1 /* Skipping input from a FILE stream.
2 Copyright (C) 2007-2012 Free Software Foundation, Inc.
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.
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.
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/>. */
20 #include "freadseek.h"
25 #include "freadahead.h"
28 #include "stdio-impl.h"
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). */
34 freadptrinc (FILE *fp, size_t increment)
36 /* Keep this code in sync with freadptr! */
37 #if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
38 fp->_IO_read_ptr += increment;
39 #elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, MacOS X, Cygwin */
42 #elif defined __EMX__ /* emx+gcc */
43 fp->_ptr += increment;
44 fp->_rcount -= increment;
45 #elif defined __minix /* Minix */
46 fp_->_ptr += increment;
47 fp_->_count -= increment;
48 #elif defined _IOERR /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, mingw, NonStop Kernel */
49 fp_->_ptr += increment;
50 fp_->_cnt -= increment;
51 #elif defined __UCLIBC__ /* uClibc */
52 # ifdef __STDIO_BUFFERS
53 fp->__bufpos += increment;
57 #elif defined __QNX__ /* QNX */
58 fp->_Next += increment;
59 #elif defined __MINT__ /* Atari FreeMiNT */
60 fp->__bufp += increment;
61 #elif defined EPLAN9 /* Plan9 */
63 #elif defined SLOW_BUT_NO_HACKS /* users can define this */
65 #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 freadseek (FILE *fp, size_t offset)
72 size_t total_buffered;
78 /* Seek over the already read and buffered input as quickly as possible,
79 without doing any system calls. */
80 total_buffered = freadahead (fp);
81 /* This loop is usually executed at most twice: once for ungetc buffer (if
82 present) and once for the main buffer. */
83 while (total_buffered > 0)
87 if (freadptr (fp, &buffered) != NULL && buffered > 0)
89 size_t increment = (buffered < offset ? buffered : offset);
91 freadptrinc (fp, increment);
95 total_buffered -= increment;
96 if (total_buffered == 0)
99 /* Read one byte. If we were reading from the ungetc buffer, this
100 switches the stream back to the main buffer. */
101 if (fgetc (fp) == EOF)
109 /* Test whether the stream is seekable or not. */
111 if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
113 /* FP refers to a regular file. fseek is most efficient in this case. */
114 return fseeko (fp, offset, SEEK_CUR);
118 /* FP is a non-seekable stream, possibly not even referring to a file
119 descriptor. Read OFFSET bytes explicitly and discard them. */
124 size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
125 if (fread (buf, 1, count, fp) < count)
135 /* EOF, or error before or while reading. */
139 /* Encountered EOF. */