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/* Threshold at which to diagnose ELOOP. Generic version.
Copyright (C) 2012-2017 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#ifndef _ELOOP_THRESHOLD_H
#define _ELOOP_THRESHOLD_H 1
#include <limits.h>
#include <sys/param.h>
/* POSIX specifies SYMLOOP_MAX as the "Maximum number of symbolic
links that can be reliably traversed in the resolution of a
pathname in the absence of a loop." This makes it a minimum that
we should certainly accept. But it leaves open the possibility
that more might sometimes work--just not "reliably".
For example, Linux implements a complex policy whereby there is a
small limit on the number of direct symlink traversals (a symlink
to a symlink to a symlink), but larger limit on the total number of
symlink traversals overall. Hence the SYMLOOP_MAX number should be
the small one, but the limit library functions enforce on users
should be the larger one.
So, we use the larger of the reported SYMLOOP_MAX (if any) and our
own constant MIN_ELOOP_THRESHOLD, below. This constant should be
large enough that it never rules out a file name and directory tree
that the underlying system (i.e. calls to 'open' et al) would
resolve successfully. It should be small enough that actual loops
are detected without a huge number of iterations. */
#ifndef MIN_ELOOP_THRESHOLD
# define MIN_ELOOP_THRESHOLD 40
#endif
/* Return the maximum number of symlink traversals to permit
before diagnosing ELOOP. */
static inline unsigned int __attribute__ ((const))
__eloop_threshold (void)
{
#ifdef SYMLOOP_MAX
const int symloop_max = SYMLOOP_MAX;
#else
/* The function is marked 'const' even though we use memory and
call a function, because sysconf is required to return the
same value in every call and so it must always be safe to
call __eloop_threshold exactly once and reuse the value. */
static long int sysconf_symloop_max;
if (sysconf_symloop_max == 0)
sysconf_symloop_max = __sysconf (_SC_SYMLOOP_MAX);
const unsigned int symloop_max = (sysconf_symloop_max <= 0
? _POSIX_SYMLOOP_MAX
: sysconf_symloop_max);
#endif
return MAX (symloop_max, MIN_ELOOP_THRESHOLD);
}
#endif /* eloop-threshold.h */
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