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<a name="1204479"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">17.3 Open file identification</font>
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<a name="1204480"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Once a file has been opened, it needs to be identified to subsequent operations.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204481"> </a>Open files at the ANSI layer are identified by a <font size=2 face=Courier><strong>FILE</strong></font> structure.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204482"> </a>Open files at the POSIX layer are identified by an integer value known as a <font size=2 face=Courier><strong><em>file descriptor</em></strong></font>. This value starts at 0.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204483"> </a>Device drivers may also define their own identifiers.</font></p>
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<a name="1204484"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">These values are allocated by the appropriate file open functions, and returned to the user. They are passed as parameters to other functions from the same API.</font>
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<a name="1204485"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">In general, a single open file should only be accessed using one technique. For example, if a file is opened using POSIX <font size=2 face=Courier><strong>open()</strong></font> then it should be read using POSIX <font size=2 face=Courier><strong>read()</strong></font>. Mixing access techniques may cause data to be lost as it can confuse buffering systems.</font>
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<a name="1204486"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">Standard files</font>
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<a name="1204487"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">When the C run-time system starts up, three standard files are automatically opened. These can be used at either the ANSI or POSIX levels.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204488"> </a>At the ANSI level three <font size=2 face=Courier><strong>FILE</strong></font> pointers exist: <font size=2 face=Courier><strong>stdin</strong></font> (standard input), <font size=2 face=Courier><strong>stdout</strong></font> (standard output) and <font size=2 face=Courier><strong>stderr</strong></font> (standard error output).</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204489"> </a>These correspond to the three POSIX level file handles 0, 1 and 2 respectively.</font></p>
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