Tests a specific event flag and returns immediately if the flag
    is set; otherwise, the process is placed in a wait state until
    the event flag is set.

    Format

      SYS$WAITFR  efn

    C Prototype

      int sys$waitfr  (unsigned int efn);

1 - Argument

 efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of the event flag for which to wait. The efn argument is a
    longword containing this number; however, $WAITFR uses only the
    low-order byte.