libc/stdio/ungetc.c

   1 /* Copyright (C) 2004       Manuel Novoa III    <mjn3@codepoet.org>
   2  *
   3  * GNU Library General Public License (LGPL) version 2 or later.
   4  *
   5  * Dedicated to Toni.  See uClibc/DEDICATION.mjn3 for details.
   6  */
   7
   8 #include "_stdio.h"
   9
  10 /* Having ungotten characters implies the stream is reading.
  11  * The scheme used here treats the least significant 2 bits of
  12  * the stream's modeflags member as follows:
  13  *   0 0   Not currently reading.
  14  *   0 1   Reading, but no ungetc() or scanf() push back chars.
  15  *   1 0   Reading with one ungetc() char (ungot[1] is 1)
  16  *         or one scanf() pushed back char (ungot[1] is 0).
  17  *   1 1   Reading with both an ungetc() char and a scanf()
  18  *         pushed back char.  Note that this must be the result
  19  *         of a scanf() push back (in ungot[0]) _followed_ by
  20  *         an ungetc() call (in ungot[1]).
  21  *
  22  * Notes:
  23  *   scanf() can NOT use ungetc() to push back characters.
  24  *     (See section 7.19.6.2 of the C9X rationale -- WG14/N897.)
  25  */
  26
  27 int ungetc(int c, register FILE *stream)
  28 {
  29         __STDIO_AUTO_THREADLOCK_VAR;
  30
  31         __STDIO_AUTO_THREADLOCK(stream);
  32         __STDIO_STREAM_VALIDATE(stream);
  33
  34 #ifdef __UCLIBC_HAS_STDIO_GETC_MACRO__
  35         /* If buffered narrow reading with no ungot slots filled, and if not
  36          * ungetting a different char than the one last read from the buffer,
  37          * we can simply decrement the position and not worry about disabling
  38          * the getc macros.  This will cut down on overhead in applications
  39          * that use getc/ungetc extensively (like gcc). */
  40         /* NOTE: If we can use getc, then we are buffered narrow reading with
  41          * no ungot slots filled. */
  42         if (__STDIO_STREAM_CAN_USE_BUFFER_GET(stream)
  43                 && (c != EOF)
  44                 && (stream->__bufpos > stream->__bufstart)
  45                 && (stream->__bufpos[-1] == ((unsigned char)c))
  46                 ) {
  47                 --stream->__bufpos;
  48                 __STDIO_STREAM_CLEAR_EOF(stream); /* Must clear end-of-file flag. */
  49         } else
  50 #endif
  51         /* Note: Even if c == EOF, we need to initialize/verify the
  52          * stream's orientation and ensure the stream is in reading
  53          * mode (if readable and properly oriented). */
  54         if ((!__STDIO_STREAM_IS_NARROW_READING(stream)
  55                  && __STDIO_STREAM_TRANS_TO_READ(stream, __FLAG_NARROW))
  56                 || ((stream->__modeflags & __FLAG_UNGOT)
  57                         && ((stream->__modeflags & 1) || stream->__ungot[1]))
  58                 ) {
  59                 c = EOF;
  60         } else if (c != EOF) {
  61                 __STDIO_STREAM_DISABLE_GETC(stream);
  62
  63                 /* Flag this as a user ungot, as scanf does the necessary fixup. */
  64                 stream->__ungot[1] = 1;
  65                 stream->__ungot[(++stream->__modeflags) & 1] = c;
  66
  67                 __STDIO_STREAM_CLEAR_EOF(stream); /* Must clear end-of-file flag. */
  68         }
  69
  70         __STDIO_STREAM_VALIDATE(stream);
  71         __STDIO_AUTO_THREADUNLOCK(stream);
  72
  73         return c;
  74 }
  75 libc_hidden_def(ungetc)