1  /* Read the contents of a symbolic link.
       2     Copyright (C) 2003-2007, 2009-2023 Free Software Foundation, Inc.
       3  
       4     This file is free software: you can redistribute it and/or modify
       5     it under the terms of the GNU Lesser General Public License as
       6     published by the Free Software Foundation; either version 2.1 of the
       7     License, or (at your option) any later version.
       8  
       9     This file is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      12     GNU Lesser General Public License for more details.
      13  
      14     You should have received a copy of the GNU Lesser General Public License
      15     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      16  
      17  #include <config.h>
      18  
      19  /* Specification.  */
      20  #include <unistd.h>
      21  
      22  #include <errno.h>
      23  #include <string.h>
      24  #include <sys/stat.h>
      25  
      26  #if !HAVE_READLINK
      27  
      28  /* readlink() substitute for systems that don't have a readlink() function,
      29     such as DJGPP 2.03 and mingw32.  */
      30  
      31  ssize_t
      32  readlink (char const *file, _GL_UNUSED char *buf,
      33            _GL_UNUSED size_t bufsize)
      34  {
      35    struct stat statbuf;
      36  
      37    /* In general we should use lstat() here, not stat().  But on platforms
      38       without symbolic links, lstat() - if it exists - would be equivalent to
      39       stat(), therefore we can use stat().  This saves us a configure check.  */
      40    if (stat (file, &statbuf) >= 0)
      41      errno = EINVAL;
      42    return -1;
      43  }
      44  
      45  #else /* HAVE_READLINK */
      46  
      47  # undef readlink
      48  
      49  /* readlink() wrapper that uses correct types, for systems like cygwin
      50     1.5.x where readlink returns int, and which rejects trailing slash,
      51     for Solaris 9.  */
      52  
      53  ssize_t
      54  rpl_readlink (char const *file, char *buf, size_t bufsize)
      55  {
      56  # if READLINK_TRAILING_SLASH_BUG
      57    size_t file_len = strlen (file);
      58    if (file_len && file[file_len - 1] == '/')
      59      {
      60        /* Even if FILE without the slash is a symlink to a directory,
      61           both lstat() and stat() must resolve the trailing slash to
      62           the directory rather than the symlink.  We can therefore
      63           safely use stat() to distinguish between EINVAL and
      64           ENOTDIR/ENOENT, avoiding extra overhead of rpl_lstat().  */
      65        struct stat st;
      66        if (stat (file, &st) == 0 || errno == EOVERFLOW)
      67          errno = EINVAL;
      68        return -1;
      69      }
      70  # endif /* READLINK_TRAILING_SLASH_BUG */
      71  
      72    ssize_t r = readlink (file, buf, bufsize);
      73  
      74  # if READLINK_TRUNCATE_BUG
      75    if (r < 0 && errno == ERANGE)
      76      {
      77        /* Try again with a bigger buffer.  This is just for test cases;
      78           real code invariably discards short reads.  */
      79        char stackbuf[4032];
      80        r = readlink (file, stackbuf, sizeof stackbuf);
      81        if (r < 0)
      82          {
      83            if (errno == ERANGE)
      84              {
      85                /* Clear the buffer, which is good enough for real code.
      86                   Thankfully, no test cases try short reads of enormous
      87                   symlinks and what would be the point anyway?  */
      88                r = bufsize;
      89                memset (buf, 0, r);
      90              }
      91          }
      92        else
      93          {
      94            if (bufsize < r)
      95              r = bufsize;
      96            memcpy (buf, stackbuf, r);
      97          }
      98      }
      99  # endif
     100  
     101    return r;
     102  }
     103  
     104  #endif /* HAVE_READLINK */