lib/freadseek.c

   1 /* Skipping input from a FILE stream.
   2    Copyright (C) 2007-2024 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation; either version 2.1 of the
   7    License, or (at your option) any later version.
   8
   9    This file 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 Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public License
  15    along with this program.  If not, see <https://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_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
  40   /* GNU libc, BeOS, Haiku, Linux libc5 */
  41   fp->_IO_read_ptr += increment;
  42 #elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
  43   /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
  44   fp_->_p += increment;
  45   fp_->_r -= increment;
  46 #elif defined __EMX__               /* emx+gcc */
  47   fp->_ptr += increment;
  48   fp->_rcount -= increment;
  49 #elif defined __minix               /* Minix */
  50   fp_->_ptr += increment;
  51   fp_->_count -= increment;
  52 #elif defined _IOERR                /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
  53   fp_->_ptr += increment;
  54   fp_->_cnt -= increment;
  55 #elif defined __UCLIBC__            /* uClibc */
  56 # ifdef __STDIO_BUFFERS
  57   fp->__bufpos += increment;
  58 # else
  59   abort ();
  60 # endif
  61 #elif defined __QNX__               /* QNX */
  62   fp->_Next += increment;
  63 #elif defined __MINT__              /* Atari FreeMiNT */
  64   fp->__bufp += increment;
  65 #elif defined EPLAN9                /* Plan9 */
  66   fp->rp += increment;
  67 #elif defined SLOW_BUT_NO_HACKS     /* users can define this */
  68 #else
  69  #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."
  70 #endif
  71 }
  72
  73 int
  74 freadseek (FILE *fp, size_t offset)
  75 {
  76   size_t total_buffered;
  77   int fd;
  78
  79   if (offset == 0)
  80     return 0;
  81
  82   /* Seek over the already read and buffered input as quickly as possible,
  83      without doing any system calls.  */
  84   total_buffered = freadahead (fp);
  85   /* This loop is usually executed at most twice: once for ungetc buffer (if
  86      present) and once for the main buffer.  */
  87   while (total_buffered > 0)
  88     {
  89       size_t buffered;
  90
  91       if (freadptr (fp, &buffered) != NULL && buffered > 0)
  92         {
  93           size_t increment = (buffered < offset ? buffered : offset);
  94
  95           freadptrinc (fp, increment);
  96           offset -= increment;
  97           if (offset == 0)
  98             return 0;
  99           total_buffered -= increment;
 100           if (total_buffered == 0)
 101             break;
 102         }
 103       /* Read one byte.  If we were reading from the ungetc buffer, this
 104          switches the stream back to the main buffer.  */
 105       if (fgetc (fp) == EOF)
 106         goto eof;
 107       offset--;
 108       if (offset == 0)
 109         return 0;
 110       total_buffered--;
 111     }
 112
 113   /* Test whether the stream is seekable or not.  */
 114   fd = fileno (fp);
 115   if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
 116     {
 117       /* FP refers to a regular file.  fseek is most efficient in this case.  */
 118       return fseeko (fp, offset, SEEK_CUR);
 119     }
 120   else
 121     {
 122       /* FP is a non-seekable stream, possibly not even referring to a file
 123          descriptor.  Read OFFSET bytes explicitly and discard them.  */
 124       char buf[4096];
 125
 126       do
 127         {
 128           size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
 129           if (fread (buf, 1, count, fp) < count)
 130             goto eof;
 131           offset -= count;
 132         }
 133       while (offset > 0);
 134
 135       return 0;
 136    }
 137
 138  eof:
 139   /* EOF, or error before or while reading.  */
 140   if (ferror (fp))
 141     return EOF;
 142   else
 143     /* Encountered EOF.  */
 144     return 0;
 145 }