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   9 <title>16.11: Redirection (<TT>freopen</TT>)</title>
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  15 <H2>16.11: Redirection (<TT>freopen</TT>)</H2>
  16
  17 <p>For some programs,
  18 standard input and standard output are enough,
  19 and these programs can get by using just
  20 <TT>getchar</TT>, <TT>putchar</TT>, <TT>printf</TT>, etc.,
  21 and letting any input/output redirection be handled
  22 by the user and the operating system
  23 (perhaps using command-line redirection such as <TT>&lt;</TT> and <TT>&gt;</TT>).
  24 Other programs handle all file manipulation themselves,
  25 opening files with <TT>fopen</TT> and maintaining file pointer
  26 (<TT>FILE *</TT>) variables recording the streams to which all
  27 input and output is done
  28 (with <TT>getc</TT>, <TT>putc</TT>, <TT>fprintf</TT>, etc.).
  29 </p><p>Occasionally,
  30 a program has to be rewritten in a hurry,
  31 to allow it to read or write a named file
  32 without manipulating file pointers and changing
  33 every call to <TT>getchar</TT> to <TT>getc</TT>,
  34 every call to <TT>printf</TT> to <TT>fprintf</TT>, etc.
  35 In these cases, the function <TT>freopen</TT> comes in handy:
  36 it reopens an existing stream on a new file.
  37 The prototype is
  38 <pre>
  39         FILE *freopen(char *filename, char *mode, FILE *fp)
  40 </pre>
  41 <TT>freopen</TT> is about like <TT>fopen</TT>,
  42 except that rather than allocating a new stream,
  43 it uses
  44 (and returns)
  45 the caller-supplied stream <TT>fp</TT>.
  46 For example,
  47 to redirect a program's output to a file ``from within,''
  48 you could call
  49 <pre>
  50         freopen(filename, "w", stdout);
  51 </pre>
  52 </p><p>A disadvantage of <TT>freopen</TT>
  53 is that there's generally no way to undo it;
  54 you can't
  55 change your mind later and
  56 make <TT>stdin</TT> or <TT>stdout</TT> go back to where they had been
  57 before you called <TT>freopen</TT>.
  58 In situations where you want to be able to swich back and forth between streams,
  59 it's much better if you <em>can</em> chase down and
  60 change
  61 every call to <TT>getchar</TT> to <TT>getc</TT>,
  62 every call to <TT>printf</TT> to <TT>fprintf</TT>, etc.,
  63 and then use some <TT>FILE *</TT> variable under your control
  64 (typically with a name like <TT>ifp</TT> or <TT>ofp</TT>)
  65 so that you can set it
  66
  67 to point to a file by calling <TT>fopen</TT>,
  68 and later back to <TT>stdin</TT> or <TT>stdout</TT> by simply reassigning it.
  69 </p><hr>
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  78 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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