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         #include <unistd.h>
         access(const char *path, int mode);
         eaccess(const char *path, int mode);
         faccessat(int fd, const char *path, int mode, int flag);


         The access() and eaccess() system calls check the accessibility of the
         file named by the path argument for the access permissions indicated by
         the mode argument.  The value of mode is either the bitwise-inclusive OR
         of the access permissions to be checked (R_OK for read permission, W_OK
         for write permission, and X_OK for execute/search permission), or the
         existence test (F_OK).
         For additional information, see the File Access Permission section of
         The eaccess() system call uses the effective user ID and the group access
         list to authorize the request; the access() system call uses the real
         user ID in place of the effective user ID, the real group ID in place of
         the effective group ID, and the rest of the group access list.
         The faccessat() system call is equivalent to access() except in the case
         where path specifies a relative path.  In this case the file whose acces-
         sibility is to be determined is located relative to the directory associ-
         ated with the file descriptor fd instead of the current working direc-
         tory.  If faccessat() is passed the special value AT_FDCWD in the fd
         parameter, the current working directory is used and the behavior is
         identical to a call to access().  Values for flag are constructed by a
         bitwise-inclusive OR of flags from the following list, defined in
                 The checks for accessibility are performed using the effective
                 user and group IDs instead of the real user and group ID as
                 required in a call to access().
         Even if a process's real or effective user has appropriate privileges and
         indicates success for X_OK, the file may not actually have execute per-
         mission bits set.  Likewise for R_OK and W_OK.


         Upon successful completion, the value 0 is returned; otherwise the
         value -1 is returned and the global variable errno is set to indicate the
                            file system.
         [ETXTBSY]          Write access is requested for a pure procedure (shared
                            text) file presently being executed.
         [EACCES]           Permission bits of the file mode do not permit the
                            requested access, or search permission is denied on a
                            component of the path prefix.
         [EFAULT]           The path argument points outside the process's allo-
                            cated address space.
         [EIO]              An I/O error occurred while reading from or writing to
                            the file system.
         Also, the faccessat() system call may fail if:
         [EBADF]            The path argument does not specify an absolute path
                            and the fd argument is neither AT_FDCWD nor a valid
                            file descriptor.
         [EINVAL]           The value of the flag argument is not valid.
         [ENOTDIR]          The path argument is not an absolute path and fd is
                            neither AT_FDCWD nor a file descriptor associated with
                            a directory.


         The access() system call is a potential security hole due to race condi-
         tions and should never be used.  Set-user-ID and set-group-ID applica-
         tions should restore the effective user or group ID, and perform actions
         directly rather than use access() to simulate access checks for the real
         user or group ID.  The eaccess() system call likewise may be subject to
         races if used inappropriately.
         access() remains useful for providing clues to users as to whether opera-
         tions make sense for particular filesystem objects (e.g. 'delete' menu
         item only highlighted in a writable folder ... avoiding interpretation of
         the st_mode bits that the application might not understand -- e.g. in the
         case of AFS).  It also allows a cheaper file existence test than stat(2).


         chmod(2), intro(2), stat(2)


         The access() system call is expected to conform to ISO/IEC 9945-1:1990
         ("POSIX.1").  The faccessat() system call follows The Open Group Extended
         API Set 2 specification.


         The access() function appeared in Version 7 AT&T UNIX.  The faccessat()
         system call appeared in FreeBSD 8.0.

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