146 lines
4.4 KiB
C
146 lines
4.4 KiB
C
/* Skipping input from a FILE stream.
|
|
Copyright (C) 2007-2020 Free Software Foundation, Inc.
|
|
|
|
This program is free software: you can redistribute it and/or modify
|
|
it under the terms of the GNU General Public License as published by
|
|
the Free Software Foundation; either version 3 of the License, or
|
|
(at your option) any later version.
|
|
|
|
This program 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 General Public License for more details.
|
|
|
|
You should have received a copy of the GNU 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, 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;
|
|
}
|