/* 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, 2009-2011 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>,
   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 */