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(libc.info)Unreading

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Unreading
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   In parser programs it is often useful to examine the next character
in the input stream without removing it from the stream.  This is called
"peeking ahead" at the input because your program gets a glimpse of the
input it will read next.

   Using stream I/O, you can peek ahead at input by first reading it and
then "unreading" it (also called  "pushing it back" on the stream).
Unreading a character makes it available to be input again from the
stream, by  the next call to `fgetc' or other input function on that
stream.

Unreading Idea

An explanation of unreading with pictures.

How Unread

How to call `ungetc' to do unreading.