lib/readlink.c

   1 /* Read the contents of a symbolic link.
   2    Copyright (C) 2003-2007, 2009-2024 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 # if defined __CYGWIN__
 102   /* On Cygwin 3.3.6, readlink("/dev/null") returns "\\Device\\Null", which
 103      is unusable.  Better fail with EINVAL.  */
 104   if (r > 0 && strncmp (file, "/dev/", 5) == 0 && buf[0] == '\\')
 105     {
 106       errno = EINVAL;
 107       return -1;
 108     }
 109 # endif
 110
 111   return r;
 112 }
 113
 114 #endif /* HAVE_READLINK */