access - check user's permissions for a file


     #include <unistd.h>

     int access(const char *pathname, int mode));


     access checks whether the process would be allowed to  read,
     write  or test for existence of the file (or other file sys-
     tem object) whose name is pathname.  If pathname is  a  sym-
     bolic  link permissions of the file referred to by this sym-
     bolic link are tested.

     mode is a mask consisting of one or more of R_OK, W_OK, X_OK

     R_OK, W_OK and X_OK request checking whether the file exists
     and  has  read, write and execute permissions, respectively.
     F_OK just requests checking for the existence of the file.

     The tests depend  on  the  permissions  of  the  directories
     occurring in the path to the file, as given in pathname, and
     on the permissions of directories and files referred  to  by
     symbolic links encountered on the way.

     The check is done with  the  process's  real  uid  and  gid,
     rather  than with the effective ids as is done when actually
     attempting an operation.  This is to allow set-UID  programs
     to easily determine the invoking user's authority.

     Only access bits are checked, not the file type or contents.
     Therefore,  if  a  directory  is  found to be "writable," it
     probably means that files can be created in  the  directory,
     and  not that the directory can be written as a file.  Simi-
     larly, a DOS file may be found to be "executable,"  but  the
     execve(2) call will still fail.


     On success (all  requested  permissions  granted),  zero  is
     returned.   On  error  (at least one bit in mode asked for a
     permission that is denied, or some other error occurred), -1
     is returned, and errno is set appropriately.


     EACCES  The requested access would be denied to the file  or
             search  permission  is  denied  to one of the direc-
             tories in pathname.

     EROFS   Write permission was  requested  for  a  file  on  a
             read-only filesystem.

     EFAULT  pathname  points  outside  your  accessible  address

     EINVAL  mode was incorrectly specified.

             pathname is too long.

     ENOENT  A directory component in pathname  would  have  been
             accessible but does not exist or was a dangling sym-
             bolic link.

     ENOTDIR A component used as a directory in pathname is  not,
             in fact, a directory.

     ENOMEM  Insufficient kernel memory was available.

     ELOOP   Too many symbolic links were encountered in  resolv-
             ing pathname.

     EIO     An I/O error occurred.


     access returns an error if any of the access  types  in  the
     requested  call fails, even if other types might be success-

     access may not work correctly on NFS file systems  with  UID
     mapping  enabled,  because UID mapping is done on the server
     and hidden from the client, which checks permissions.

     Using access to check if a user is authorized to e.g. open a
     file  before actually doing so using open(2) creates a secu-
     rity hole, because the user might  exploit  the  short  time
     interval between checking and opening the file to manipulate




     stat(2), open(2), chmod(2), setuid(2), setgid(2).