sysdeps/unix/sysv/linux/readlinkat.c

   1 /* Copyright (C) 2005, 2006 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, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    02111-1307 USA.  */
  18
  19 #include <errno.h>
  20 #include <fcntl.h>
  21 #include <stddef.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25 #include <sysdep.h>
  26 #include <unistd.h>
  27 #include <kernel-features.h>
  28
  29
  30 /* Read the contents of the symbolic link PATH relative to FD into no
  31    more than LEN bytes of BUF.  */
  32 ssize_t
  33 readlinkat (fd, path, buf, len)
  34      int fd;
  35      const char *path;
  36      char *buf;
  37      size_t len;
  38 {
  39   int result;
  40
  41 #ifdef __NR_readlinkat
  42 # ifndef __ASSUME_ATFCTS
  43   if (__have_atfcts >= 0)
  44 # endif
  45     {
  46       result = INLINE_SYSCALL (readlinkat, 4, fd, path, buf, len);
  47 # ifndef __ASSUME_ATFCTS
  48       if (result == -1 && errno == ENOSYS)
  49         __have_atfcts = -1;
  50       else
  51 # endif
  52         return result;
  53     }
  54 #endif
  55
  56 #ifndef __ASSUME_ATFCTS
  57   char *pathbuf = NULL;
  58
  59   if (fd != AT_FDCWD && path[0] != '/')
  60     {
  61       size_t pathlen = strlen (path);
  62       static const char procfd[] = "/proc/self/fd/%d/%s";
  63       /* Buffer for the path name we are going to use.  It consists of
  64          - the string /proc/self/fd/
  65          - the file descriptor number
  66          - the file name provided.
  67          The final NUL is included in the sizeof.   A bit of overhead
  68          due to the format elements compensates for possible negative
  69          numbers.  */
  70       size_t buflen = sizeof (procfd) + sizeof (int) * 3 + pathlen;
  71       pathbuf = __alloca (buflen);
  72
  73       __snprintf (pathbuf, buflen, procfd, fd, path);
  74       path = pathbuf;
  75     }
  76
  77   INTERNAL_SYSCALL_DECL (err);
  78
  79   result = INTERNAL_SYSCALL (readlink, err, 3, path, buf, len);
  80
  81   if (__builtin_expect (INTERNAL_SYSCALL_ERROR_P (result, err), 0))
  82     {
  83       __atfct_seterrno (INTERNAL_SYSCALL_ERRNO (result, err), fd, pathbuf);
  84       result = -1;
  85     }
  86
  87   return result;
  88 #endif
  89 }