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  53. <a name="ungetc_002d_002d_002dpush-data-back-into-a-stream"></a>
  54. <h3 class="section">4.68 <code>ungetc</code>&mdash;push data back into a stream</h3>
  55. <a name="index-ungetc"></a>
  56. <a name="index-_005fungetc_005fr"></a>
  57. <p><strong>Synopsis</strong>
  58. </p><div class="example">
  59. <pre class="example">#include &lt;stdio.h&gt;
  60. int ungetc(int <var>c</var>, FILE *<var>stream</var>);
  61. int _ungetc_r(struct _reent *<var>reent</var>, int <var>c</var>, FILE *<var>stream</var>);
  62. </pre></div>
  63. <p><strong>Description</strong><br>
  64. <code>ungetc</code> is used to return bytes back to <var>stream</var> to be read again.
  65. If <var>c</var> is EOF, the stream is unchanged. Otherwise, the unsigned
  66. char <var>c</var> is put back on the stream, and subsequent reads will see
  67. the bytes pushed back in reverse order. Pushed byes are lost if the
  68. stream is repositioned, such as by <code>fseek</code>, <code>fsetpos</code>, or
  69. <code>rewind</code>.
  70. </p>
  71. <p>The underlying file is not changed, but it is possible to push back
  72. something different than what was originally read. Ungetting a
  73. character will clear the end-of-stream marker, and decrement the file
  74. position indicator. Pushing back beyond the beginning of a file gives
  75. unspecified behavior.
  76. </p>
  77. <p>The alternate function <code>_ungetc_r</code> is a reentrant version. The
  78. extra argument <var>reent</var> is a pointer to a reentrancy structure.
  79. </p>
  80. <br>
  81. <p><strong>Returns</strong><br>
  82. The character pushed back, or <code>EOF</code> on error.
  83. </p>
  84. <br>
  85. <p><strong>Portability</strong><br>
  86. ANSI C requires <code>ungetc</code>, but only requires a pushback buffer of one
  87. byte; although this implementation can handle multiple bytes, not all
  88. can. Pushing back a signed char is a common application bug.
  89. </p>
  90. <p>Supporting OS subroutines required: <code>sbrk</code>.
  91. </p>
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