access


SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       access()  checks  whether the calling process can access the file path-
       name.  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  eas-
       ily 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, 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.

ERRORS
       access() shall fail if:

       EACCES The requested access would be denied to the file, or search per-
              mission 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
              filesystem.

              cuted.

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

NOTES
       Warning:  Using access() to check if a user is authorized to, for exam-
       ple, open a file before actually doing so using open(2) creates a secu-
       rity  hole,  because  the  user  might  exploit the short time interval
       between checking and opening the file to manipulate it.  For this  rea-
       son,  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_SYM-
       LINK_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.   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.

       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.

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 filesystem.  Since  ker-
       nel 2.6.20, access() honors this flag.

Linux                             2013-09-13                         ACCESS(2)
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