146 lines
4.4 KiB
C
146 lines
4.4 KiB
C
/* Skipping input from a FILE stream.
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Copyright (C) 2007-2022 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2.1 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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#include <config.h>
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/* Specification. */
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#include "freadseek.h"
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#include <stdlib.h>
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#include <unistd.h>
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#include "freadahead.h"
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#include "freadptr.h"
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#include "stdio-impl.h"
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/* Increment the in-memory pointer. INCREMENT must be at most the buffer size
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returned by freadptr().
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This is very cheap (no system calls). */
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static void
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freadptrinc (FILE *fp, size_t increment)
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{
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/* Keep this code in sync with freadptr! */
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#if HAVE___FREADPTRINC /* musl libc */
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__freadptrinc (fp, increment);
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#elif defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
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/* GNU libc, BeOS, Haiku, Linux libc5 */
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fp->_IO_read_ptr += increment;
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#elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
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/* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
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fp_->_p += increment;
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fp_->_r -= increment;
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#elif defined __EMX__ /* emx+gcc */
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fp->_ptr += increment;
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fp->_rcount -= increment;
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#elif defined __minix /* Minix */
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fp_->_ptr += increment;
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fp_->_count -= increment;
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#elif defined _IOERR /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
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fp_->_ptr += increment;
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fp_->_cnt -= increment;
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#elif defined __UCLIBC__ /* uClibc */
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# ifdef __STDIO_BUFFERS
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fp->__bufpos += increment;
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# else
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abort ();
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# endif
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#elif defined __QNX__ /* QNX */
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fp->_Next += increment;
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#elif defined __MINT__ /* Atari FreeMiNT */
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fp->__bufp += increment;
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#elif defined EPLAN9 /* Plan9 */
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fp->rp += increment;
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#elif defined SLOW_BUT_NO_HACKS /* users can define this */
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#else
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#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."
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#endif
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}
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int
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freadseek (FILE *fp, size_t offset)
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{
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size_t total_buffered;
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int fd;
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if (offset == 0)
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return 0;
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/* Seek over the already read and buffered input as quickly as possible,
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without doing any system calls. */
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total_buffered = freadahead (fp);
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/* This loop is usually executed at most twice: once for ungetc buffer (if
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present) and once for the main buffer. */
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while (total_buffered > 0)
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{
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size_t buffered;
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if (freadptr (fp, &buffered) != NULL && buffered > 0)
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{
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size_t increment = (buffered < offset ? buffered : offset);
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freadptrinc (fp, increment);
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offset -= increment;
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if (offset == 0)
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return 0;
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total_buffered -= increment;
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if (total_buffered == 0)
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break;
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}
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/* Read one byte. If we were reading from the ungetc buffer, this
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switches the stream back to the main buffer. */
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if (fgetc (fp) == EOF)
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goto eof;
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offset--;
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if (offset == 0)
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return 0;
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total_buffered--;
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}
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/* Test whether the stream is seekable or not. */
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fd = fileno (fp);
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if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
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{
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/* FP refers to a regular file. fseek is most efficient in this case. */
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return fseeko (fp, offset, SEEK_CUR);
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}
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else
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{
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/* FP is a non-seekable stream, possibly not even referring to a file
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descriptor. Read OFFSET bytes explicitly and discard them. */
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char buf[4096];
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do
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{
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size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
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if (fread (buf, 1, count, fp) < count)
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goto eof;
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offset -= count;
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}
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while (offset > 0);
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return 0;
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}
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eof:
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/* EOF, or error before or while reading. */
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if (ferror (fp))
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return EOF;
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else
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/* Encountered EOF. */
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return 0;
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}
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