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| <h4 class="subsection">12.10.2 Using <code>ungetc</code> To Do Unreading</h4> |
| |
| <p>The function to unread a character is called <code>ungetc</code>, because it |
| reverses the action of <code>getc</code>. |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>ungetc</b> (<var>int c, FILE *stream</var>)<var><a name="index-ungetc-1004"></a></var><br> |
| <blockquote><p>The <code>ungetc</code> function pushes back the character <var>c</var> onto the |
| input stream <var>stream</var>. So the next input from <var>stream</var> will |
| read <var>c</var> before anything else. |
| |
| <p>If <var>c</var> is <code>EOF</code>, <code>ungetc</code> does nothing and just returns |
| <code>EOF</code>. This lets you call <code>ungetc</code> with the return value of |
| <code>getc</code> without needing to check for an error from <code>getc</code>. |
| |
| <p>The character that you push back doesn't have to be the same as the last |
| character that was actually read from the stream. In fact, it isn't |
| necessary to actually read any characters from the stream before |
| unreading them with <code>ungetc</code>! But that is a strange way to write a |
| program; usually <code>ungetc</code> is used only to unread a character that |
| was just read from the same stream. The GNU C library supports this |
| even on files opened in binary mode, but other systems might not. |
| |
| <p>The GNU C library only supports one character of pushback—in other |
| words, it does not work to call <code>ungetc</code> twice without doing input |
| in between. Other systems might let you push back multiple characters; |
| then reading from the stream retrieves the characters in the reverse |
| order that they were pushed. |
| |
| <p>Pushing back characters doesn't alter the file; only the internal |
| buffering for the stream is affected. If a file positioning function |
| (such as <code>fseek</code>, <code>fseeko</code> or <code>rewind</code>; see <a href="File-Positioning.html#File-Positioning">File Positioning</a>) is called, any pending pushed-back characters are |
| discarded. |
| |
| <p>Unreading a character on a stream that is at end of file clears the |
| end-of-file indicator for the stream, because it makes the character of |
| input available. After you read that character, trying to read again |
| will encounter end of file. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: wint_t <b>ungetwc</b> (<var>wint_t wc, FILE *stream</var>)<var><a name="index-ungetwc-1005"></a></var><br> |
| <blockquote><p>The <code>ungetwc</code> function behaves just like <code>ungetc</code> just that it |
| pushes back a wide character. |
| </p></blockquote></div> |
| |
| <p>Here is an example showing the use of <code>getc</code> and <code>ungetc</code> to |
| skip over whitespace characters. When this function reaches a |
| non-whitespace character, it unreads that character to be seen again on |
| the next read operation on the stream. |
| |
| <pre class="smallexample"> #include <stdio.h> |
| #include <ctype.h> |
| |
| void |
| skip_whitespace (FILE *stream) |
| { |
| int c; |
| do |
| /* <span class="roman">No need to check for </span><code>EOF</code><span class="roman"> because it is not</span> |
| <code>isspace</code><span class="roman">, and </span><code>ungetc</code><span class="roman"> ignores </span><code>EOF</code><span class="roman">.</span> */ |
| c = getc (stream); |
| while (isspace (c)); |
| ungetc (c, stream); |
| } |
| </pre> |
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