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   9 <title>12.3 Predefined Streams</title>
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  15 <H2>12.3 Predefined Streams</H2>
  16
  17 <p>Besides the file pointers which we explicitly open by calling <TT>fopen</TT>,
  18 there are also three predefined streams.
  19 <TT>stdin</TT> is a constant file pointer corresponding to standard input,
  20 and <TT>stdout</TT> is
  21 a constant file pointer corresponding to standard output.
  22 Both of these can be used anywhere a file pointer is called for;
  23 for example,
  24 <TT>getchar()</TT> is the same as <TT>getc(stdin)</TT>
  25 and <TT>putchar(c)</TT> is the same as <TT>putc(c, stdout)</TT>.
  26 The third predefined stream is <TT>stderr</TT>.
  27 Like <TT>stdout</TT>,
  28 <TT>stderr</TT> is typically connected to the screen by default.
  29 The difference is that <TT>stderr</TT> is not redirected
  30 when the standard output is redirected.
  31 For example,
  32 under Unix or MS-DOS,
  33 when you invoke
  34 <pre>
  35         program &gt; filename
  36 </pre>
  37 anything printed to <TT>stdout</TT>
  38 is redirected to the file <TT>filename</TT>,
  39 but anything printed to <TT>stderr</TT> still goes to the screen.
  40 The intent behind <TT>stderr</TT> is
  41 that it is the ``standard error output'';
  42 error messages printed to it will not disappear into an output file.
  43 For example,
  44 a more realistic way to print an error message
  45 when a file can't be opened would be
  46 <pre>
  47         if((ifp = fopen(filename, "r")) == NULL)
  48                 {
  49                 fprintf(stderr, "can't open file %s\n", filename);
  50                 <I>exit or return</I>
  51                 }
  52 </pre>
  53 where <TT>filename</TT> is a string variable
  54 indicating the file name to be opened.
  55 Not only is the error message printed to <TT>stderr</TT>,
  56 but it is also more informative
  57 in that it mentions the name of the file that couldn't be opened.
  58 (We'll see another example in the next chapter.)
  59
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  69 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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