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       access - check real user's permissions for a file


       #include <unistd.h>

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


       access()  checks whether the calling process can access the file pathname.  If pathname 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, 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.


       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.


       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

       ELOOP  Too many symbolic links were encountered in resolving pathname.

              pathname is too long.

       ENOENT A component of pathname does not exist or is a dangling symbolic link.

              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.

              Write access was requested to an executable which is being executed.


       SVr4, 4.3BSD, POSIX.1-2001.


       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.   (In  the  example  just
       described, a safer alternative would be to temporarily switch the process's effective user
       ID to the real ID and then call open(2).)

       access() always dereferences symbolic links.  If you need to check the  permissions  on  a
       symbolic link, use faccessat(2) with the flag AT_SYMLINK_NOFOLLOW.

       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  an  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 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.


       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.


       chmod(2),  chown(2),  faccessat(2), open(2), setgid(2), setuid(2), stat(2), euidaccess(3),
       credentials(7), path_resolution(7)


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