object. If it can't, returns immediately with an error.
 113
 114 \wxheading{Return value}
 115
 116 One of:
 117
 118 \twocolwidtha{7cm}
 119 \begin{twocollist}\itemsep=0pt
 120 \twocolitem{{\bf wxMUTEX\_NO\_ERROR}}{There was no error.}
 121 \twocolitem{{\bf wxMUTEX\_DEAD\_LOCK}}{A deadlock situation was detected.}
 122 \twocolitem{{\bf wxMUTEX\_BUSY}}{The mutex is already locked by another thread.}
 123 \end{twocollist}
 124
 125 \membersection{wxMutex::Unlock}\label{wxmutexunlock}
 126
 127 \func{wxMutexError}{Unlock}{\void}
 128
 129 Unlocks the mutex object.
 130
 131 \wxheading{Return value}
 132
 133 One of:
 134
 135 \twocolwidtha{7cm}
 136 \begin{twocollist}\itemsep=0pt
 137 \twocolitem{{\bf wxMUTEX\_NO\_ERROR}}{There was no error.}
 138 \twocolitem{{\bf wxMUTEX\_DEAD\_LOCK}}{A deadlock situation was detected.}
 139 \twocolitem{{\bf wxMUTEX\_BUSY}}{The mutex is already locked by another thread.}
 140 \twocolitem{{\bf wxMUTEX\_UNLOCKED}}{The calling thread tries to unlock an unlocked mutex.}
 141 \end{twocollist}
 142