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

       access, faccessat - check user's permissions for a file

       #include <unistd.h>

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

       #include <fcntl.h>           /* Definition of AT_* constants */
       #include <unistd.h>

       int faccessat(int dirfd, const char *pathname, int mode, int flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

       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.

       The  faccessat()  system  call  operates  in  exactly  the  same way as
       access(), except for the differences described here.

       If the pathname given in pathname is relative, then it  is  interpreted
       relative  to  the  directory  referred  to by the file descriptor dirfd
       (rather than relative to the current working directory of  the  calling
       process, as is done by access() for a relative pathname).

       If  pathname  is relative and dirfd is the special value AT_FDCWD, then
       pathname is interpreted relative to the current  working  directory  of
       the calling process (like access()).

       If pathname is absolute, then dirfd is ignored.

       flags  is  constructed  by ORing together zero or more of the following

              Perform access checks using the effective user  and  group  IDs.
              By default, faccessat() uses the real IDs (like access()).

              If  pathname  is a symbolic link, do not dereference it: instead
              return information about the link itself.

       See openat(2) for an explanation of the need for faccessat().

       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() and faccessat() 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.

              pathname is too long.

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

              A  component  used as a directory in pathname is not, in fact, a

       EROFS  Write permission  was  requested  for  a  file  on  a  read-only

       access() and faccessat() 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 exe-

       The following additional errors can occur for faccessat():

       EBADF  dirfd is not a valid file descriptor.

       EINVAL Invalid flag specified in flags.

              pathname is relative and dirfd is a file descriptor referring to
              a file other than a directory.

       faccessat()  was  added  to Linux in kernel 2.6.16; library support was
       added to glibc in version 2.4.

       access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       faccessat(): POSIX.1-2008.

       Warning: Using these calls 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 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-

       These calls return 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.

       These  calls  may not work correctly on NFSv2 filesystems with UID map-
       ping enabled, because UID mapping is done on the server and hidden from
       the  client, which checks permissions.  (NFS versions 3 and higher per-
       form the check on the server.)  Similar  problems  can  occur  to  FUSE

   C library/kernel ABI differences
       The  raw  faccessat() system call takes only the first three arguments.
       The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are  actually  implemented
       within  the glibc wrapper function for faccessat().  If either of these
       flags is specified, then the wrapper  function  employs  fstatat(2)  to
       determine access permissions.

   Glibc notes
       On older kernels where faccessat() is unavailable (and when the AT_EAC-
       CESS and AT_SYMLINK_NOFOLLOW flags are not specified), the glibc  wrap-
       per  function  falls  back  to the use of access().  When pathname is a
       relative pathname, glibc constructs a pathname based  on  the  symbolic
       link in /proc/self/fd that corresponds to the dirfd argument.

       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,  these  calls  ignored  the  effect of the
       MS_NOEXEC flag if it was used to mount(2)  the  underlying  filesystem.
       Since kernel 2.6.20, the MS_NOEXEC is honored

       chmod(2),  chown(2),  open(2),  setgid(2),  setuid(2), stat(2), euidac-
       cess(3), credentials(7), path_resolution(7), symlink(7)

       This page is part of release 3.74 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

Linux                             2014-08-19                         ACCESS(2)

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