access

ACCESS(2)                  Linux Programmer's Manual                 ACCESS(2)



NAME
       access - check user's permissions for a file

SYNOPSIS
       #include <unistd.h>

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

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

       mode is a mask consisting of one or more of R_OK, W_OK, X_OK and  F_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.  There-
       fore, 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.  Similarly, a DOS file may be found to  be  "exe-
       cutable," but the execve(2) call will still fail.

       If  the process has appropriate privileges, an implementation may indi-
       cate success for X_OK even if none of the execute file permission  bits
       are set.

RETURN VALUE
       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 appro-
       priately.

ERRORS
       access shall fail if:

       EACCES The requested access would be denied to the file or search  per-
              mission is denied to one of the directories in pathname.

       ELOOP  Too  many symbolic links were encountered in resolving pathname.

       ENAMETOOLONG
              pathname is too long.

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

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

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

       access may fail if:

       EFAULT pathname points outside your accessible address space.

       EINVAL mode was incorrectly specified.

       EIO    An I/O error occurred.

       ENOMEM Insufficient kernel memory was available.

       ETXTBSY
              Write  access was requested to an executable which is being exe-
              cuted.

RESTRICTIONS
       access returns an error if any of the access  types  in  the  requested
       call fails, even if other types might be successful.

       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 security hole, because
       the  user  might  exploit  the short time interval between checking and
       opening the file to manipulate it.

CONFORMING TO
       SVID, AT&T, POSIX, X/OPEN, BSD 4.3

SEE ALSO
       stat(2), open(2), chmod(2), chown(2), setuid(2), setgid(2)



Linux                             2002-04-23                         ACCESS(2)