access(2) - NetBSD Manual Pages

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ACCESS(2)                 NetBSD System Calls Manual                 ACCESS(2)


NAME

     access -- check access permissions of a file or pathname


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

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


DESCRIPTION

     The access() function checks the accessibility of the file named by path
     for the access permissions indicated by mode.  The value of mode is 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.  All components of the pathname
     path are checked for access permissions (including F_OK).

     The real user ID is used in place of the effective user ID and the real
     group access list (including the real group ID) are used in place of the
     effective ID for verifying permission.

     If a process has super-user privileges and indicates success for R_OK or
     W_OK, the file may not actually have read or write permission bits set.
     If a process has super-user privileges and indicates success for X_OK, at
     least one of the user, group, or other execute bits is set.  (However,
     the file may still not be executable.  See execve(2).)


RETURN VALUES

     If path cannot be found or if any of the desired access modes would not
     be granted, then a -1 value is returned; otherwise a 0 value is returned.


ERRORS

     Access to the file is denied if:

     [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.  The owner of a file has
                        permission checked with respect to the ``owner'' read,
                        write, and execute mode bits, members of the file's
                        group other than the owner have permission checked
                        with respect to the ``group'' mode bits, and all oth-
                        ers have permissions checked with respect to the
                        ``other'' mode bits.

     [EFAULT]           path points outside the process's allocated address
                        space.

     [EIO]              An I/O error occurred while reading from or writing to
                        the file system.

     [ELOOP]            Too many symbolic links were encountered in translat-
                        ing the pathname.

     [ENAMETOOLONG]     A component of a pathname exceeded {NAME_MAX} charac-
                        ters, or an entire path name exceeded {PATH_MAX} char-
                        acters.

     [ENOENT]           The named file does not exist.

     [ENOTDIR]          A component of the path prefix is not a directory.

     [EROFS]            Write access is requested for a file on a read-only
                        file system.

     [ETXTBSY]          Write access is requested for a pure procedure (shared
                        text) file presently being executed.


SEE ALSO

     chmod(2), execve(2), stat(2), secure_path(3)


STANDARDS

     The access() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').


SECURITY CONSIDERATIONS

     The access() system call is a potential security hole due to race condi-
     tions.  It 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 access() system call may however have some value in providing clues
     to users as to whether certain operations make sense for a particular
     filesystem object.  Arguably it also allows a cheaper file existence test
     than stat(2).

NetBSD 6.1.2                      May 3, 2010                     NetBSD 6.1.2