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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 path-
       name is a symbolic link, it is dereferenced.

       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, respec-
       tively.

       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 permission 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  inac-
       cessible,  then  the  access() call will fail, regardless of the permissions 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 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 "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  non-
       directory  file, then the only access() test that returns -1 is when mode is speci-
       fied 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),  euidac-
       cess(3), credentials(7), path_resolution(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2007-07-10                         ACCESS(2)

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