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



NAME
       access - check real user's permissions for a file

SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       access() checks whether the calling process can access the file pathname.  If pathname is a symbolic link, it is derefer-
       enced.

       The mode specifies the accessibility check(s) to be performed, and is either the value F_OK, or a mask consisting of  the
       bitwise  OR of one or more of R_OK, W_OK, and X_OK.  F_OK tests for the existence of the file.  R_OK, W_OK, and X_OK test
       whether the file exists and grants read, write, and execute permissions, respectively.

       The check is done using the calling process's real UID and GID, rather than the effective IDs as is  done  when  actually
       attempting  an  operation (e.g., open(2)) on the file.  This allows set-user-ID programs to easily determine the invoking
       user's authority.

       If the calling process is privileged (i.e., its real UID is zero), then an X_OK check is successful for a regular file if
       execute permission is enabled for any of the file owner, group, or other.

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

ERRORS
       access() shall fail if:

       EACCES The requested access would be denied to the file, or search permission is denied for one of the directories in the
              path prefix of pathname.  (See also path_resolution(7).)

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ENAMETOOLONG
              pathname is too long.

       ENOENT A component of pathname does not exist or is 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 file system.

       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 executed.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

NOTES
       Warning:  Using  access()  to  check  if a user is authorized to, for example, 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.  For this reason, the use of this system call should be avoided.

       access()  returns  an  error if any of the access types in mode is denied, even if some of the other access types in mode
       are permitted.

       If the calling process has appropriate privileges (i.e., is superuser), POSIX.1-2001 permits implementation  to  indicate
       success for an X_OK check even if none of the execute file permission bits are set.  Linux does not do this.

       A  file  is  only  accessible  if  the permissions on each of the directories in the path prefix of pathname grant search
       (i.e., execute) access.  If any directory is inaccessible, then the access() call will fail, regardless  of  the  permis-
       sions on the file itself.

       Only access bits are checked, not the file type or contents.  Therefore, if a directory is found to be writable, it prob-
       ably 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 "executable," but the execve(2) call will still fail.

       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.

BUGS
       In kernel 2.4 (and earlier) there is some strangeness in the handling of X_OK tests for superuser.  If all categories  of
       execute  permission  are  disabled  for  a nondirectory file, then the only access() test that returns -1 is when mode is
       specified as just X_OK; if R_OK or W_OK is also specified in mode, then access() returns 0 for  such  files.   Early  2.6
       kernels (up to and including 2.6.3) also behaved in the same way as kernel 2.4.

       In  kernels  before  2.6.20,  access() ignored the effect of the MS_NOEXEC flag if it was used to mount(2) the underlying
       file system.  Since kernel 2.6.20, access() honors this flag.

SEE ALSO
       chmod(2), chown(2), faccessat(2), open(2), setgid(2), setuid(2),  stat(2),  euidaccess(3),  credentials(7),  path_resolu-
       tion(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2007-07-10                                                  ACCESS(2)