Here's the side-effecting companion of freadptr.
2008-02-29 Bruno Haible <[EMAIL PROTECTED]>
New module 'freadseek'.
* modules/freadseek: New file.
* lib/freadseek.h: New file.
* lib/freadseek.c: New file.
* MODULES.html.sh (File stream based Input/Output): Add freadseek.
========================== modules/freadseek ===============================
Description:
freadseek() function: Read and discard input from a stream.
Files:
lib/freadseek.h
lib/freadseek.c
Depends-on:
freadahead
lseek
configure.ac:
Makefile.am:
lib_SOURCES += freadseek.c
Include:
"freadseek.h"
License:
LGPL
Maintainer:
Bruno Haible
=========================== lib/freadseek.h ================================
/* Skipping input from a FILE stream.
Copyright (C) 2007-2008 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 <http://www.gnu.org/licenses/>. */
#include <stddef.h>
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
/* Assuming the stream STREAM is open for reading:
Read and discard OFFSET bytes from STREAM.
freadseek (STREAM, OFFSET) is the same as
fseek (STREAM, OFFSET, SEEK_CUR), except that the latter does not work
on non-seekable input streams (such as pipes).
Upon success, return 0.
Upon premature end of stream, return 0 (like fseek does).
Upon error, set the error indicator in the stream and return EOF.
STREAM must not be wide-character oriented. */
extern int freadseek (FILE *stream, size_t offset);
#ifdef __cplusplus
}
#endif
=========================== lib/freadseek.c ================================
/* Skipping input from a FILE stream.
Copyright (C) 2007-2008 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 <http://www.gnu.org/licenses/>. */
#include <config.h>
/* Specification. */
#include "freadseek.h"
#include <stdlib.h>
#include <unistd.h>
#include "freadahead.h"
int
freadseek (FILE *fp, size_t offset)
{
size_t buffered;
int fd;
if (offset == 0)
return 0;
/* Increment the in-memory pointer. This is very cheap (no system calls). */
buffered = freadahead (fp);
if (buffered > 0)
{
size_t increment = (buffered < offset ? buffered : offset);
/* Keep this code in sync with freadahead and freadptr! */
#if defined _IO_ferror_unlocked /* GNU libc, BeOS */
fp->_IO_read_ptr += increment;
#elif defined __sferror /* FreeBSD, NetBSD, OpenBSD, MacOS X,
Cygwin */
fp->_p += increment;
fp->_r -= increment;
#elif defined _IOERR /* AIX, HP-UX, IRIX, OSF/1, Solaris, mingw
*/
# if defined __sun && defined _LP64 /* Solaris/{SPARC,AMD64} 64-bit */
# define fp_ ((struct { unsigned char *_ptr; \
unsigned char *_base; \
unsigned char *_end; \
long _cnt; \
int _file; \
unsigned int _flag; \
} *) fp)
fp_->_ptr += increment;
fp_->_cnt -= increment;
# else
fp->_ptr += increment;
fp->_cnt -= increment;
# endif
#elif defined __UCLIBC__ /* uClibc */
# ifdef __STDIO_BUFFERS
fp->__bufpos += increment;
# else
abort ();
# endif
#elif defined __QNX__ /* QNX */
fp->_Next += increment;
#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
offset -= increment;
if (offset == 0)
return 0;
}
/* 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 fseek (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)
{
if (ferror (fp))
/* EOF, or error before or while reading. */
return EOF;
else
/* Encountered EOF. */
return 0;
}
offset -= count;
}
while (offset > 0);
return 0;
}
}
