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<title>17.7.10 Concurrency support </title>
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<a name="1204656"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">17.7.10 Concurrency support</font>
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<a name="1204657"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The POSIX I/O code can be set to callback to user supplied mutex functions, which allows the internal data structures to be protected. These callbacks are registered using the function:</font>
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<a name="1204658"> </a><font size=2 face=Courier><strong>void* device_setmutexfn(DeviceMutexCallbackFn <font size=2 face=Courier><strong><em>fn</em></strong></font>, void* <font size=2 face=Courier><strong><em>data</em></strong></font>);<br></strong></font>
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<a name="1204659"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><font size=2 face=Courier><strong><em>fn</em></strong></font> must have the prototype:</font>
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<a name="1204660"> </a><font size=2 face=Courier><strong>void DeviceMutexCallbackFn(_ST_mutexop, void*);<br></strong></font>
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<a name="1204661"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">where <font size=2 face=Courier><strong>_ST_mutexop</strong></font> is <font size=2 face=Courier><strong>_ST_mutexop_lock</strong></font> to lock the mutex and <font size=2 face=Courier><strong>_ST_mutexop_unlock</strong></font> to unlock it.</font>
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<a name="1204662"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><em>Note: This lock is always released before calling into any functions in a device driver. This means that the device driver must be responsible for protecting its own data structures from concurrent access by multiple threads.</em></font>
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<a name="1204663"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">When OS20 is used, the application does not have to manage the POSIX I/O mutex; a mutex function is provided and registered with <font size=2 face=Courier><strong>device_setmutexfn()</strong></font> by the OS20 function <font size=2 face=Courier><strong>kernel_initialize()</strong></font>. However, the application may override this default mutex function after <font size=2 face=Courier><strong>kernel_initialize()</strong></font> has been called. The OS20 source file that implements concurrency support for device I/O is supplied in the file:</font>
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<a name="1204664"> </a><font size=2 face=Courier><strong>src/os20/clibprot/deviceprot.c<br></strong></font>
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<a name="1204665"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">To provide alternative mutual exclusion, or if using an operating system other than OS20, concurrency support may be set up manually.</font>
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<a name="1208117"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><em>Note: If setting up concurrency support for POSIX I/O functions then concurrency support must also be provided for the debug support functions if they are used. See <a href="libsdeb3.htm#1204431"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Section⌨️ 快捷键说明
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