1 /* Skipping input from a FILE stream.
2 Copyright (C) 2007-2013 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 HAVE___FREADPTRINC /* musl libc */
38 __freadptrinc (fp, increment);
39 #elif defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
40 fp->_IO_read_ptr += increment;
41 #elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin */
44 #elif defined __EMX__ /* emx+gcc */
45 fp->_ptr += increment;
46 fp->_rcount -= increment;
47 #elif defined __minix /* Minix */
48 fp_->_ptr += increment;
49 fp_->_count -= increment;
50 #elif defined _IOERR /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, mingw, NonStop Kernel */
51 fp_->_ptr += increment;
52 fp_->_cnt -= increment;
53 #elif defined __UCLIBC__ /* uClibc */
54 # ifdef __STDIO_BUFFERS
55 fp->__bufpos += increment;
59 #elif defined __QNX__ /* QNX */
60 fp->_Next += increment;
61 #elif defined __MINT__ /* Atari FreeMiNT */
62 fp->__bufp += increment;
63 #elif defined EPLAN9 /* Plan9 */
65 #elif defined SLOW_BUT_NO_HACKS /* users can define this */
67 #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."
72 freadseek (FILE *fp, size_t offset)
74 size_t total_buffered;
80 /* Seek over the already read and buffered input as quickly as possible,
81 without doing any system calls. */
82 total_buffered = freadahead (fp);
83 /* This loop is usually executed at most twice: once for ungetc buffer (if
84 present) and once for the main buffer. */
85 while (total_buffered > 0)
89 if (freadptr (fp, &buffered) != NULL && buffered > 0)
91 size_t increment = (buffered < offset ? buffered : offset);
93 freadptrinc (fp, increment);
97 total_buffered -= increment;
98 if (total_buffered == 0)
101 /* Read one byte. If we were reading from the ungetc buffer, this
102 switches the stream back to the main buffer. */
103 if (fgetc (fp) == EOF)
111 /* Test whether the stream is seekable or not. */
113 if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
115 /* FP refers to a regular file. fseek is most efficient in this case. */
116 return fseeko (fp, offset, SEEK_CUR);
120 /* FP is a non-seekable stream, possibly not even referring to a file
121 descriptor. Read OFFSET bytes explicitly and discard them. */
126 size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
127 if (fread (buf, 1, count, fp) < count)
137 /* EOF, or error before or while reading. */
141 /* Encountered EOF. */