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<title>17.7.2 Opening a device </title>
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<h3>
<a name="1204562"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">17.7.2 Opening a device</font>
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<p>
<a name="1204563"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The device may be opened with either the ANSI call, for example:</font>
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<a name="1204564"> </a><font size=2 face=Courier><strong>FILE *f;<br>f = fopen("mydevice:", "r");<br></strong></font>
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<a name="1204566"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">or the POSIX call, for example:</font>
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<a name="1204567"> </a><font size=2 face=Courier><strong>int fd;<br>fd = open("mydevice:", O_RDONLY);<br></strong></font>
<p>
<a name="1204569"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Each of these function calls executes a call to the open callback function <font size=2 face=Courier><strong>my_device_open</strong></font> with the following arguments:</font>
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<a name="1204570"> </a><font size=2 face=Courier><strong>long int r = my_device_open("", O_RDONLY);<br></strong></font>
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<a name="1204571"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This callback function performs device-specific functionality. The open callback should return <font size=2 face=Courier><strong>-1</strong></font> if it fails to open the device or any other long integer handle on success. The value of the handle returned on success is arbitrary and device-specific and it is passed to many of the callback functions registered in the <font size=2 face=Courier><strong>dev_op_t</strong></font> and <font size=2 face=Courier><strong>file_operations_t</strong></font> structures for operations on an open file. This allows a filesystem to be implemented on a device.</font>
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<a name="1208145"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">For example, the following code:</font>
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<a name="1204573"> </a><font size=2 face=Courier><strong>int fd, fd2;<br>fd = open("mydevice:/mydir/myfile", O_RDONLY);<br>fd2 = open("mydevice:/mydir/myfile_2", O_RDONLY);<br></strong></font>
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<a name="1204576"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">calls:</font>
</p>
<a name="1204577"> </a><font size=2 face=Courier><strong>m = my_device_open("/mydir/myfile", O_RDONLY);<br>n = my_device_open("/mydir/myfile_2", O_RDONLY);<br></strong></font>
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<a name="1204579"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">which return respectively, non-negative long integer handles <font size=2 face=Courier><strong>m</strong></font> and <font size=2 face=Courier><strong>n</strong></font> on success. See <a href="posix11.htm#1204585"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Section⌨️ 快捷键说明
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