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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, or mode is F_OK and the file exists), zero
       is returned.  On error (at least one bit in mode asked for a permission that is denied, or
       mode  is  F_OK and the file does not exist, 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 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.

              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 accessible only 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 NFSv2 filesystems with UID mapping enabled, because UID
       mapping  is done on the server and hidden from the client, which checks permissions.  (NFS
       versions 3 and higher perform the check on the server.)  Similar  problems  can  occur  to
       FUSE mounts.


       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 filesystem.  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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