sysdeps/unix/sysv/linux/readlinkat.c

   1 /* Copyright (C) 2005-2014 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library 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 GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <fcntl.h>
  20 #include <stddef.h>
  21 #include <stdio.h>
  22 #include <stdlib.h>
  23 #include <string.h>
  24 #include <sysdep.h>
  25 #include <unistd.h>
  26 #include <kernel-features.h>
  27
  28
  29 /* Read the contents of the symbolic link PATH relative to FD into no
  30    more than LEN bytes of BUF.  */
  31 ssize_t
  32 readlinkat (fd, path, buf, len)
  33      int fd;
  34      const char *path;
  35      char *buf;
  36      size_t len;
  37 {
  38   int result;
  39
  40 #ifdef __NR_readlinkat
  41 # ifndef __ASSUME_ATFCTS
  42   if (__have_atfcts >= 0)
  43 # endif
  44     {
  45       result = INLINE_SYSCALL (readlinkat, 4, fd, path, buf, len);
  46 # ifndef __ASSUME_ATFCTS
  47       if (result == -1 && errno == ENOSYS)
  48         __have_atfcts = -1;
  49       else
  50 # endif
  51         return result;
  52     }
  53 #endif
  54
  55 #ifndef __ASSUME_ATFCTS
  56   char *pathbuf = NULL;
  57
  58   if (fd != AT_FDCWD && path[0] != '/')
  59     {
  60       size_t pathlen = strlen (path);
  61       if (__builtin_expect (pathlen == 0, 0))
  62         {
  63           __set_errno (ENOENT);
  64           return -1;
  65         }
  66
  67       static const char procfd[] = "/proc/self/fd/%d/%s";
  68       /* Buffer for the path name we are going to use.  It consists of
  69          - the string /proc/self/fd/
  70          - the file descriptor number
  71          - the file name provided.
  72          The final NUL is included in the sizeof.   A bit of overhead
  73          due to the format elements compensates for possible negative
  74          numbers.  */
  75       size_t buflen = sizeof (procfd) + sizeof (int) * 3 + pathlen;
  76       pathbuf = __alloca (buflen);
  77
  78       __snprintf (pathbuf, buflen, procfd, fd, path);
  79       path = pathbuf;
  80     }
  81
  82   INTERNAL_SYSCALL_DECL (err);
  83
  84   result = INTERNAL_SYSCALL (readlink, err, 3, path, buf, len);
  85
  86   if (__builtin_expect (INTERNAL_SYSCALL_ERROR_P (result, err), 0))
  87     {
  88       __atfct_seterrno (INTERNAL_SYSCALL_ERRNO (result, err), fd, pathbuf);
  89       result = -1;
  90     }
  91
  92   return result;
  93 #endif
  94 }
  95 libc_hidden_def (readlinkat)