chown.c

glibc 库, 不仅可以学习使用库函数,还可以学习函数的具体实现,是提高功力的好资料

/* chown() compatibility.
   Copyright (C) 1998, 2000, 2002, 2003, 2005 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

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

#include <kernel-features.h>

/*
  In Linux 2.1.x the chown functions have been changed.  A new function lchown
  was introduced.  The new chown now follows symlinks - the old chown and the
  new lchown do not follow symlinks.
  This file emulates chown() under the old kernels.
*/

int
__chown (const char *file, uid_t owner, gid_t group)
{
#if __ASSUME_LCHOWN_SYSCALL
  return INLINE_SYSCALL (chown, 3, file, owner, group);
#else
  int err;
  int old_errno;
  char link[PATH_MAX + 2];
  char path[2 * PATH_MAX + 4];
  int loopct;
  size_t filelen;
  static int libc_old_chown = 0 /* -1=old linux, 1=new linux, 0=unknown */;

  if (libc_old_chown == 1)
    return INLINE_SYSCALL (chown, 3, __ptrvalue (file), owner, group);

  old_errno = errno;

# ifdef __NR_lchown
  if (libc_old_chown == 0)
    {
      err = INLINE_SYSCALL (chown, 3, __ptrvalue (file), owner, group);
      if (err != -1 || errno != ENOSYS)
	{
	  libc_old_chown = 1;
	  return err;
	}
      libc_old_chown = -1;
    }
# endif

  err = __readlink (file, link, PATH_MAX + 1);
  if (err == -1)
    {
      __set_errno (old_errno);
      return __lchown (file, owner, group);
    }

  filelen = strlen (file) + 1;
  if (filelen > sizeof (path))
    {
      __set_errno (ENAMETOOLONG);
      return -1;
    }
  memcpy (path, file, filelen);

  /* 'The system has an arbitrary limit...'  In practise, we'll hit
     ENAMETOOLONG before this, usually.  */
  for (loopct = 0; loopct < 128; ++loopct)
    {
      size_t linklen;

      if (err >= PATH_MAX + 1)
	{
	  __set_errno (ENAMETOOLONG);
	  return -1;
	}

      link[err] = 0;  /* Null-terminate string, just-in-case.  */

      linklen = strlen (link) + 1;

      if (link[0] == '/')
	memcpy (path, link, linklen);
      else
	{
	  filelen = strlen (path);

	  while (filelen > 1 && path[filelen - 1] == '/')
	    --filelen;
	  while (filelen > 0 && path[filelen - 1] != '/')
	    --filelen;
	  if (filelen + linklen > sizeof (path))
	    {
	      errno = ENAMETOOLONG;
	      return -1;
	    }
	  memcpy (path + filelen, link, linklen);
	}

      err = __readlink (path, link, PATH_MAX + 1);

      if (err == -1)
	{
	  __set_errno (old_errno);
	  return __lchown (path, owner, group);
	}
    }
  __set_errno (ELOOP);
  return -1;
#endif
}
libc_hidden_def (__chown)

#include <shlib-compat.h>
versioned_symbol (libc, __chown, chown, GLIBC_2_1);