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/* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
Copyright (C) 2001, 2003, 2004 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 2, 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; see the file COPYING.
If not, write to the Free Software Foundation,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
/* Written by Jim Meyering <jim@meyering.net> */
#if HAVE_CONFIG_H
# include <config.h>
#endif
#include "xreadlink.h"
#include <stdio.h>
#include <errno.h>
#include <limits.h>
#include <sys/types.h>
#include <stdlib.h>
#if HAVE_UNISTD_H
# include <unistd.h>
#endif
#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)
#endif
#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
#endif
#define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
#include "xalloc.h"
/* Call readlink to get the symbolic link value of FILENAME.
SIZE is a hint as to how long the link is expected to be;
typically it is taken from st_size. It need not be correct.
Return a pointer to that NUL-terminated string in malloc'd storage.
If readlink fails, return NULL (caller may use errno to diagnose).
If malloc fails, or if the link value is longer than SSIZE_MAX :-),
give a diagnostic and exit. */
char *
xreadlink (char const *filename, size_t size)
{
/* The initial buffer size for the link value. A power of 2
detects arithmetic overflow earlier, but is not required. */
size_t buf_size = size < MAXSIZE ? size + 1 : MAXSIZE;
while (1)
{
char *buffer = xmalloc (buf_size);
ssize_t r = readlink (filename, buffer, buf_size);
size_t link_length = r;
/* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
with errno == ERANGE if the buffer is too small. */
if (r < 0 && errno != ERANGE)
{
int saved_errno = errno;
free (buffer);
errno = saved_errno;
return NULL;
}
if (link_length < buf_size)
{
buffer[link_length] = 0;
return buffer;
}
free (buffer);
if (buf_size <= MAXSIZE / 2)
buf_size *= 2;
else if (buf_size < MAXSIZE)
buf_size = MAXSIZE;
else
xalloc_die ();
}
}
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