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/* Read symbolic links into a buffer without size limitation, relative to fd.

   Copyright (C) 2001, 2003-2004, 2007, 2009-2011 Free Software Foundation,

   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
   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 Paul Eggert, Bruno Haible, and Jim Meyering.  */

#include <config.h>

#include "careadlinkat.h"

#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/* Define this independently so that stdint.h is not a prerequisite.  */
#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)

#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))

#include "allocator.h"

/* Get the symbolic link value of FILENAME and put it into BUFFER, with
   size BUFFER_SIZE.  This function acts like readlink  but has
   readlinkat's signature.  */
careadlinkatcwd (int fd, char const *filename, char *buffer,
                 size_t buffer_size)
  /* FD must be AT_FDCWD here, otherwise the caller is using this
     function in contexts for which it was not meant for.  */
  if (fd != AT_FDCWD)
    abort ();
  return readlink (filename, buffer, buffer_size);

/* Assuming the current directory is FD, get the symbolic link value
   of FILENAME as a null-terminated string and put it into a buffer.
   If FD is AT_FDCWD, FILENAME is interpreted relative to the current
   working directory, as in openat.

   If the link is small enough to fit into BUFFER put it there.
   BUFFER's size is BUFFER_SIZE, and BUFFER can be null
   if BUFFER_SIZE is zero.

   If the link is not small, put it into a dynamically allocated
   buffer managed by ALLOC.  It is the caller's responsibility to free
   the returned value if it is nonnull and is not BUFFER.  A null
   ALLOC stands for the standard allocator.

   The PREADLINKAT function specifies how to read links.  It operates
   like POSIX readlinkat()
   but can assume that its first argument is the same as FD.

   If successful, return the buffer address; otherwise return NULL and
   set errno.  */

char *
careadlinkat (int fd, char const *filename,
              char *buffer, size_t buffer_size,
              struct allocator const *alloc,
              ssize_t (*preadlinkat) (int, char const *, char *, size_t))
  char *buf;
  size_t buf_size;
  size_t buf_size_max =
    SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
  char stack_buf[1024];

  if (! alloc)
    alloc = &stdlib_allocator;

  if (! buffer_size)
      /* 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().  */
      buffer = stack_buf;
      buffer_size = sizeof stack_buf;

  buf = buffer;
  buf_size = buffer_size;

      /* Attempt to read the link into the current buffer.  */
      ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
      size_t link_size;
      if (link_length < 0)
          /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
             with errno == ERANGE if the buffer is too small.  */
          int readlinkat_errno = errno;
          if (readlinkat_errno != ERANGE)
              if (buf != buffer)
                  alloc->free (buf);
                  errno = readlinkat_errno;
              return NULL;

      link_size = link_length;

      if (link_size < buf_size)
          buf[link_size++] = '\0';

          if (buf == stack_buf)
              char *b = (char *) alloc->allocate (link_size);
              buf_size = link_size;
              if (! b)
              memcpy (b, buf, link_size);
              buf = b;
          else if (link_size < buf_size && buf != buffer && alloc->reallocate)
              /* Shrink BUF before returning it.  */
              char *b = (char *) alloc->reallocate (buf, link_size);
              if (b)
                buf = b;

          return buf;

      if (buf != buffer)
        alloc->free (buf);

      if (buf_size <= buf_size_max / 2)
        buf_size *= 2;
      else if (buf_size < buf_size_max)
        buf_size = buf_size_max;
      else if (buf_size_max < SIZE_MAX)
          errno = ENAMETOOLONG;
          return NULL;
      buf = (char *) alloc->allocate (buf_size);
  while (buf);

  if (alloc->die)
    alloc->die (buf_size);
  errno = ENOMEM;
  return NULL;

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