lang/C/the.ansi.c.programming.language/c.programming.notes/sx12c.html

   1 <!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
   2 <!-- This collection of hypertext pages is Copyright 1995-7 by Steve Summit. -->
   3 <!-- This material may be freely redistributed and used -->
   4 <!-- but may not be republished or sold without permission. -->
   5 <html>
   6 <head>
   7 <link rev="owner" href="mailto:scs@eskimo.com">
   8 <link rev="made" href="mailto:scs@eskimo.com">
   9 <title>12.3 Predefined Streams</title>
  10 <link href="sx12b.html" rev=precedes>
  11 <link href="sx12d.html" rel=precedes>
  12 <link href="sx12.html" rev=subdocument>
  13 </head>
  14 <body>
  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
  60 </p><hr>
  61 <p>
  62 Read sequentially:
  63 <a href="sx12b.html" rev=precedes>prev</a>
  64 <a href="sx12d.html" rel=precedes>next</a>
  65 <a href="sx12.html" rev=subdocument>up</a>
  66 <a href="top.html">top</a>
  67 </p>
  68 <p>
  69 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
  70 // <a href="copyright.html">Copyright</a> 1995-1997
  71 // <a href="mailto:scs@eskimo.com">mail feedback</a>
  72 </p>
  73 </body>
  74 </html>