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(libc.info)Unreading Idea

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What Unreading Means
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   Here is a pictorial explanation of unreading.  Suppose you have a
stream reading a file that contains just six characters, the letters
`foobar'.  Suppose you have read three characters so far.  The
situation looks like this:

     f  o  o  b  a  r
              ^

so the next input character will be `b'.

   If instead of reading `b' you unread the letter `o', you get a
situation like this:

     f  o  o  b  a  r
              |
           o--
           ^

so that the next input characters will be `o' and `b'.

   If you unread `9' instead of `o', you get this situation:

     f  o  o  b  a  r
              |
           9--
           ^

so that the next input characters will be `9' and `b'.