lib/freadseek.c

   1 /* Skipping input from a FILE stream.
   2    Copyright (C) 2007-2017 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  16
  17 #include <config.h>
  18
  19 /* Specification.  */
  20 #include "freadseek.h"
  21
  22 #include <stdlib.h>
  23 #include <unistd.h>
  24
  25 #include "freadahead.h"
  26 #include "freadptr.h"
  27
  28 #include "stdio-impl.h"
  29
  30 /* Increment the in-memory pointer.  INCREMENT must be at most the buffer size
  31    returned by freadptr().
  32    This is very cheap (no system calls).  */
  33 static void
  34 freadptrinc (FILE *fp, size_t increment)
  35 {
  36   /* Keep this code in sync with freadptr!  */
  37 #if HAVE___FREADPTRINC              /* musl libc */
  38   __freadptrinc (fp, increment);
  39 #elif defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
  40   fp->_IO_read_ptr += increment;
  41 #elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
  42   /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
  43   fp_->_p += increment;
  44   fp_->_r -= increment;
  45 #elif defined __EMX__               /* emx+gcc */
  46   fp->_ptr += increment;
  47   fp->_rcount -= increment;
  48 #elif defined __minix               /* Minix */
  49   fp_->_ptr += increment;
  50   fp_->_count -= increment;
  51 #elif defined _IOERR                /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, mingw, MSVC, NonStop Kernel, OpenVMS */
  52   fp_->_ptr += increment;
  53   fp_->_cnt -= increment;
  54 #elif defined __UCLIBC__            /* uClibc */
  55 # ifdef __STDIO_BUFFERS
  56   fp->__bufpos += increment;
  57 # else
  58   abort ();
  59 # endif
  60 #elif defined __QNX__               /* QNX */
  61   fp->_Next += increment;
  62 #elif defined __MINT__              /* Atari FreeMiNT */
  63   fp->__bufp += increment;
  64 #elif defined EPLAN9                /* Plan9 */
  65   fp->rp += increment;
  66 #elif defined SLOW_BUT_NO_HACKS     /* users can define this */
  67 #else
  68  #error "Please port gnulib freadseek.c to your platform! Look at the definition of getc, getc_unlocked on your system, then report this to bug-gnulib."
  69 #endif
  70 }
  71
  72 int
  73 freadseek (FILE *fp, size_t offset)
  74 {
  75   size_t total_buffered;
  76   int fd;
  77
  78   if (offset == 0)
  79     return 0;
  80
  81   /* Seek over the already read and buffered input as quickly as possible,
  82      without doing any system calls.  */
  83   total_buffered = freadahead (fp);
  84   /* This loop is usually executed at most twice: once for ungetc buffer (if
  85      present) and once for the main buffer.  */
  86   while (total_buffered > 0)
  87     {
  88       size_t buffered;
  89
  90       if (freadptr (fp, &buffered) != NULL && buffered > 0)
  91         {
  92           size_t increment = (buffered < offset ? buffered : offset);
  93
  94           freadptrinc (fp, increment);
  95           offset -= increment;
  96           if (offset == 0)
  97             return 0;
  98           total_buffered -= increment;
  99           if (total_buffered == 0)
 100             break;
 101         }
 102       /* Read one byte.  If we were reading from the ungetc buffer, this
 103          switches the stream back to the main buffer.  */
 104       if (fgetc (fp) == EOF)
 105         goto eof;
 106       offset--;
 107       if (offset == 0)
 108         return 0;
 109       total_buffered--;
 110     }
 111
 112   /* Test whether the stream is seekable or not.  */
 113   fd = fileno (fp);
 114   if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
 115     {
 116       /* FP refers to a regular file.  fseek is most efficient in this case.  */
 117       return fseeko (fp, offset, SEEK_CUR);
 118     }
 119   else
 120     {
 121       /* FP is a non-seekable stream, possibly not even referring to a file
 122          descriptor.  Read OFFSET bytes explicitly and discard them.  */
 123       char buf[4096];
 124
 125       do
 126         {
 127           size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
 128           if (fread (buf, 1, count, fp) < count)
 129             goto eof;
 130           offset -= count;
 131         }
 132       while (offset > 0);
 133
 134       return 0;
 135    }
 136
 137  eof:
 138   /* EOF, or error before or while reading.  */
 139   if (ferror (fp))
 140     return EOF;
 141   else
 142     /* Encountered EOF.  */
 143     return 0;
 144 }