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/* Skipping input from a FILE stream.
Copyright (C) 2007-2023 Free Software Foundation, Inc.
This file is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
This file is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
#include <config.h>
/* Specification. */
#include "freadseek.h"
#include <stdlib.h>
#include <unistd.h>
#include "freadahead.h"
#include "freadptr.h"
#include "stdio-impl.h"
/* Increment the in-memory pointer. INCREMENT must be at most the buffer size
returned by freadptr().
This is very cheap (no system calls). */
static void
freadptrinc (FILE *fp, size_t increment)
{
/* Keep this code in sync with freadptr! */
#if HAVE___FREADPTRINC /* musl libc */
__freadptrinc (fp, increment);
#elif defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
/* GNU libc, BeOS, Haiku, Linux libc5 */
fp->_IO_read_ptr += increment;
#elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
/* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
fp_->_p += increment;
fp_->_r -= increment;
#elif defined __EMX__ /* emx+gcc */
fp->_ptr += increment;
fp->_rcount -= increment;
#elif defined __minix /* Minix */
fp_->_ptr += increment;
fp_->_count -= increment;
#elif defined _IOERR /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
fp_->_ptr += increment;
fp_->_cnt -= increment;
#elif defined __UCLIBC__ /* uClibc */
# ifdef __STDIO_BUFFERS
fp->__bufpos += increment;
# else
abort ();
# endif
#elif defined __QNX__ /* QNX */
fp->_Next += increment;
#elif defined __MINT__ /* Atari FreeMiNT */
fp->__bufp += increment;
#elif defined EPLAN9 /* Plan9 */
fp->rp += increment;
#elif defined SLOW_BUT_NO_HACKS /* users can define this */
#else
#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."
#endif
}
int
freadseek (FILE *fp, size_t offset)
{
size_t total_buffered;
int fd;
if (offset == 0)
return 0;
/* Seek over the already read and buffered input as quickly as possible,
without doing any system calls. */
total_buffered = freadahead (fp);
/* This loop is usually executed at most twice: once for ungetc buffer (if
present) and once for the main buffer. */
while (total_buffered > 0)
{
size_t buffered;
if (freadptr (fp, &buffered) != NULL && buffered > 0)
{
size_t increment = (buffered < offset ? buffered : offset);
freadptrinc (fp, increment);
offset -= increment;
if (offset == 0)
return 0;
total_buffered -= increment;
if (total_buffered == 0)
break;
}
/* Read one byte. If we were reading from the ungetc buffer, this
switches the stream back to the main buffer. */
if (fgetc (fp) == EOF)
goto eof;
offset--;
if (offset == 0)
return 0;
total_buffered--;
}
/* Test whether the stream is seekable or not. */
fd = fileno (fp);
if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
{
/* FP refers to a regular file. fseek is most efficient in this case. */
return fseeko (fp, offset, SEEK_CUR);
}
else
{
/* FP is a non-seekable stream, possibly not even referring to a file
descriptor. Read OFFSET bytes explicitly and discard them. */
char buf[4096];
do
{
size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
if (fread (buf, 1, count, fp) < count)
goto eof;
offset -= count;
}
while (offset > 0);
return 0;
}
eof:
/* EOF, or error before or while reading. */
if (ferror (fp))
return EOF;
else
/* Encountered EOF. */
return 0;
}
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