<null value>

── NULL ───────────────────────────────────────────────────────────────┤

A null value can be used to uninitialize procedure references, pointers, array references, structure block references, or connection block references to null through the use of <pointer assignment>, <array reference assignment>, <procedure reference assignment>, <structure block reference assignment>, or <connection block reference assignment>. NULL can also be used as an actual parameter for a formal procedure, array, call-by-value pointer parameter, structure block variable parameter, or connection block parameter.

When used in a procedure reference assignment, if NULL is specified and there is an environment called NULL, then a reference to the procedure called NULL is assigned. If NULL is specified and there is no environment called NULL, then a NULL value is assigned to the procedure reference variable.

If a pointer or array reference variable contains NULL and an attempt to use that variable is made, a program interrupt occurs. If the procedure reference variable is invoked while it is NULL, a program interrupt occurs.