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/* areadlink.c -- readlink wrapper to return the link name in malloc'd storage
Unlike xreadlink and xreadlink_with_size, don't ever call exit.
Copyright (C) 2001, 2003-2007, 2009-2011 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/>. */
/* Written by Jim Meyering <jim@meyering.net>
and Bruno Haible <bruno@clisp.org>. */
#include <config.h>
/* Specification. */
#include "areadlink.h"
#include <errno.h>
#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
#endif
/* Use the system functions, not the gnulib overrides in this file. */
#undef malloc
#undef realloc
/* The initial buffer size for the link value. A power of 2
detects arithmetic overflow earlier, but is not required. */
enum {
INITIAL_BUF_SIZE = 1024
};
/* Call readlink to get the symbolic link value of FILENAME.
Return a pointer to that NUL-terminated string in malloc'd storage.
If readlink fails, return NULL and set errno.
If realloc fails, or if the link value is longer than SIZE_MAX :-),
return NULL and set errno to ENOMEM. */
char *
areadlink (char const *filename)
{
/* Allocate the initial buffer on the stack. This way, in the common
case of a symlink of small size, we get away with a single small malloc()
instead of a big malloc() followed by a shrinking realloc(). */
char initial_buf[INITIAL_BUF_SIZE];
char *buffer = initial_buf;
size_t buf_size = sizeof initial_buf;
while (1)
{
/* Attempt to read the link into the current buffer. */
ssize_t link_length = readlink (filename, buffer, buf_size);
/* 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 (link_length < 0 && errno != ERANGE)
{
if (buffer != initial_buf)
{
int saved_errno = errno;
free (buffer);
errno = saved_errno;
}
return NULL;
}
if ((size_t) link_length < buf_size)
{
buffer[link_length++] = '\0';
/* Return it in a chunk of memory as small as possible. */
if (buffer == initial_buf)
{
buffer = (char *) malloc (link_length);
if (buffer == NULL)
{
/* It's easier to set errno to ENOMEM than to rely on the
'malloc-posix' gnulib module. */
errno = ENOMEM;
return NULL;
}
memcpy (buffer, initial_buf, link_length);
}
else
{
/* Shrink buffer before returning it. */
if ((size_t) link_length < buf_size)
{
char *smaller_buffer = (char *) realloc (buffer, link_length);
if (smaller_buffer != NULL)
buffer = smaller_buffer;
}
}
return buffer;
}
if (buffer != initial_buf)
free (buffer);
buf_size *= 2;
if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
{
errno = ENOMEM;
return NULL;
}
buffer = (char *) malloc (buf_size);
if (buffer == NULL)
{
/* It's easier to set errno to ENOMEM than to rely on the
'malloc-posix' gnulib module. */
errno = ENOMEM;
return NULL;
}
}
}
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