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/* areadlinkat.c -- readlinkat wrapper to return malloc'd link name
Unlike xreadlinkat, only call exit on failure to change directory.
Copyright (C) 2001, 2003-2007, 2009-2010 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>,
and Eric Blake <ebb9@byu.net>. */
#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
#if HAVE_READLINKAT
/* 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 readlinkat to get the symbolic link value of FILENAME relative to FD.
Return a pointer to that NUL-terminated string in malloc'd storage.
If readlinkat fails, return NULL and set errno (although failure to
change directory will issue a diagnostic and exit).
If realloc fails, or if the link value is longer than SIZE_MAX :-),
return NULL and set errno to ENOMEM. */
char *
areadlinkat (int fd, 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 = readlinkat (fd, 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)
/* errno is 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)
/* errno is ENOMEM. */
return NULL;
}
}
#else /* !HAVE_READLINKAT */
/* It is more efficient to change directories only once and call
areadlink, rather than repeatedly call the replacement
readlinkat. */
# define AT_FUNC_NAME areadlinkat
# define AT_FUNC_F1 areadlink
# define AT_FUNC_POST_FILE_PARAM_DECLS /* empty */
# define AT_FUNC_POST_FILE_ARGS /* empty */
# define AT_FUNC_RESULT char *
# define AT_FUNC_FAIL NULL
# include "at-func.c"
# undef AT_FUNC_NAME
# undef AT_FUNC_F1
# undef AT_FUNC_POST_FILE_PARAM_DECLS
# undef AT_FUNC_POST_FILE_ARGS
# undef AT_FUNC_RESULT
# undef AT_FUNC_FAIL
#endif /* !HAVE_READLINKAT */
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